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This absurd video on Reddit of the Earth blowing up while Apollo astronauts were walking on the Moon got me thinking.

Assuming something catastrophic happened to the Lunar Module the moment the two Apollo astronauts stepped out onto the surface of the moon, would they've run out of oxygen before the carbon dioxide scrubbers failed, or the other way around?

uhoh
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Bob516
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    I love the relativistic speed the fragments of the earth reach the moon. – Bob516 Jun 25 '21 at 00:27
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    Not to mention the fact that the impact of those fragments doesn't seem to do more than stirring up some dust... – leftaroundabout Jun 25 '21 at 08:07
  • Item: does not a LiHO based CO2 scrubber become very dangerous if totally saturated? I seem to recall that they do not just stop working , but start outgassing all sorts of nastyness due to condensation forming as they cool off after saturation. Thus suit designers would build in a hefty excess margin for safety. Better to asphyxiate without oxygen, than to explode due to malf scrubber. – CuteKItty_pleaseStopBArking Jun 25 '21 at 09:47
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    If the amount of oxygen available was small enough that carbon dioxide levels would not reach dangerous levels before you ran out, you wouldn't have scrubbers. The fact that it was needed would indicate that the oxygen would outlast the breathability of the air. – JDługosz Jun 25 '21 at 14:11
  • @leftaroundabout Not entirely implausible. If you're thinking about this, keep in mind that depends on there being an atmosphere. Without air, explosions wouldn't have nearly the impact that they would on earth. – Darrel Hoffman Jun 25 '21 at 20:47
  • @DarrelHoffman the kinetic energy needs to go somewhere either way, if not in the atmosphere then on hitting the ground. And as the XKCD explains, a relativistic collision will actually have aspects of a nuclear explosion. A nuke has a pretty large lethality radius, regardless of whether or not there's an atmosphere to propagate the mechanical shock wave! – leftaroundabout Jun 25 '21 at 21:03
  • @JDługosz This question asks which issue would reach lethality first, and CO2 causes effects at rather low concentrations long before it's lethal. – Acccumulation Jun 26 '21 at 02:55
  • @Acccumulation yea... is that supposed to be a reaction to my comment? Hmm... you're saying that it might have been dangerous without scrubbers, but not lethal? – JDługosz Jun 28 '21 at 13:58

2 Answers2

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We have some data about the Apollo Space Suit from https://www.hq.nasa.gov/alsj/ALSJ-FlightPLSS.pdf

The Apollo Portable Life Support System By Kenneth S. Thomas :

  • O2 Storage Quantity 1.0 lbs

  • LiOH Quantity 2.7 lbs

From Wikipedia :

  • One gram of anhydrous lithium hydroxide can remove 450 cm3 of carbon dioxide gas.

  • 1 lbs is 0.45359237 kg

  • the density of oxygen is 1.429 g/L

We can calculate the volume of oxygen and carbon dioxide now:

There is 1 lbs oxygen or 0.454 kg or 317.7 l at 1 bar.

2.7 lbs LiOH is 1.225 kg binding 551 L of CO2 at 1 bar.

(The suit pressure is about 0.3 bar instead of 1.0 bar, but there is no influence on the comparison.)

So we have more CO2 binding capacity than available oxygen, oxygen will run out first.

To limit the CO2 level in the suit at the end of a long EVA, it is necessary to use only about 50 % of the scrubber. If 75 % or more of the scrubber would be used, CO2 removal could not keep up with CO2 production at peak metabolic rate. CO2 partial pressure into helmet (Maximum) was 10 mm Hg after 3 hours and 15 mm Hg after 4 hours.

Some oxygen is lost by leakage but this does not change the result.

Fred
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Uwe
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  • I think more than just "some" leakage from the suits, they were not particularly airtight. On apollo 12, Pete Conrad's suit was leaking 0.25PSI per minute. (from base of 3PSI, this basically trying to empty his suit's air volume in 12 minutes, that's a lot). https://history.nasa.gov/alsj/TM-2005-213610.pdf – CuteKItty_pleaseStopBArking Jun 25 '21 at 08:04
  • @PcMan The suit leakage rate was 200 scc/min, we need something comparable 0.25PSI per minute. I would prefer both leakage rates in volume per minute, not pressure drop per minute. scc means standard cubic centimeters at a pressure of 1 bar. – Uwe Jun 25 '21 at 09:07
  • Ah lovely.. yes, that is a much more usable figure. So the leak rate was enough to empty the total oxygen in 26.5 hours. As the supply (with no leaks) would last something less than 12 hours, the leaking is apparently less significant than actual consumption. Say about... 8.25 hours until suit oxygen depletion? Which seems to line up with the "8 hours duration" wiki states for the extended PLSS – CuteKItty_pleaseStopBArking Jun 25 '21 at 09:45
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Edit: Uwe posted his answer 8 seconds before mine. I've run his numbers using moles, and come to the same conclusion.

There is 1.0 lb O$_2$ x (1000 g / 2.2 lb) x (mol O$_2$ / 32.0 g O$_2$) = 14.2 mol O$_2$ available.

The cartridge contains 2.7 lb LiOH x (1000 g / 2.2 lb) x (mol LiOH / 23.95 g LiOH) = 51.2 mol LiOH.

2 moles of LiOH scrub 1 mole of CO$_2$. This means 25.6 mol CO$_2$ can be scrubbed.

Since 6 mol O$_2$ are metabolized to 6 mol CO$_2$ -- a 1:1 ratio -- this means that the O$_2$ will run out before the CO$_2$.

Original answer:

It depends. All of the suits leaked to some degree, even though they weren't supposed to. The only suit that leaked badly enough to be considered a failure was Apollo 12 commander Charles Conrad's. Leaks caused oxygen to be drawn from the supply at a faster rate, to maintain suit pressure.

However, in most cases, the CO$_2$ scrubber will indeed fail before the oxygen supply runs out. A 1% increase in CO$_2$ concentration is deadly; a 1% decrease in O$_2$ is not.

It's also possible for the cooling water to run out first and the astronauts to overheat, depending on physical activity level and solar exposure.

DrSheldon
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