(2.3.1-006) Baseline Air Pressure

The atmosphere in an orbital habitat will necessarily need to be maintained at a certain air pressure.

The air pressure on International Space Station (ISS) is set to 101.3 Kpa which is the terrestrial sea level average^{[1] [2]}.

While members of the ISS crew could stay healthy even with the pressure at a lower level, the equipment on the Station is very sensitive to pressure. If the pressure were to drop too far, it could cause problems with the Station equipment.

The ISS also contains tanks containing nitrogen which can be released in order to maintain the air pressure in the event of a leak.

Many terrestrial closed biosphere experiments (BoK: 4.1) use the external ambient air pressure as the baseline internal air pressure. This is typically done to minimise pressure differentials which could generate excessive leakage. Biosphere 2 used a system of large lungs to cope with the expansion and contraction of the atmosphere due to the daily temperature swings.

So, how then does one decide on a baseline internal pressure for an orbital habitat. Since the atmosphere of an orbital habitat will be much larger than is typical of space craft very probably the determining factor will be the mass of the gasses themselves. One might expect that the internal pressure of an orbital habitat will be set as high as is possible given the material material availability subject to a hard upper limit of terrestrial sea level air pressure.

Should all biomes should be at the same air pressure? This is a tradeoff as differing internal air pressures imply each biome is a container capable of maintaining its own air pressure. Such a requirement would require much more structural material for its construction. However, multiple biospheres each with their own internal systems would provide a redundancy nicely consistent with Rule 1 of the Three Laws of Orbital Habitats (1.1-001).

References:

1. Wikipedia, ISS ECLSS , accessed 2024-05-18

2. NASA, The Air Up There , accessed 2023-05-20