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The Wikipedia page for the International Space Station says that it has a fairly Earth-like, sea-level atmosphere: 21% oxygen, balance nitrogen at 101.3 kPa. Supposedly it's because a pure-oxygen environment is dangerous as in the Apollo 1 disaster, but in that case "pure-oxygen" meant 1.15 atm of O2. It seems like a pure, 0.21 atm O2 atmosphere (or even lower) with no inert balance gas should be fine for people and would be ~ 80% less structurally demanding.

One strange aspect might be an effective drop in boiling point to ~ 60 °C, but I'm not sure if anyone would be boiling water for a tea up there. My presumption is that it's not for any safety or human reason but solely for the sake of making ISS experiments more similar (and directly comparable, save for the microgravity environment) to those on Earth. Am I not considering something?

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    $\begingroup$ Tangent: blog post and paper: An independent assessment of the technical feasibility of the mars one mission plan. Note the bit about the 'fire safety threshold'. Granted, ISS doesn't have to worry about growing their own food (and thus producing oxygen... still an interesting topical read. $\endgroup$ – user5892 Oct 25 '14 at 0:48
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    $\begingroup$ Lots of interesting information on this page particularly this image which partially refutes Rory. 100% O2 at about 3 - 9 psia is physiologically safe (assuming you've gotten the nitrogen out of your bloodstream). Other concerns still apply, though. $\endgroup$ – hobbs Oct 25 '14 at 5:47
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    $\begingroup$ Here is an in-depth article on various constraints. spaceflightsystems.grc.nasa.gov/repository/NRA/… I note that simple cooling constraints are called out as forcing a minimum of 7.35psi. $\endgroup$ – BowlOfRed Oct 28 '14 at 0:47
  • $\begingroup$ While human body depends on absolute oxygen content, meaning pressure drop requires increase of oxygen content, ability of substances to burn depends on relative atmosphere composition: 100% oxygen at 0.3 bar creates risk of fire only a little smaller than at 1bar, and enormously higher than a 21% O2-N2 mix. In other words, increasing the oxygen content, even with the reduced pressure, drastically increases risk of fire. $\endgroup$ – SF. May 13 '16 at 21:06
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    $\begingroup$ Breathing pure oxygen at 1 or even 1.15 atm is not healthy when done for days and weeks, see en.wikipedia.org/wiki/Oxygen_toxicity#Lung_toxicity therefore the partial pressure of oxygen should be less than 0.5 bar. $\endgroup$ – Uwe Aug 30 '17 at 20:15
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Am I not considering something?

Yes. You are not considering Mir, Soyuz, and the Space Shuttle.

The International Space Station is a multinational program, jointly led by the US and Russia. While the US and Russia had to compromise on many design decisions, the makeup of the breathing atmosphere was not one of them. The decision to pressurize the ISS to one atmosphere with a standard mix of nitrogen and oxygen was probably one of the easiest design decisions agreed upon by those two countries. The Mir space station, the Soyuz capsules, and the Space Shuttle were all pressurized to one atmosphere. Making the ISS breathing atmosphere be anything but one standard atmosphere would have required extensive redesigns of the Soyuz capsule and the Shuttle, and would have precluded reuse of the Mir environmental control systems.

The real question then is why the breathing atmosphere in Mir, Soyuz, and the Space Shuttle is a standard atmosphere, both in terms of pressure and composition. There are significant advantages to a reduced pressure, pure oxygen environment. Such an environment reduces spacecraft mass, structural integrity issues, and complexity. A pure oxygen environment eliminates the need to carry nitrogen tanks, eliminates the need to carefully monitor the oxygen/nitrogen mix, and eliminates the possibility of the bends (decompression sickness). The reduced pressure means the spacecraft can be a bit less bulky as well. There are additional advantages, particularly with respect to EVAs. Both the Soviet Union and the US initially planned to use pure oxygen breathing atmospheres.

The Mercury, Gemini, and Apollo breathing atmospheres were pure oxygen. The Apollo 1 fire modified how that pure oxygen atmosphere was attained, but it did not change that the breathing atmosphere was transitioned to pure oxygen shortly after launch. The issues associated with a pure oxygen breathing atmosphere made NASA shift to having some nitrogen in the Skylab breathing atmosphere, but not much. The Skylab breathing air was 75% oxygen, 25% nitrogen. The use of a pure breathing atmosphere in the Apollo spacecraft continued to the very end, which created challenges for the Apollo-Soyuz test mission.

