[Space] High O2, low pressure atmosphere.

[Kilmore]

Imagine a planet with a thin atmosphere (about 25% of Earth's), but with the same amount of Oxygen in the atmosphere as Earth. My guess is that the air would be hard to breathe as there is not enough atmospheric "body" to be able to breathe normally. So explorers would have to wear tanks of inert gasses to combine with the ambient atmosphere. Helium is the cheapest and less likely to cause nastiness than Nitrogen, and it has the added benefit of making the characters' voices squeaky.

Are my guesses correct? Also, will the air be any more combustible? What about cosmic radiation?

Thanks in advance.

[scc]

No, 25% pressure of Earth should be fine, as long as the oxygen percentage has been upped to compensate, so multiply the oxygen percentage by 4 and things should be fine, the only problem will the be bends when first breathing

[Ulzgoroth]

The basic business of breathing pretty much just cares about the partial pressures of oxygen and carbon dioxide. So long as the first is high enough and the second is low enough, total pressure won't really matter.

One place low total pressure can hurt you is in creating injurious pressure differentials between the body interior and the atmosphere. I don't know how low it has to go, but low enough and you start breaking blood vessels in the skin, and more critically in the lungs as well.

The other thing of course is messing up dissolved gasses in the blood, but I don't think any others are physiologically important, so you could probably equilibrate that, though you might need to decompress gradually.


Divers need inert gas pressure because they have to be able to inflate their lungs while under heavy underwater pressure. It'd be very hard to breath in from a quarter-atm gas source while your body is under 2 atm pressure. But someone out in the thin air won't have that problem.

[Dammann]

There is even an argument for using lower pressure, higher O2 air on space ships. Lower pressure means that hull punctures will be less explosive.

[Anthony]

Originally Posted by Kilmore

Imagine a planet with a thin atmosphere (about 25% of Earth's), but with the same amount of Oxygen in the atmosphere as Earth. My guess is that the air would be hard to breathe as there is not enough atmospheric "body" to be able to breathe normally.

No, that won't cause particularly major effects.

Originally Posted by Kilmore

So explorers would have to wear tanks of inert gasses to combine with the ambient atmosphere. Helium is the cheapest and less likely to cause nastiness than Nitrogen, and it has the added benefit of making the characters' voices squeaky.

Nitrogen at the pressures required isn't a notable risk.

Originally Posted by Kilmore

Are my guesses correct? Also, will the air be any more combustible? What about cosmic radiation?

Due to the lack of buffer gases to pull heat away from a flame, fires will catch more easily, and some things will be flammable that wouldn't be in our atmosphere. The reduced atmospheric mass will certainly cut down on the radiation shielding provided by the atmosphere, though as long as the planet has a reasonably strong magnetic field the doses should remain tolerable.

[Flyndaran]

That would be an atmosphere of almost pure oxygen. Hope you like conflagrations and explosions.
For space walks they use pure oxygen and around 1/3 atmosphere pressure. Otherwise they couldn't bend their suit's joints.

[Nereidalbel]

That's an air density humans can acclimate to. Fires will be much hotter than on Earth, so, any civilizations would have no trouble with smelting. And, unless there is a very strong magnetic field and a rather thick ozone layer, life will have to adapt to higher doses of radiation than we're used to.

[Dinadon]

Originally Posted by Dammann

There is even an argument for using lower pressure, higher O2 air on space ships. Lower pressure means that hull punctures will be less explosive.

I'm not certain 1 atmosphere is enough for that to be a worry.

Also, a terminology nitpick, if you're worrying about explosive decompressions you probably have more important concerns that atmospheric pressure. An explosive decompression is by definition quick, and most likely in conjunction with a major structural flaw of some sort. A rapid decompression, on the other hand, is the sort that would benefit from this. (And I think a slow decompression might actually be worse, given that to be categorised as such can include the fact it isn't noticed until too late)

EDIT:
Also, space suits already operate with lower pressure, higher oxygen.

[johndallman]

Originally Posted by Dammann

There is even an argument for using lower pressure, higher O2 air on space ships. Lower pressure means that hull punctures will be less explosive.

Well, Apollo used 1/3 of an atmosphere of pure oxygen, since that meant the hull didn't need to be so strong to hold the pressure. This was important on the Lunar Module, which was very weight-critical.

However, at take-off they needed to start with full atmospheric pressure, and originally used a full atmosphere of pure oxygen. This is a very serious fire risk, and is why the Apollo 1 fire was so bad. That got changed to starting with Earth-normal atmosphere and a gradual transition during take-off and assent to the 1/3 atmosphere of pure oxygen conditions.

[RyanW]

Originally Posted by Dammann

There is even an argument for using lower pressure, higher O2 air on space ships. Lower pressure means that hull punctures will be less explosive.

If by "an argument for" you mean "was standard practice for decades" then yes. Apollo used (during spaceflight, but not ascent/reentry) pure oxygen at 1/3 atmosphere. EVA suits still use this, but most modern vehicles use a similar nitrogen/oxygen blend as is found at sea level.