Would there be some benefit to changing the chemical composition of 'air' in a space station/space suit (for the station: it's creating about .6 amount of gravity compared to Earth gravity via spin). Since we have to create/carry the chemicals up from Earth's gravity well anyway, would there be a way or a need to change the composition when doing spacefaring things? Imagine technology is pretty similar to where we are now.

  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Apr 17 at 20:14
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    $\begingroup$ This might be a better question for the space expoloration stack exchange. $\endgroup$ Apr 17 at 20:40
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    $\begingroup$ What research have you done such that you need to ask the question? A quick Google search revealed some interesting information here, here, here and our sister site Space.se. I suspect between those links you'll find your answer. Please remember, research is required before using Stack Exchange. $\endgroup$
    – JBH
    Apr 17 at 20:40
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    $\begingroup$ @JBH there's limited room in my brains to fit magic in there these days. $\endgroup$ Apr 17 at 20:41
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    $\begingroup$ The Apollo missions famously used pure oxygen at 0.2 atmospheres, with the idea of avoiding to haul useless nitrogen to the Moon and back. (And needing less strong walls, because the pressure was only 0.2 atm instead of 1 atm.) $\endgroup$
    – AlexP
    Apr 18 at 0:07

3 Answers 3


TL;DR: it depends, but plain old sea-level air pressure and gas mix is easy and safe, and there are no obvious alternatives that are cheaper or simpler.

Longer answer:

There are several key requirements for your atmosphere:

  • there needs to be enough pressure for regular human breathing to get gas into the lungs.
  • there needs to be a high-enough oxygen partial pressure for human lungs to work, but not so high it risks oxygen toxicity.
  • it needs not to be an explosion or fire hazard (which probably it won't be, so long as you don't try to mix in hydrogen, or have a potentially toxic amount of oxygen)

There's a bit of wiggle room in that requirement... Skylab had a 75% oxygen, 25% nitrogen atmosphere, but at about 1/3 regular sea-level pressure so there's no greater risk of oxygen toxicity, though (as @Yakk pointed out in the comments) fire risk is elevated (but not as much as with 1 atm of pure O2).

edit: following some comments by @JiříBaum who pointed me at NASA-STD-3001 (which recommends a minimum of 30% diluent gas, more than Skylab), I've come across Bounding the Spacecraft Atmosphere Design Space for Future Exploration Missions which adds an additional important requirement:

  • For long durations (in excess of two weeks), provide a physiologically inert gas to prevent atelectasis

Absorption atelectasis can occur when an alveolus in your lungs is filled entirely with oxygen. All the oxygen may be absorbed, which causes the alveolus to collapse, which means it can't do useful things like transfer CO2 out of your bloodstream, and it may not reinflate on your next breath. The longest Apollo missions were a little over 12 days long, which is probably where the 2 week limit came from, and why Skylab got its unusual gas mix... the unabsorbed neutral gas keeps the alveolae inflated and functioning correctly, and the Skylab missions were much longer than the Apollo ones.

NASA eventually ended up with regular Earth sea-level gas mix and pressure on the ISS because ultimately it is just the easiest thing to work with... co-operation with other space agencies is straightforward (its what Mir used), and there's no awkward off-gassing period for people travelling to the station (because if you go from 1 atmosphere of pressure to 0.3 too quickly, you risk the bends which can be debilitating, disabling or fatal). Higher pressures make ventilation more effective, which is good for the temperature and gas mixture in your habitat, with excesses of hot or cold or damp or CO2 or dust being easier to even out and clean up when there's a higher mass of air being pushed around.

Current soft spacesuits, on the other hand, need to be at a low pressure otherwise they become too difficult for the wearers to bend their limbs. Low pressure in turn requires increased oxygen percentages, and that means that there's a long preparation period prior to EVA for astronauts to adjust to the changed gas mixture. The Apollo spacecraft had a pure-oxygen atmosphere internally which made that adjustment easier, but had to take-off from Earth with an oxygen-nitrogen mix to avoid a repeat of the Apollo I fire.

If you can make a good mechanical counterpressure suit, you can avoid the need to have a reduced suit gas pressure, and so have the same mix everywhere without having to do careful decompression procedures, etc.

Since we have to create/carry the chemicals up from Earth's gravity well anyway,

That depends. You can get oxygen from ice, and there's a lot of that in space pretty much everywhere from Earth on outwards. You can get oxygen from lunar regolith and Martian perchlorates if you visit those places. Asteroids are a good source too, and it is a waste product from metal refining.

Shipping up fresh oxygen from Earth isn't too hard, because water is useful, reasonable dense, quite stable and nontoxic. Neutral gas diluents are much less convenient though, and they're harder to replace. Nitrogen gas isn't very dense, the liquid requires cryogenic storage, and other chemical forms like ammonia or hydrazine are reactive and toxic to boot.

There are a few other gasses that can be used for the neutral bit, but they're also inconvenient to ship and awkward to come by elsewhere in the inner solar system. Argon and neon and helium don't form nice compounds and the next nearest convenient and substantial atmospheres containing them are out past the asteroid belt. Helium and argon can also increase decompression sickness risks. Mars doesn't have much atmosphere to spare. Nitrogen does form compounds, but there's not a lot of that on the Moon. Mars seems a potentially useful source of nitrates, but exploiting Martian minerals seems like an expensive and difficult option unless your space station is right there.

