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I was doing some research regarding the Soyuz as I'm currently planning a summer project in which I would build a close replica of a Soyuz when the question popped into my mind, "What would happen if there was a freak accident in which that the ISS needed to be evacuated and there was ONLY one space craft available?" I'll leave y'all's response on the matter to either be regarding the Crew Dragon or the Soyuz.

James Ervin
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By design that will never happen. There are always enough return seats for the crew.

This is exactly why

  • the whole crew of one of the visiting vehicles gets in it whenever it undocks, even when it is only being moved from one docking port of the station to another

  • the crews retreat to their vehicles in fire / leak / toxic atmosphere emergencies

  • the crews retreat to their vehicles if a collision risk is discovered too late to execute a safe distancing maneuver

See also Can a single Soyuz return a crew of six back to Earth?

Organic Marble
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What would happen if there was a freak accident in which that the ISS needed to be evacuated and there was ONLY one space craft available?

Carrying that to an even greater extreme, what would happen if there was a freak accident in which the ISS needed to be evacuated and no space craft was available? For example, suppose two human-qualified vehicles are docked with the ISS. Suppose an unseen 10 cm diameter asteroid hits one of those escape vehicles at 30 km/s. It would rip right through that vehicle. Suppose it ripped open a propellant tank. Kaboom! Suppose some of the resulting debris opened holes in the ISS, and other chunks opened holes in the other escape vehicle.

The answer is simple: The astronauts and cosmonauts on the ISS will die. Being an astronaut, cosmonaut, or taikonaut is not exactly the safest career choice. One of the standard questions potential astronauts are asked is whether they are willing to die given that over 10% of astronauts have died on the job. Potential astronauts have to be willing to accept that possible outcome. They also have to be not willing to desire that possible outcome.

The ISS is intended to be two fault tolerant regarding loss of life at the three sigma level with respect to reasonably plausible failures. The ISS is not even one fault tolerant with respect to some outrageously implausible root causes such as my hypothetical 10 cm asteroid. It's also noteworthy that the three sigma level is not that high of a bar; modern Earth-bound manufacturing is moving toward making the devices they make safe and reliable at the six sigma level.

Bottom line: There are semi-plausible scenarios where astronauts or cosmonauts will die. This is a risk that astronauts and cosmonauts sign on for, but it also is a risk that they work very aggressively to counteract.

David Hammen
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    Either you die, or you beat Felix Baumgartner's record ;-). – Peter - Reinstate Monica Apr 16 '21 at 13:08
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    @Peter-ReinstateMonica You'd need the delta-v to de-orbit, first. Might work if last they ate was cabbage soup. – Polygnome Apr 16 '21 at 14:43
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    @Polygnome A job for Jo Nesbo's Doctor Proctor! :-) – Peter - Reinstate Monica Apr 16 '21 at 16:28
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    Do you have a source for that 10% statistic? I know there's already a follow-up question but anyway. – Everyday Astronaut Apr 16 '21 at 21:17
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    @EverydayAstronaut My sources are personal. I've been fortunate to know three fantastic people who progressed fairly highly in the very competitive astronaut candidate selection process. While none made it to the stage of being an ASCAN (astronaut candidate), they did progress far enough so as to have been subjected to various forms of what I would call torture. Some of the torture was physical (e.g., being centrifuged), but most was psychological. – David Hammen Apr 17 '21 at 10:16
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    Asking "are you willing to die" is a form of psychological torture, particularly when backed up with probabilities close to those of Russian roulette. Whether that number is valid is a different question. All three of those candidate astronauts (as opposed to astronaut candidates) told me about being asked such a question by NASA. – David Hammen Apr 17 '21 at 10:20
  • 10% only because of one vehicle that was unsafe at any speed. One day they'll find the Intrepid sunk at its moorings (despite resting on silt), and when they investigate, the root cause will trace back to the Space Shuttle somehow lol... – Harper - Reinstate Monica Apr 18 '21 at 16:26
  • @Harper-ReinstateMonica 355 individuals flew on the shuttle and 14 got killed by it. Admittedly my math isn't great but that's about 4% Not sure folks getting killed in the space program is the best thing to 'lol' about – Organic Marble Apr 18 '21 at 17:06
  • @OrganicMarble Astronauts have also died in training exercises (e.g., Apollo 1), in training flights / experimental flights (e.g., various T-38 and experimental aircraft accidents), and even in required commercial travel. It's a good deal more than 14 deaths. In addition, your 355 number is high. This is a per career statistic rather than a per flight statistic. The two astronauts who flew on the Shuttle seven times only count as two instead of fourteen. There were a lot of astronauts who flew the Shuttle more than once. – David Hammen Apr 18 '21 at 17:44
  • @DavidHammen my 355 number is exact. I said 355 "individuals" flew on the shuttle, and that is correct. https://www.nasa.gov/pdf/566250main_2011.07.05%20SHUTTLE%20ERA%20FACTS.pdf There are many ways to slice and dice the numbers depending on what your desired result is. – Organic Marble Apr 18 '21 at 18:01
  • @OrganicMarble you lifted my "lol" out of context, so no "high horse" for you... – Harper - Reinstate Monica Apr 18 '21 at 18:06
  • @Harper-ReinstateMonica cool, now work on your 10% – Organic Marble Apr 18 '21 at 18:10
  • @DavidHammen Your answer says "given that over 10% of astronauts have died on the job" which I find misleading ... given that your comment says "Whether that number is valid is a different question". Anyway thanks for the interesting story about those 3 people's experiences. – Everyday Astronaut May 08 '21 at 21:01
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There WAS a different concept for returning folks from orbit in emergencies that was considered, called MOOSE, but it never got out of the planning stages.

MOOSE

DrSheldon
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Rob
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