The Soviet space program switched from a pure oxygen atmosphere to standard atmosphere very early on. Valentin Bondarenko died in a pure oxygen fire three weeks before Yuri Gagarin's historic flight. Having a standard atmosphere mix drastically reduces the likelihood and severity of fires, and also greatly simplifies the pre-launch process. A pure oxygen atmosphere requires extensive pre-breathing to purge nitrogen from the bloodstream. A standard atmosphere meant the cosmonauts could enter the capsule without wearing a helmet and they were physiologically ready to go.

NASA eventually learned these lessons, too. The Space Shuttle used a standard atmosphere. Having a standard atmosphere in the ISS was the only logical decision.

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    $\begingroup$ @DaveNay -- The ISS has leaks. The mechanisms used to remove CO2 from the breathing atmosphere concentrate CO2 and vent that concentrated gas to space. The vented gas is not pure CO2; it still contains some oxygen and nitrogen. The joints between modules leak breathing atmosphere. The tiny little gaps around windows leak breathing atmosphere. Oxygen is easily replaced; simply electrolyze some water and vent the hydrogen to space. Nitrogen? That's not so easy. It needs to be hauled up to the ISS as compressed nitrogen gas. Most Soyuz launches carry nitrogen to the ISS as part of their manifest. $\endgroup$ – David Hammen Oct 25 '14 at 14:01
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    $\begingroup$ Wouldn't you be able to carry the nitrogen up as N2H4 (hydrazine)? Doesn't even need to be electrolyzed. In fact, a fuel cell can produce nitrogen and electricity from it. As a side benefit, it can also be used for thrusters. Also, why bother replacing the nitrogen? Even if the pre-launch benefits justify launching with a standard atmosphere, it doesn't seem to justify keeping it standard. And for the return trip you'd probably be able to just pressurize the Soyuz on its way down. $\endgroup$ – MSalters Oct 27 '14 at 13:15
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    $\begingroup$ "The Skylab breathing air was 75% oxygen, 25% nitrogen" this is very interesting. Presumably the total pressure was then about 1/3atm to keep the partial pressure of O2 at about its Earthly level of 0.2atm to avoid O2 toxicity, is that right? This suggests then that any long term effect of breathing a low pressure atmosphere of the right O2 concentration is very slow to show itself. $\endgroup$ – WetSavannaAnimal Mar 22 '15 at 9:39
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    $\begingroup$ We don't all live at sea level. The inhabitants of Denver Colorado live a mile above; jet aircraft transition to about a 7,000-8,000 foot cabin altitude (simply pressurizing the ambient air) as they climb to cruise. They don't maintain a sea-level pressure cabin because it reduces structural loads, making the airframe lighter. So why wouldn't spacecraft and/or ISS use a similar "high elevation" internal environment? $\endgroup$ – Anthony X Dec 22 '16 at 14:43
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    $\begingroup$ You must remember that hydrazine doesn't cleanly decompose to H2 and N2 and nothing else. There's a lot of quite toxic byproducts. $\endgroup$ – SF. Sep 7 '17 at 13:37
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Rory mentions oxygenation rate which is an excellent point but there's additional reasons why not keep ISS atmosphere at a lower pressure - thermal convection and air cycling. Pressure at roughly one atmosphere means that the ventilation system on the station works better and no pockets of carbon dioxide or even carbon monoxide build up, which would be dangerous to astronauts. The air is easier recycled / replenished and mixed with oxygen (electrolysis of water) and carbon oxides removed from it (Sabatier reaction). The ventilation system also works more reliably at higher pressure and its parts lasts longer between failures. Astronauts / cosmonauts also exercise quite a bit on the station to combat adverse effects of prolonged stay in microgravity on human body, so air pressure also helps them shed excess body heat. Overheating is stressful to the body, lowers performance and can be deadly. And they use all kinds of equipment that requires air cooling too, and it would definitely complicate biology experiments or even render their results useless.

Nitrogen is also relatively cheap to deliver to the station since it's not really a consumable within the life support system and is only lost at a low rate to its inefficiencies, and is also used for all kinds of other things both on the station as well as visiting spacecraft (purging atmosphere, non-toxic fire suppressant, ullage gas in storage tanks to provide fluid / gas pressure,...). So it would be delivered to the station anyway. But in theory, if it made sense from the logistics standpoint, it could be replaced with some other inert and non-toxic gases like, say, Argon. Especially if they would for some reason decide to keep the atmosphere at a much increased pressure, where nitrogen narcosis might become a problem. But they won't do that, there's no good reason to and the station probably couldn't support it structurally without coming dangerously close its ability to maintain pressure and not lose it to space.

