How do astronauts breathe in space?
The oxygen is made on board by splitting water with electricity. The harder half of the problem — and the one nobody mentions — is taking the carbon dioxide back out.
5 min read
There is no air in space, so a crew has to bring their atmosphere with them and then keep it breathable. Carrying six months of oxygen in tanks is possible but heavy, and heavy is the one thing spaceflight cannot afford. So the space station makes most of its own.
Making oxygen from water
Run an electric current through water and it comes apart into hydrogen and oxygen. That is electrolysis, and it is a school science demonstration doing a life-critical job 420 km up.
The electricity comes from the station’s solar arrays. The oxygen goes into the cabin for the crew. The hydrogen is either vented overboard or fed to a reactor that combines it with exhaled carbon dioxide to make more water — recovering oxygen that would otherwise be thrown away. Two systems do this work: NASA’s Oxygen Generation System in the Destiny module, operational since 2007, and the Russian Elektron.
Which means water is the real supply
If oxygen comes from water, then water is what actually has to be kept topped up — and every litre launched from Earth costs a great deal. So the station recycles it, from humidity in the air, from sweat, and from urine.
In 2023 NASA reported reaching 98% water recovery aboard the station. As one engineer put it: collect 100 pounds of water, lose two, and the rest keeps going around. That recycled water is the same water being split for oxygen.
The half nobody mentions
Running out of oxygen is the danger everyone imagines. In a sealed metal can, the more immediate one is carbon dioxide. Every exhale adds some, and in a closed volume it climbs to dangerous concentrations long before the oxygen runs low.
So the air is continuously scrubbed. The Carbon Dioxide Removal Assembly draws cabin air through beds of zeolite, a porous mineral whose structure traps CO₂ molecules. When a bed fills, it is heated and vented to space to clear it, and another takes over. The Russian Vozdukh system does the same job by a similar route.
Outside the station
A spacesuit is a spacecraft you wear, and it repeats the same three jobs on a smaller scale: supply oxygen, remove carbon dioxide, hold pressure. It runs at a much lower pressure than the cabin, which is why an astronaut breathes pure oxygen beforehand to purge nitrogen from their blood — the same precaution a scuba diver takes against the bends, for the same reason.
The pattern is worth noticing. Almost nothing aboard is a one-way supply. Water becomes oxygen, exhaled carbon dioxide becomes water again, and the loop closes a little more tightly each time. That is the technology any long trip to Mars depends on.
Common questions
How do astronauts breathe in space?
The space station makes oxygen by electrolysis — passing solar-generated electricity through water to split it into hydrogen and oxygen. The oxygen goes into the cabin, and the air is continuously scrubbed of carbon dioxide.
Where does the oxygen on the ISS come from?
From water, split by NASA's Oxygen Generation System and the Russian Elektron. The water itself is recycled — NASA reached 98% water recovery aboard the station in 2023.
How is carbon dioxide removed from the space station?
The Carbon Dioxide Removal Assembly pulls cabin air through beds of zeolite, a porous mineral that traps CO₂. A full bed is heated and vented to space to clear it while another bed takes over.