NASA's Oxygen Systems
Everyone knows there is no oxygen in space. So how do the astronauts aboard the International Space Station (ISS) get their oxygen? The answer to this question is a little more complicated than one would expect.
The entire atmosphere on the ISS is controlled via the Environmental Control and Life Support System (ECLSS). The ECLSS also controls fire detection and suppression, waste management and water supply. The water recovery system uses waste water from showers and sinks to make oxygen. This is done through a process called electrolysis where water is separated into oxygen and hydrogen. The system cycles the oxygen back into the ISS through the Oxygen Generation System (OGS). The hydrogen, which is explosive, is vented out of the ISS.
If the OGS fails, the Russians have their own oxygen generating system called Elektron. This system was first used on the Mir space station and also uses electrolysis to produce oxygen. Both the OGS and Elektron systems have had trouble since being brought aboard the ISS.
In the case of both the OGS and Elektron systems failing at the same time, there is a third system in place. The Vika oxygen generator is another Russian system from the Mir space station. Vika is a chemical oxygen generator which uses canisters of solid lithium perchlorate. Astronauts burn these canisters similar to a large candle, but instead of smoke they give off fresh oxygen. Each Vika canister holds enough oxygen to support one ISS crew member for a full day.
Finally, when all else fails, the crew of the space station can use oxygen brought in from supply ships like Progress. When a supply ship docks with the space station, they can pump in oxygen and nitrogen to fill pressurized oxygen tanks aboard the ISS. When activated, the system mixes the two gases and releases them as breathable air.
With all of these systems in place for creating oxygen, one has to wonder what happens with all of the unwanted gases also created. The Air Revitalization System contains multiple assemblies to remove dangerous gases from the ISS atmosphere. The goal is to keep the ISS atmosphere similar to the atmosphere of Earth. This is done not only for comfort of the crew, but for their safety as well. The other option would be to simply use pure oxygen, but the high flammability of pure oxygen makes this a dangerous idea.