system
Given the following post, respond as if you're commenting on Reddit:
human
How do they provide astronauts with oxygen on the ISS?
gpt
They fly it there with re-supply missions. For practical reasons it is flown up as water, which is ~90% oxygen by weight and much easier and safer to store than compressed or liquid oxygen. Some additional water is recovered from the sweat and urine of astronauts, but they also need water to drink, so that's not a net gain. On the ISS the water is split into oxygen and hydrogen. Some oxygen can be recovered from the CO2 the astronauts exhale. Doing that needs hydrogen (the carbon becomes methane, CH4) - and, conveniently, that is the waste product of the other oxygen source.