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The International Space Station typically has one or more (manned) spacecraft docked. This is obviously to ferry crew to and from the station for each of the Expeditions. The Soyuz currently fulfils this role, but during its operation the Space Shuttle was also used.

In the event of an emergency the docked craft can be used to evacuate the ISS and return the crew safely to Earth.

At any point has there ever not been the capacity for a full evacuation?

Common sense says no - every crew member went up in a seat therefore there should always be one available for them to evacuate. Also crew safety is clearly the primary concern.

However, not all launches are full, not all landings are full and some astronauts stay for multiple Expeditions, meaning the number of crew doesn't always match the number of available seats. This assumes that a full evacuation means every crew member has a seat.

NASA had a plan for a permanent escape vehicle, but this was cancelled.

Edit: I'd like to be certain that there were no situations (for example during relocation of a Soyuz craft - though this case has been addressed). We can also ignore EVAs where technically prompt evacuation would have been difficult. Granted, this may be difficult to prove definitively.

Edit 2: Although the conclusions seem pretty clear, I'm still not entirely convinced that there was never a short period without full evacuation capacity. There is some good discussion on NASASpaceflight on the rationale behind always ensuring evacuation. This paper also details the studies into a dedicated ACRV.

In the absence of an official word from one of the organisations involved, or unless someone can find a cross-referenced chronology of all astronaut activity, I'll accept the top answer as it's certain to be correct!

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  • $\begingroup$ Unfortunately NASA has chosen to not make the ISS flight rules publicly available. The definitive answer is in there. There's a timeline of every docking and undocking here: nasa.gov/feature/… but it would be a lot of work to go through and check where each crewmember was at the time. Most of the items on the web site link to a press release which give more details. Here's the first Soyuz relocation, showing how the whole crew got in.nasa.gov/centers/johnson/news/station/2001/iss01-06.html $\endgroup$ – Organic Marble Jun 14 '18 at 13:27
  • $\begingroup$ @OrganicMarble Yeh it's a shame, I'd love to see one like the Shuttle flight rules. I guess it's withheld beacuse of the international aspect? Hopefully it'll be released after decommission (not that I'm looking forward to that day)! Wow, yeh it'd be a bit of an undertaking to go through all that! $\endgroup$ – Jack Jun 14 '18 at 13:46
  • $\begingroup$ I'd like that too! There was always so much more quality material released on Shuttle than ISS. The Shuttle flight rules and a lot of the stuff on the JSC FDF page didn't get published until after Columbia though. The shuttle flight rules before that had a big warning printed in them PUBLISHING ON ANY PUBLIC ELECTRONIC FORUM IS EXPRESSLY FORBIDDEN. Why? $\endgroup$ – Organic Marble Jun 14 '18 at 13:52
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It never actually happened, but there was a case when this situation had the potential for happening.

That was the last shuttle flight, STS-135.

If a critical flaw in the thermal protection system of the Orbiter had been detected, the shuttle crew would have had to hang out in the ISS until they could be rotated home using spare Soyuz capability. They all got measured for the custom Soyuz seat liners to cover this case.

This was because there was no more available Orbiters for a rescue flight!

The stay could potentially have been quite long for the last crewmember, months, maybe a year if I remember correctly.

This was a major reason for having the final crew size be only four astronauts.

Reference 1 - Houston Chronicle (may not be available in some regions)

