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If the Apollo ascent vehicle hadn't been able to take off from the Moon, how long was the crew expected to have survived? And what would the likely natural cause of death be?

I've heard that partial decompression or deoxygenation would be a painless way to commit suicide, but I wonder what essential life support they would've run out of first.

(Imagine that the crew of Apollo 10 went a bit funny in their heads and agreed to steal Armstrong's glory, knowing they could land, but not return.)

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  • $\begingroup$ The life support system needed oxygen, electric power and the scrubber to remove CO2. But oxygen consumption and CO2 production depends on physical activity and also mental state. The deadly partial pressure of both gases may not predicted precisely. It is not possible to predict the survival time exactly. $\endgroup$ – Uwe Oct 7 '18 at 10:12
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The lunar excursion module (LEM) was designed to support two astronauts for 45 hours, with two depressurization cycles.

Batteries lasted longer than that, as shown on Apollo 13, so if they weren't able to take off, they would have run out of air first.

This and more details about the LEM are available on Wikipedia:

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  • $\begingroup$ If life support was designed for 45 hours, there should be some reserves. They could not run out of air (pure oxygen without nitrogen), the oxygen partial pressure would decrease while the carbon dioxide pressure would increase. The combination of low oxygen and high CO2 would be harmful at last. $\endgroup$ – Uwe Oct 7 '18 at 11:12
  • $\begingroup$ "the oxygen partial pressure would decrease while the carbon dioxide pressure would increase" -- technically, yes. Informally, "run out of air." As to margins, the only thing I've read is that the astronauts could modulate their breathing or skip one or both of the depressurization cycles. Doing the latter for Apollo 13 extended the available air to 90 hours. $\endgroup$ – RickNZ Oct 7 '18 at 11:26
  • $\begingroup$ To modulate their breathing may reduce air consumption when using an open loop SCUBA system but not at a closed loop life support. To increase oxygen consumption, physical activity should be reduced as much as possible. But for Apollo 13 the usage of the carbon dioxide scrubbers of the CM in the LM was essential to keep the CO2 level breathable. $\endgroup$ – Uwe Oct 7 '18 at 11:47
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Apollo 17 - LM-12 (Challenger)

LM-5 (Lunar Module Eagle - Apollo 11) to LM-8 (Antares - Apollo 14) had enough battery and oxygen to operate normally for around 48 hours on the lunar surface. Who knows how long that could have been stretched if needed.

Starting with LM-10 (Falcon - Apollo 15), the ELM series had enough power and oxygen to last 75 hours on the lunar surface.

I was unable to tell how long LM-4 (Snoopy - Apollo 10) could last on the lunar surface. However, I have to assume it was nearly identical to LM-5, since the mission was a trial run for the Apollo 11 moon landing.

Apollo 17 spent a total of 75 hours on the lunar surface and logged 22 hours, 3 minutes, 57 seconds on EVAs outside of LM-12 (Challenger). In comparison, the total time for Apollo 11 on the lunar surface from landing to liftoff was 21 hours, 36 minutes.

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