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  • $\begingroup$ The article you link for blood oxygenation explicitly mentions partial pressures repeatedly, which, from a basic chemistry point-of-view is all that matters in any chemical reaction/equilibrium, including the scrubbing you mention. I'm skeptical of your "air mixing" claims. Cooling concerns (probably more-so of equipment versus people, who sweat, and reduced convection/conduction to the air would be mitigated by increased evaporation rates) seem like an interesting point though. $\endgroup$ – Nick T Oct 24 '14 at 17:19
  • $\begingroup$ How would the thermal mass of nitrogen compare with other gases? How about the cost per delivered kg (all gases would require some sort of container to keep them liquid, but some might require heavier containers than others). $\endgroup$ – supercat Oct 24 '14 at 19:40
  • $\begingroup$ @supercat You mean Nitrogen's specific heat capacity? Refer to this table engineeringtoolbox.com/specific-heat-capacity-gases-d_159.html It lists values that closely match ISS atmospheric pressure and temperature. Molecular Nitrogen (at 1 atm and 20°C) has a specific heat capacity slightly over Carbon Monoxide and nearly double that of Argon. And a bit over that of standard atmosphere air. Cost per kg delivered to the station would be much the same, both Nitrogen and Argon are easy to store as inert gases and are not difficult to handle. But you'd have to deliver Nitrogen anyway. $\endgroup$ – TildalWave Oct 25 '14 at 16:51
  • $\begingroup$ @TildalWave: From a fire-suppression standpoint, I think what would be important would be the ratio of specific heat per unit density; I don't know if there's a term for that. Looking at the table (thanks for the link) the specific heat of helium is five times that of nitrogen, but if nitrogen were replaced with helium, each cubic meter of gas would have only 1/7 as much mass of inert gas. $\endgroup$ – supercat Oct 27 '14 at 0:29
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    $\begingroup$ @supercat Oh that's a science in its own right. For example, there's the disassociation temperature that, if too low, might render some gas or a liquid unsuitable as a fire suppressant as it might cause explosions. Water, for example, isn't any good to fight fires with in most circumstances exactly for this reason. And there's other properties such as viscosity, thermal inertia (related to specific heat capacity but in a volumetric sense, and describing thermal conductivity),... I'm afraid tho that it's not my area. All I know is that it's complicated and is simpler to use what's know to work. $\endgroup$ – TildalWave Oct 27 '14 at 0:46
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Running a pure oxygen atmosphere at 0.21 is not going to be very healthy. Humans require atmospheric pressure within reasonable bounds of 'normal' in order to function correctly (gaseous transport across membranes etc) and a pure Oxygen atmosphere, even at lower pressures, is still going to be explosive.

Using nitrogen allows for normal atmospheric conditions and reduces that risk of explosion/fire.

As you pointed out, it does allow for experiments to be run in Earthlike conditions (if we exclude that whole gravity thing...) but that is almost certainly going to be less of an issue, after all, many experiments are run in deliberately non-Earthlike conditions, or in sealed micro-environments with their own atmosphere etc.

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    $\begingroup$ The Mercury, Gemini, and Apollo programs all used a reduced pressure pure oxygen atmosphere. The Apollo 1 fire made NASA modify how that pure oxygen atmosphere was achieved, but did not alter the fact that the breathing atmosphere became pure oxygen shortly after launch. Skylab's breathing atmosphere was 75% oxygen, 25% nitrogen. Astronauts survived for 84 days breathing this "unhealthy" mix. $\endgroup$ – David Hammen Oct 24 '14 at 15:54
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    $\begingroup$ As a diver it seems like a dubious claim that 0.21 atm of O2 with or without any inert component would be any different. We're taught 1.4-1.6 atm of oxygen is acutely bad and that it's the partial oxygen pressure that matters, regardless of any other gas in the mix. Do you have any links for any of your claims? $\endgroup$ – Nick T Oct 24 '14 at 17:11
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    $\begingroup$ There are adverse safety and operational issues with using diluent gases as well. Regarding a pure oxygen atmosphere at 1/5 atmospheric pressure being "unhealthy", citation needed. I looked. No such luck. I also looked for the trade studies that made the Shuttle lean toward using a standard atmosphere. I couldn't find that, either. I found lots of trade studies that claimed to justify the pure oxygen atmosphere on Mercury, Gemini, and Apollo. $\endgroup$ – David Hammen Oct 24 '14 at 19:38
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    $\begingroup$ -1 pure oxygen is not explosive. $\endgroup$ – Mehrdad Oct 25 '14 at 10:02
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    $\begingroup$ @Mehrdad: The point, I think, is that a full atmosphere of pure O2 creates the risk of an explosion (as proven by Apollo 1) as long as there are flammable substances in the area. Pure O2 at 0.21 atmosphere creates a lesser risk of explosion, but still some. $\endgroup$ – Keith Thompson Jun 25 '15 at 23:09
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When an object is burning in an atmosphere which is 80% nitrogen and 20% oxygen, the nitrogen will absorb a lot of the generated heat while doing nothing to assist in combustion. While it's entirely possible that some other gas would be better than nitrogen (e.g. have a greater higher thermal mass per mole for better fire-suppression characteristics, or have comparable thermal mass at lower density for lower payload weight), nitrogen has the advantage that human beings can breathe an 80% concentration of it (at standard atmospheric pressure) for extended periods of time without ill effect.