Reference 2 - spaceflightnow.com

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    $\begingroup$ Of course, if any Soyuz mission was stuck in a similar fashion, the same thing would have happened. $\endgroup$ – PearsonArtPhoto Jun 11 '18 at 19:33
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    $\begingroup$ There was the potential of a Shuttle rescue mission though, before the last one. Not quickly, of course. $\endgroup$ – Organic Marble Jun 11 '18 at 19:35
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    $\begingroup$ There was also potential for two shuttle crews to get stranded on ISS that way... $\endgroup$ – SF. Jun 11 '18 at 23:45
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    $\begingroup$ @SF. After the Columbia failure a special wiring harness was fabricated and carried in the Orbiter. This harness allowed any of the operations that were previously only manual (like dropping the gear) to be controlled by the ground. If the heat shield was critically damaged the plan was to install the harness and try and land the Orbiter at Edwards by remote control. $\endgroup$ – Organic Marble Jun 12 '18 at 0:36
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    $\begingroup$ @Jack: IIRC the only period after initial "unmanned" when ISS was without lifeboat, it was also unmanned - while moving Soyuz to a different port. The maneuver could have been performed by a single person or remote control easily, but the Soyuz was boarded by three crew members exactly for the reason so that they wouldn't be stranded in case the redocking maneuver fails. $\endgroup$ – SF. Jun 12 '18 at 11:39
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No. There has never been a point where a crew member had no way to 'leave'.

Even when the Shuttle did crew transfers, they took back 3 and left 3 behind or some combination of that approach.

The Soyuz capsule needs custom seat liners for each passenger, since landing is somewhat hard, and wrinkles and bumps will hurt a lot.

Part of finalizing the crew transfer is when they move their seat liners into their appropriate Soyuz capsule.

When the Russian segment crew is reduced to 2, from three that leaves a seat open for a possible tourist or other country to pay for a short visit. In those cases, the new Soyuz typically arrives and later the old Soyuz leaves. Thus the tourist hangs around for a week or so till the old Soyuz is set to land.

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  • $\begingroup$ Thank you. Do you possibly have any sources for this? Is this true even for operations like Soyuz relocation? I appreciate that disproof is much harder in this case! $\endgroup$ – Jack Jun 11 '18 at 18:56
  • $\begingroup$ @geoffc is right. There's a question on this site somewhere about crews landing on different vehicles than they launched on, I think there's an analysis there that might help you. $\endgroup$ – Organic Marble Jun 11 '18 at 18:57
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    $\begingroup$ And yes, for the relocations, they all had to pile into the Soyuz in case there was a failure that forced them to deorbit. $\endgroup$ – Organic Marble Jun 11 '18 at 18:58
  • $\begingroup$ @Jack I was thinking about how could prove this, and not sure you can. $\endgroup$ – geoffc Jun 11 '18 at 19:02
  • $\begingroup$ This is what I was remembering but maybe not too useful after all. space.stackexchange.com/questions/18933/… $\endgroup$ – Organic Marble Jun 11 '18 at 21:47
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For the entire time that the ISS has been occupied, there has been enough transportation to return to Earth. The only instance where anything different might have occurred was if there was a Soyuz mission when a Space Shuttle was docked. We can see the spacecraft that have visited at this chart. Wikipedia lists the number up/down from each Space Shuttle mission. STS-121 was the only mission that left more people at the ISS then it brought there. Prior to that mission, there were only 2 astronauts on the ISS. STS-121 left 3.

It seems to me that every Soyuz capsule has brought the same number of people that it took with it. Thus, there has always been a lifeboat.

For bonus, here is the number of seats on Soyuz/ Space Shuttle docked to the ISS at any one time. enter image description here

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  • $\begingroup$ Where does this chart come from? 6 Soyuz seats + 8 shuttle seats does not add up to 20. $\endgroup$ – Organic Marble Jun 14 '18 at 23:23
  • $\begingroup$ I made it. Technically there are 11 shuttle seats, and everything I can find says there were 3 soyuz on the ISS, but that does seem off... Will see if I can find a mistake in my source data. $\endgroup$ – PearsonArtPhoto Jun 15 '18 at 0:04
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    $\begingroup$ It seems like 7 might be a better choice. It seems that 11 was the max with a rescue configuration, and 7 would be a more realistic number. I must have the order wrong for one of the missions... Sigh. $\endgroup$ – PearsonArtPhoto Jun 15 '18 at 0:55
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    $\begingroup$ Found the number, replaced STS seats with 7, and made it look a bit smoother. What do you think? $\endgroup$ – PearsonArtPhoto Jun 15 '18 at 2:12
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    $\begingroup$ Those numbers "feel" a lot better! $\endgroup$ – Organic Marble Jun 15 '18 at 2:28

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