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I just re-read the book The Martian, and a question came to mind:

Why did the Ares missions use a 1 atm nitrogen-oxygen atmosphere instead of a lower pressure pure oxygen one?

AFAIK, pure oxygen is harmful to humans only when the partial pressure is higher than that on Earth. In fact, many real world space missions (including Apollo) used low pressure pure oxygen.

It would have several benefits:

  • less gas to carry around
  • less strong pressure vessels required
  • less complex Atmospheric Regulator (it would only need to cool the air to the freezing point of CO2, which is much higher than the melting point of oxygen)
  • the EVA suits would be more flexible (high pressures "inflate" the EVA suit, which makes the hand and arm joints more stiff and thus harder to work in)
  • faster airlock operation (less air to remove)

I understand why this was chosen by the author, as several plot elements hinge on having a mixed N2-O2 atmosphere. Is there any explanation or justification for why NASA made this decision in the story?

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    flammability?.. Commented May 30 at 17:23
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    @ThePopMachine During combustion, it's also the partial pressure that matters. So low pressure (0.2 atm) pure oxygen wouldn't make things more flammable than they are in Earth's atmosphere.
    – FZs
    Commented May 31 at 8:05
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    @FZs the nitrogen between the oxygen is somewhat slowing down fires (as it doesn't contribute to the fire, but still needs to be heated). So it does help against fires. Commented Jun 1 at 0:51

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I don't know that we have an author statement on this, but I've presumed it's similar to the reason the US Space Shuttle flew with 80/20 nitrogen oxygen atmosphere at 1 bar: so crews don't have to prebreathe oxygen at 1 bar before lowering cabin pressure, and so there's no need to pull a partial vacuum in transfer vessels going to the cycler.

As noted, there are downsides, but those are generally all "solve once" problems, as opposed to issues that come up with every ship coming up from Earth to the cycler.

Further, there are some significant disadvantages to low pressure pure oxygen: it has less cooling action, so temperature (especially inside a rover or EVA suit) must be kept lower, it's difficult to cook foods because the boiling point of water in 3 psia (appr. 0.2 bar) is barely above body temperature (so boiling, for instance, a potato or an egg will only heat it to that temperature). A human will dehydrate faster, too, because the water in the body is close to its boiling point, so evaporates faster (while still carrying away less heat).

Combine these, and the Shuttle program (and ISS) chose to fly with 1 bar of 80/20 mix, but run the EVA suits at 3 psi pure oxygen with astronauts prebreathing pure O2 for 30 minutes before/while suiting up for an EVA -- and there's no reason to think the Ares program would find it necessary to make different choices.

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    Not to mention Apollo 1 Commented May 30 at 15:18
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    @cybernetic.nomad, Apollo 1 was pure oxygen at 16.7 PSI, a much more dangerous situation. Pure oxygen at 3 PSI is no more of a fire hazard than normal air.
    – Mark
    Commented May 30 at 21:57
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    This is key - breathability isn't the only concern; several important mechanisms depend on atmospheric pressure, not atmpospheric composition. Your body uses evaporative cooling via sweat, but at 0.2 bar your sweat would evaporate much more readily - this might change how warm or cold you feel, (perception of heat is more complex than just air temperature; look at wet-bulb temperatures). The thin air might also mean that devices that use air cooling might struggle to move sufficient air mass to carry away their waste heat - you might not be able to use ordinary consumer laptops, for example. Commented May 31 at 14:29
  • They could at least go trimix - less gas to haul around... Any reason against it? Leaks?
    – Therac
    Commented May 31 at 16:07
  • @Therac Trimix is mainly of use at high pressures, however, to limit decompression and nitrogen narcosis. There's little if any advantage for space travel. And "less gas" is wrong -- they'd have to carry helium as well as oxygen, and scrubbers vs. oxygen and scrubber material (the nitrogen is reused, but helium gets lost much more readily).
    – Zeiss Ikon
    Commented Jun 3 at 11:07

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