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This answer to Why did they bother closing the hatch on the LM while doing EVA? links to a transcript of Apollo 11 which contains the following (find it there by searching for "radiative"):

109:41:53 Armstrong: (Laughs) A particularly good thought.

[The hatch can be opened from the outside, if necessary. The reason for almost closing the hatch is, I believe, to prevent radiative cooling of the cabin. Neither Neil or Buzz remembered any specific reason.]

[Armstrong (straight-faced) - "To avoid having somebody say 'Were you born in a barn?'"]

[Aldrin - "Now that you bring it up, what would have happened if the valve had gotten screwed up or something and it started re-pressurizing?"]

[Armstrong - "You'd never get back in."]

[Aldrin - "Did we really ever investigate that problem? (Chuckling) It probably would have been a good idea to use a brick or a camera to keep it from closing. Somebody must have thought about that."]

[I recalled that the dump valve could be opened from the outside.]

[Aldrin - "We had a handle (on the outside) to unlatch it. But, considering the difficulty we had, if you had a couple of psi (in the cabin), you'd never get it open. (Half seriously) Well, you'd get it open, but you'd never get the bent hatch closed again."]

[Actually, the handle is the weak point.]

The dump valve link then says:

Setting the internal handle to the dump position unseated the poppet. With a bacterial filter installed, the forward hatch valve could dump pressure from 5.0 to 0.08 psia in 310 seconds without cabin oxygen inflow; without a filter in place, either valve could dump cabin pressure to 0.08 psia in 180 seconds; and, with both open and no filter in use, the time was 90 seconds. Setting the handle to the closed position prevented the valve from opening at normal pressures, if the servo valve failed.

Question: Why would there be a bacterial filter in a valve used to dump the atmosphere of the lunar module before opening the hatch? Who or what was being protected from what bacteria from what source?

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    $\begingroup$ "Who [...] was being protected?" - I guess everybody who can answer that is up for the next Noble price. $\endgroup$ – asdfex Jul 29 at 6:30
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It's one of several factors to prevent Earth microorganisms from contaminating the moon. The Apollo Program Summary Report states

8.5.2.1 Lunar-surface contamination.- Nations involved in the exploration of extraterrestrial bodies have agreed to take all steps that are technically feasible to prevent the contam- ination of these bodies during exploration. The primary reasons for preventing contamination of extraterrestrial bodies are

(1) to ensure that scientific analyses for the detection of viable life originating from an extraterrestrial body can be conducted without the complications asso- ciated with terrestrial contamination of such a body, and

(2) to ensure that, if life does exist on an extraterrestrial body, the ecological balance existing on that body is not disturbed by the introduction of terrestrial microbial life-forms.

I'm not sure which "nations" had "agreed" to this (that would make another good question), but it is a reasonable precaution. Microbial contamination is a kind of Pandora's box, which can't be undone.

The Apollo crewmembers represented the prime source of contamination of the lunar surface. Three other sources were determined to be

(1) waste products such as feces, urine, and residual food;

These were mixed with germicidal liquid, placed in the jettison bags, and carried out or thrown out the hatch.

(2) viable terrestrial micro-organisms released during lunar module depressurization; and

This is the reason for the filter discussed in the question. You aren't depressurizing while you're in space; you are depressurizing before opening the hatch on the moon.

(3) micro-organisms present in the lunar module waste-water system. Procedures were defined to eliminate massive contamination of the lunar surface from these three sources.

The waste-water was mostly left over from the PLSS. It was mixed with germicidal liquid and put in the jettison bag with the solid trash.

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  • $\begingroup$ indisputable photographic evidence: space.stackexchange.com/a/33502/12102 and more about the "pills": space.stackexchange.com/a/36205/12102 $\endgroup$ – uhoh Jul 29 at 5:31
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    $\begingroup$ Is the issue one of viability, or one of being recognizable as having been "deliberately" made non-viable? I would think the main issue would be ensuring that if any kinds of microorganism are found on the moon, it would be possible to determine whether they were of terrestrial origin, or were of outside origin but were coincidentally similar to those of Earth. $\endgroup$ – supercat Jul 29 at 16:22
  • $\begingroup$ @supercat: It seems they were doing their best to prevent something from actively growing. The report admits that it is impossible to completely sterilize everything, so they used precautions that were feasible. There is no mention of identifying non-viable life, nor could they predict if/how that would be done in the future. Indeed, we can't even tell now if methane detected by Curiosity is terrestrial-based. $\endgroup$ – DrSheldon Jul 29 at 17:09
  • $\begingroup$ @DrSheldon: Has any terrestrial organism ever been observed which would be capable of surviving in a vacuum for anywhere near long enough to reproduce a full generation (an organism that was less than second away from completing reproduction when it was exposed to vacuum might "reproduce", but that shouldn't really count)? $\endgroup$ – supercat Jul 29 at 17:17
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    $\begingroup$ So then Mark Watney from The Martian is not only technically a space pirate, but also in violation of international treaties with his potato farming? $\endgroup$ – Mason Wheeler Jul 29 at 19:29

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