How is oxygen made aboard a spacecraft?
by Craig Freudenrich, Ph.D.
Astronaut Daniel W. Bursch, Expedition Four flight engineer, works on the Elektron Oxygen Generator in the Zvezda Service Module on the International Space Station (ISS).
On Earth, we have a constant supply of fresh air. We breathe in oxygen and breathe out carbon dioxide. These gases get recycled by plants through the process of photosynthesis. The plants take in carbon dioxide and release oxygen. It's a wonderful cycle on a vast scale. But what happens in the tiny, confined cabins of spacecraft, like the space shuttle or space stations?
Most spacecraft carry their own supply of oxygen with them and may have one backup system. However, the missions of these spacecraft last a short time, on the order of days to two weeks. In contrast, the International Space Station (ISS) was designed for long-term spaceflight and has been in orbit since 1998. So how is oxygen made aboard the ISS? It's handled in one of three ways, using oxygen generators, pressurized oxygen tanks or solid fuel oxygen generators (also called oxygen candles).
At the cathode, a type of reaction called reduction occurs. Electrons (e-) from the cathode combine with the water (H2O) to make hydrogen gas (H2) and hydroxide ions (OH-): 2H2O (l) + 2e- ->H2 (g) + 2 OH- (aq).
The electricity is generated by the station's solar panels and supplied to the oxygen generators through the station's power grid. The water gets delivered to the station from Earth by Progress supply ships and the space shuttle. Water also gets reclaimed by condensers that remove water vapor from the cabin air (astronauts exhale water vapor). Finally, water can be recycled from the astronauts' urine by the ECLSS unit. The hydrogen gas made in the electrolysis process gets vented into space and the oxygen gas is circulated into the cabin air.
Now let's look at the other ways by which the ISS makes oxygen.
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