A Primer on Long-Duration Life Support

Summary

Life support is the biggest technical obstacle to the human exploration of Mars.This fact makes people mad, because there are all kinds of other obstacles that are fun to solve (orbital refueling, landing heavy payloads, making rocket fuel out of Martian air), and life support has a fun factor of zero. It is a thankless world of dodgy sensors, failing bearings, and bacteria trying to grow on absolutely everything. But if we want to get to Mars alive, we need for this stuff to work.An astronaut in space needs 840 grams of oxygen, 2.8 kilos of water, and 1.8 kilos of dried food a day to stay alive. They also appreciate the little touches, like water to wash with (0.7 kg), fresh clothes (1.5 kg per week), wet wipes (0.2 kg/day) and a toilet (1.4 kg for canisters and wipes). On the output side, each astronaut exhales around a kilo of carbon dioxide and pees out a liter and a half of urine. They also produce a fairly small quantity of feces and menses (though many women opt to medically induce amenorrhea during space flight). On shorter missions like Apollo (~12 days) or Shuttle flights (~14 days) it makes sense to pack everything a crew needs with no attempts at recycling. In this paradigm, carbon dioxide can be scrubbed from the cabin air with disposable lithium hydroxide cartridges; everything else is carried along in the space version of a picnic basket. On longer missions, trying to carry single-use supplies gets unwieldy. A crew of four on a 1,000 day mission to Mars would need 48 tons of consumables, about equal to the mass of the entire spacecraft. And even if mass were unlimited, there simply wouldn’t be enough room to fit everything on board.So past a threshold of about 30 days, you have to make some attempt at recycling.From the diagram above, you can see water is the obvious place to start. Four to five kilos of the stuff flow through each astronaut every day, and you can reclaim most of that amount just by condensing cabin air on a cold surface in the spacec...