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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.

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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?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Apr 19 at 12:23
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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.

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    $\begingroup$ Either you die, or you beat Felix Baumgartner's record ;-). $\endgroup$ – Peter - Reinstate Monica Apr 16 at 13:08
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    $\begingroup$ @Peter-ReinstateMonica You'd need the delta-v to de-orbit, first. Might work if last they ate was cabbage soup. $\endgroup$ – Polygnome Apr 16 at 14:43
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    $\begingroup$ Do you have a source for that 10% statistic? I know there's already a follow-up question but anyway. $\endgroup$ – Everyday Astronaut Apr 16 at 21:17
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    $\begingroup$ @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. $\endgroup$ – David Hammen Apr 17 at 10:16
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    $\begingroup$ 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. $\endgroup$ – David Hammen Apr 17 at 10:20
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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

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