Given all that, and Earthlike breathing mix is safe, and readily available from the place you're probably going to get most of your nitrogen from, there's no particular reason not to use it. It also eases travel too and from Earth, which seems like an important destination.

  • $\begingroup$ For Nitrogen availability, Ammonia is cheap and pretty easy to store and transport. If you bring a tank of Oxygen and a tank of Ammonia from Earth to a space colony, you can burn them to produce electricity, and collect the Water and Nitrogen gas byproducts to be used as thier own useful resources. Inversely, if you have "extra" Nitrogen in the Air, you can spend energy to go the other way producing Ammonia (a useful fertilizer and cleaning agent) and more breathable oxygen. Ammonia is also common on most planets and moons from Jupiter outwards. $\endgroup$
    – Nosajimiki
    Apr 17 at 22:07
  • $\begingroup$ There are also medical issues with pure oxygen, so the breathing mix for long-term use needs at least 25-50% nitrogen, depending on how far you're willing to push things $\endgroup$ Apr 18 at 12:01
  • $\begingroup$ @JiříBaum chronic oxygen toxicity is an issue only if the partial pressure of oxygen is too high. A pure oxygen atmosphere at reduced pressure doesn't have that issue. I'm not aware of reports of long term health issues from exposure to low pressure pure oxygen, but following the end of the Apollo and Skylab missions I don't think anyone has spent long in that sort of atmosphere, so I wouldn't expect there to be much medical data. $\endgroup$ Apr 18 at 14:10
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    $\begingroup$ "There's a bit of wiggle room in the last requirement... Skylab had a 75% oxygen, 25% nitrogen atmosphere, but at about 1/3 regular sea-level pressure so there's no greater risk of fire or oxygen toxicity. " The partial pressure of oxygen doesn't determine the risk of fire; the other gasses in the air act as a kind of "insulator" and bleed off heat. When you have low pressure pure oxygen, things are a LOT more flammable than the same partial pressure 20% oxygen. $\endgroup$
    – Yakk
    Apr 18 at 19:05
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    $\begingroup$ @JiříBaum thanks for that reference, that was super helpful. I've found some associated NASA stuff (not referenced from STD-3001, frustratingly) that states that "For long durations (in excess of two weeks), provide a physiologically inert gas to prevent atelectasis". Evidently, the Apollo flights were short enough that the low-pressure pure O2 was not considered too problematic. $\endgroup$ Apr 19 at 15:01

The big tradeoff is mixing vs mass.

Humans can do fine at about a quarter of atmospheric pressure as long as you hold the molarity of oxygen constant. Fire and other chemical reactions are not a hazard even though the new mixture is mostly oxygen; reactivity is influenced by molarity, not percentage.

This saves you a tiny bit of nitrogen mass, but the big win is the ability to slightly reduce the amount of metal and plastic needed: nitrogen tanks, monitoring and mixing equipment, airlock pumps which can be smaller if they need to pump less air to make a vacuum, structures which can be lighter and smaller if they have to separate lower pressure from vacuum.

The big losses are:

  • lose most of the ability to use the air to regulate temperature with convection and
  • need more and bigger fans to disperse other gases, such as CO2 from breathing, from building up to dangerous levels in places, since less total gas means less random thermodynamic mixing.

"Lighter and smaller" is the only item in either column which really applies to space suits, so your space suits should probably operate at about a quarter of atmospheric pressure.

For your space ships, with items in both the plus and minus column, it's up to you whether you want it to be more technologically convenient for your astronauts to use low pressure and mostly oxygen, earthlike pressure of nitrox, or something in between.

If your astronauts are British, they will demand a space ship pressure of at least 0.66 atmospheres, so that they can brew tea at the optimal temperature.

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    $\begingroup$ You can make them use pressurecookers for tea. Or use them to drive development of fuel cells that deliver suitable outlet temperatures and chemistry. $\endgroup$ Apr 17 at 21:31
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    $\begingroup$ "Fire and other chemical reactions are not a hazard even though the new mixture is mostly oxygen; reactivity is influenced by molarity, not percentage." This is not 100% true. The nitrogen in the air absorbs heat which inhibits chemical reactions. Low pressure O2 is safer than 1 atm of O2, but not as safe as 1 atm of Nitrogen rich air. $\endgroup$
    – Nosajimiki
    Apr 17 at 21:47
  • $\begingroup$ Nice touch with the tea, spot on. $\endgroup$ Apr 18 at 0:42
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    $\begingroup$ If your astronauts are Italian, they will want 0.66 atmospheres or their pasta will stay stiff. Pasta would make a lovely climbers' food, but it doesn't go soft in the Grepon Hut. $\endgroup$ Apr 18 at 7:27
  • $\begingroup$ @RichardKirk TIL! presumably precooked and dehydrated would work OK for non-italian astronauts. $\endgroup$ Apr 19 at 15:18

In real life, the International Space Station takes two different approaches to this.

When they work outside in their space suits, they are given pure oxygen at one third atmospheric pressure. That way they can get the same amount of oxygen into their blood, but the joints of their balloon-like suits are easier to bend than they would be if inflated to atmospheric pressure.

Onboard the station, they use (more or less) Earth air (ie 21% oxygen) at Earth sea-level pressure. That's what people evolved with, so we know it's adequate for long-term use. Also, I think maybe fire hazards are worse in pure oxygen.

Having two very different atmospheres like this is a hassle. The astronauts have to adjust slowly to the pressure change, over a few hours, so they don't get the bends.


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