Permanent Human Settlement of the Earth, Space and Ocean Frontiers

Wednesday, December 27, 2006

The Air we Breathe

Ralph Buttigieg

Sydney, NSW

We have all seen the movie. The astronaut puts on a spacesuit, enters the airlock then makes a quick exist to the outside. The reality is much more complicated. Because of the mobility problems mentioned in the previous article space suits are kept at low pressure. In the case of the shuttle suit 4.3 psi. The Shuttle and Space Station on the other hand are at sea level pressure, 14.7 psi. A quick drop to spacesuit pressure would be a sure way to give the astronaut a bad case of Decompression Sickness, the bends.

To avoid this the astronauts have to preform a lengthy procedure before they can commence a EVA. In the case of the Shuttle it involves pre-breathing pure oxygen for an hour then reducing the cabin pressure to 10.2 psi for 24 hours before a final 75 minute oxygen prebreathe. ISS astronauts also use the Shuttle suit and they cope with a 2 hours and 20 minutes pre-breathe but that includes a 10 minutes of vigorous exercise which helps in removing the nitrogen. Now a Moon or Mars base will require astronauts to do frequent outside excursions, time consuming pre breathing procedures will need to be avoided if the astronauts are actually going to explore, not just stay indoors.

One possible solution is to reduce the habitats atmospheric pressure. A good percentage of the human race does not live at the 14.7 psi of sea level. Residents of Mexico City live with a 28% reduction in pressure , at Lhasa,Tibet (altitude 3,800 meters) people cope with with 42% reduction. With proper acclimatization, most people seem to cope with these low air pressure. In a habitat we can do even better as the oxygen partial pressure can be maintained by increasing the O2 percentage. The Apollo command module used a 100% oxygen atmosphere at 5psi. But there are problems with very low pressure, lower the pressure and you lower the boiling point so getting a hot meal becomes a problem. More importantly the risk of fire increases and getting rid of waste heat becomes more difficult.

Another way to to use tougher spacesuits that can handle higher pressure. Originally the American ISS astronauts were supposed to use a spacesuit that could handle 8psi. But that proved more difficult to build then first thought so the Shuttle suit was used. There has been considerable work on hard suits over the years but these just worse the cost and mobility problems I have previously mentioned.

There is no easy solution to this problem. Real world trials are just going to have been done to find a compromise between low habitat atmospheric pressure and suit design to allow effective base operation.