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Low pressure can imply decompression sickness. Maintaining enough O2 is not sufficient; when overall pressure drops, gases dissolved in the blood (especially nitrogen) regain their freedom, and bubbles form.
Given the price involved in sending a live human being up in orbit, it would not be very rational to first keep him in a decompression chamber for a few days; maintaining such a chamber in the ISS would also use substantial space, which is a scarce resource up there. Alternatively, decompression would be done prior to the flight, which would be technologically challenging (the vessel would have to be kept at low pressure at all times in the pre-flight phase).

In the early vessels like Apollo, DCS was solved by "pre-breathing", i.e. having astronauts breathe pure O2 for half an hour before launch; and, more generally, by assuming that the astronauts were tough guys who could suck it up. Some problems remained which can explain why "normal" air was used in the Space Shuttle and the ISS.

Less seriously, some experiments on the ISS involve live subjects (e.g. small animals) that would not necessarily accommodate a low pressure; results would be biased. Unless, again, compression chambers are used.

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    $\begingroup$ I would think nitrogen purging could be accomplished earth-side by having astronauts in an atmosphere with a mix of O2 and He and then having them breathe such an atmosphere in launch vehicles. While a pre-breathing requirement would mean that astronauts couldn't be sent up without short notice, I wouldn't think it would have to shorten the duration of astronauts' trips to space. $\endgroup$ – supercat Oct 24 '14 at 19:43
  • $\begingroup$ You should not only read en.wikipedia.org/wiki/Decompression_sickness but also en.wikipedia.org/wiki/Altitude_sickness . DCS is not a problem for Everest climbers but altitude sickness really is. Not low pressure causes DCS, but a sudden pressure drop can do when to much nitrogen was solved in the blood and the tissues. I delete the wrong sentence about DCS and the Everest. $\endgroup$ – Uwe Aug 30 '17 at 20:38
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It is simpler to design because things behave like on Earth, and less chance to have things go horribly wrong.

To add to what was already mentioned - less problems with overheating, and fires are less dangerous... also lower air pressure lowers the boiling point of water. Also simpler to not have to transition from earth's surface to different atmosphere.

Down sides - structural of several times the air pressure, no need for nitrogen and balancing nitrogen, harder and more time consuming to do space walks - in space suit they use low pressure pure oxygen type atmosphere, more risk of decompression sickness (nitrogen boiling in blood), it may actually be easier to get rid of co2 if only oxygen and co2 in atmosphere, etc...

A permanent space colony might use atmosphere like you suggest for such reasons... there are ways to adapt, eg if overheating is a problem then you lower temp of habitat, eg 5 degrees Celsius rather than 20 degrees. At times you want to lose less heat - means you need less food and burn less oxygen, metabolism can slow down. A colony might have constant wind/airflow coming from roof and going into holes in floor as one way to help you stay on ground in zero gravity and take care of problems like spilling a liquid, would also help take care of you overheating.

But people are not so serious about thinking long term "colony"... if they were they would have a system that uses plants or similar to recycle the co2 into o2 and food rather than expensively have to ship up tonnes of consumables to keep astronauts alive... current system works for a few astronauts but would be unsustainable/too expensive for a colony of 100 or 1000 people. Typical people, especially in government do not take risks/do stuff different because you suffer much more for failure than you can hope to gain from success in trying to do things newer/better.

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Consider that humans "outgas" in many ways and this process increases as the ambient pressure decreases. As we expel the excess gases produced they mix with the available mass of existing gases in the environment. If you operate at a lower pressure your "exhaust" will have a greater impact on the existing mass of gasses. NASA does not advertise this fact but you can bet that it is plenty ripe enough on the ISS at one atmosphere. Removing these contaminants takes time and the processing is more efficient at higher pressures.

Missions on ISS are much longer term than in the past and the cumulative impact of the environment has more opportunity to multiply the effect.

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  • $\begingroup$ This reads more like a comment than an answer - you're not responding to the why question title. $\endgroup$ – Jan Doggen Mar 15 '18 at 20:32
  • $\begingroup$ @JanDoggen: It seems to be a deliberately partial answer, considering only one difficulty of a low-pressure atmosphere, but partial answers are not comments and do not need to be deleted. $\endgroup$ – Nathan Tuggy Mar 15 '18 at 20:40

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