How is oxygen made aboard a spacecraft?

Creating Oxygen Aboard the ISS
Water electrolysis
Water electrolysis

As you now know, getting oxygen aboard the International Space Station (ISS) is handled in one of three ways, using oxygen generators, pressurized oxygen tanks or solid fuel oxygen generators. On the previous page, we talked about the oxygen generators. Now, let's talk about the other two methods.

In the second method, oxygen is not made, but rather delivered to the ISS from Earth. When Progress supply ships, European automated transfer vehicles, or the U.S. space shuttle dock at the station, they pump oxygen into pressurized tanks at the airlock nodes. They also pump nitrogen gas into other pressurized tanks at those airlocks. The station's atmospheric controls mix the gases in the correct proportions to Earth's atmosphere and circulate the mixture through the cabin.

The third method is a backup system that makes oxygen through chemical reactions. The system is called the solid fuel oxygen generator (SFOG) and is located in the station's service module (Zvezda). The SFOG, which is also called oxygen candles or chlorate candles, has canisters that contain a mixture of powdered sodium chlorate (NaClO3) and iron (Fe) powder. When the SFOG is ignited, the iron "burns" at 1112 degrees F (600 degrees C), which supplies the heat energy required for the reaction. The sodium chlorate breaks down into sodium chloride (table salt- NaCl) and oxygen gas (O2). Some of the oxygen combines with iron to form iron oxide (FeO):


NaClO3 (s) + Fe (s) -> 3O2 (g) + NaCl (s) + FeO (s)

The SFOG supplies 6.5 man-hours of oxygen per kilogram of the mixture. Russian spacesuits also make oxygen using SFOGs.

In future space stations or space colonies, NASA scientists hope to create oxygen and eliminate carbon dioxide naturally by growing plants. The plants would supply breathable air and be a food source for the astronauts. One problem that must be solved, though, is how to grow large numbers of plants in small spaces -- living space aboard a space station is limited.

Related HowStuffWorks Articles


  • American Chemistry Council, "Sodium Chlorate: Providing Emergency Oxygen" January 2007.
  • Bovard, RM, "Oxygen Sources for Space Flights" Aerospace Medicine 31: 407-412, 1960
  • Launius, RD, "Space Stations: Base Camps to the Stars" Smithsonian Books, Washington, DC, 2003
  • Reference Guide to ISS