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I recently read multiple sources that indicate human-occupied vehicles (Space Shuttle, ISS, Soyuz, etc) are all pressurized to 1 atmosphere of pressure (approx 100 kPa).

Since I believe that people function just fine to an altitude of 8,000 ft, or about 70 kPa, why aren't vehicles pressurized to less than one atmosphere?

(At 70kPa, I believe the atmosphere would be approximately .15 partial pressure of O2 + .56 partial N2 because 0.70 * 0.21 == 0.15 ).

The ratio of N2 / O2 would be the same, but there'd just be a lot less gas around, which leads to various benefits. There would be less O2/N2 gas to launch into orbit, or existing supplies would last longer.

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    $\begingroup$ Atmospheric pressure on spacecraft can be (and has been) varied (see here). $\endgroup$
    – called2voyage ♦
    Jun 19, 2015 at 14:22
  • $\begingroup$ Because the ISS is pressurized to approximately 100 kPa by standard, any vessel docking with it must be similarly pressurized. Now, a good question would be why the ISS standard pressure was set to 1 atmosphere. $\endgroup$
    – called2voyage ♦
    Jun 19, 2015 at 14:24
  • $\begingroup$ @called2voyage: That is exactly my question: Why are ISS, and all other vehicles, apparently at 1 Atm? $\endgroup$
    – abelenky
    Jun 19, 2015 at 14:27
  • $\begingroup$ Related question $\endgroup$
    – neelsg
    Jun 19, 2015 at 14:28
  • $\begingroup$ @abelenky Right, I understood that. I was giving you part of the answer. Other vehicles are because the ISS is. $\endgroup$
    – called2voyage ♦
    Jun 19, 2015 at 14:31

1 Answer 1

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  1. Safety.
  2. Comfort.

Humans can quite well survive in about 30% of Earth atmospheric pressure providing there's enough oxygen. The lower limit is somewhere between when 100% oxygen is not enough for breathing and where human blood starts boiling at body temperature - I'd have to find the exact number, but that's not far below that 30%.

The prolonged health effects are hard to predict, generally, while survivable, that's not a natural environment. But that's a secondary concern.

The primary concern is that fire only cares about percentage of oxygen, not the pressure.

The Apollo vehicles operated like that, and - as you remember - that resulted in a bad fire. Everything that isn't inherently fire-proof becomes extremely flammable. Plastic, polymer clothes, aluminium, things that would be very difficult to burn in air, and would extinguish themselves as soon as removed from flame, burn quite well in oxygen-rich atmosphere.

And it degrades materials that oxidate over time. Rubber seals, plastic foils, even small amounts of moisture makes metals rust. The atmosphere is not good for long-term missions. But primarily, a small spark or a tiny overheat may cause a very rapid and difficult to extinguish fire.

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  • $\begingroup$ I disagree with the "that's not a natural environment". Many people here on earth live at 8,000 ft. and above with absolutely no ill effects. (Those who have lived their entire lives above 14,000 ft tend to seem some long-term stunting of grown, but nothing that affects healthy adults over a 1 or 2 year period.) $\endgroup$
    – abelenky
    Jun 19, 2015 at 14:30
  • $\begingroup$ @abelenky: They live in the natural oxygen percentage though, and people who come there from lowlands need an adaptation period. Apollo operated at pressure experienced at 8000m, not 8000ft, and extra oxygen was a necessity. $\endgroup$
    – SF.
    Jun 19, 2015 at 14:46
  • $\begingroup$ oops! Looks like I confused 8,000m and 8,000ft. in my original question. Editing. $\endgroup$
    – abelenky
    Jun 19, 2015 at 14:48
  • $\begingroup$ Other than there being less inert gas to cool a possible flame, why would identical partial pressures of oxygen make everything extremely flammable? $\endgroup$
    – Nick T
    Dec 22, 2016 at 21:09
  • $\begingroup$ @NickT: fire requires oxygen; in normal case products of combustion + nitrogen deplete oxygen supply and suppress fire until more oxygen is provided through convection. With little to no nitrogen dilution of oxygen is way slower; partial or absolute pressure plays little role, while raw percentage is a deciding factor. $\endgroup$
    – SF.
    Dec 22, 2016 at 23:36

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