How is carbon dioxide eliminated aboard a spacecraft?

Interior view of "mail box" for purging carbon dioxide from Lunar Module
Interior view of the Apollo 13 Lunar Module during the trouble-plagued journey back to Earth -- the "mail box" pictured here was used for purging carbon dioxide. See more space exploration pictures.
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We produce carbon dioxide in our bodies when our cells break down food and we release it when we exhale. In the atmosphere, carbon dioxide concentrations are approximately 0.04 percent. However, in the confined cabins of spacecraft, like the space shuttle or space stations, the carbon dioxide concentration can get much higher, which poses a problem because carbon dioxide is toxic. As carbon dioxide concentration in the air around you increases, you will suffer certain symptoms:

  • At 1 percent - drowsiness
  • At 3 percent - impaired hearing, increased heart rate and blood pressure, stupor
  • At 5 percent - shortness of breath, headache, dizziness, confusion
  • At 8 percent - unconsciousness, muscle tremors, sweating
  • Above 8 percent - death

On Earth, plants remove carbon dioxide through the process of photosynthesis. The plants take in carbon dioxide and release oxygen. However, in a spacecraft, carbon dioxide must be removed from the cabin air through chemical processes. Most spacecraft rely solely on removing the carbon dioxide with canisters that contain powdered lithium hydroxide. When air containing carbon dioxide (CO2) gets passed through the canister, it combines with the lithium hydroxide (LiOH) to form lithium carbonate (Li2CO3) and water (H2O).


CO2 (g) + 2LiOH (s) -> Li2CO3 (s) + 3 H2O (l)

Once all the lithium hydroxide is used up, the canister must be replaced and discarded. Perhaps, the most famous example of using lithium hydroxide canisters occurred on the Apollo 13 mission.

After an explosion crippled the command module, the astronauts lived in the lunar module while the spacecraft returned to Earth. The lunar module used round lithium hydroxide canisters, while the command module used square ones. With three astronauts breathing the air in a space designed for only two, the lunar module canisters were quickly used up, but the astronauts could not exchange them readily because of the different shapes. So, engineers at Mission Control had to devise a way to adapt the air flow from the lunar module through the square lithium hydroxide canisters. They were able to rig a system using hoses, socks, plastic bags and duct tape -- saving the astronauts from carbon dioxide-induced death.

Lithium hydroxide canisters aren't the only solution -- keep reading to find out how SCUBA equipment works in space.



SCUBA in Space

Lithium hydroxide canisters aren't the only CO2 problem solver in space. The International Space Station (ISS) uses lithium hydroxide canisters but it also has a newer technology that uses molecular sieves to absorb carbon dioxide. SCUBA re-breathers and personal oxygen units used by firefighters and miners must also remove carbon dioxide. Some rebreathers use lithium hydroxide canisters. But others use a reaction involving potassium superoxide (KO2). When potassium superoxide combines with water vapor (H2O) and carbon dioxide (CO2) from a person's breath, it absorbs carbon dioxide and makes oxygen gas and potassium bicarbonate (KHCO3):

4KO2 (s) + 4CO2 (g) + 2H2O (g) -> 4KHCO3 (s) + 3O2 (g)


The reaction makes heat. So, you can tell when it's done because it stops heating up. This system has the added advantage of supplying oxygen as well as removing carbon dioxide.

The U.S. Destiny lab portion and Node 3 portion of the ISS contain a carbon dioxide removal assembly (CDRA). The CDRA uses molecular sieve technology to remove carbon dioxide. The molecular sieves are zeolites, crystals of silicon dioxide and aluminum dioxide. The crystals arrange themselves to form tiny screens. The openings of the screens or pores are consistent sizes that allow some molecules to enter and get trapped in the sieves. In the CDRA, there are four beds of two different zeolites. Zeolite 13x absorbs water, while zeolite 5A absorbs carbon dioxide. Each side of the CDRA contains a zeolite 13X connected to a zeolite 5A bed. As the air passes through the zeolite 13X bed, water gets trapped and removed from the air. The dried air goes into the zeolite 5A bed where carbon dioxide gets trapped and removed. The outgoing air is then dry and free from carbon dioxide.

Unlike lithium hydroxide canisters, which get used up and discarded, the zeolites in the CDRA can be regenerated. Electrical heating elements within the beds heat up the zeolites and free the trapped water vapor and carbon dioxide. The carbon dioxide gets vented into outer space, while the water vapor gets condensed and recycled. The CDRA is designed with independent controls so that one half is actively removing carbon dioxide and water from the air, while the other half is regenerating. The two halves alternate. The CDRA is the primary method by which carbon dioxide gets removed from the ISS cabin air, while lithium hydroxide canisters are used as backups.

In October of 2010, a new system, called the Sabatier, was installed on the ISS. It takes carbon dioxide (CO2) that is removed by the CDRA, combines it with the hydrogen gas (H2) generated by the Russian Elektron and U.S. Environmental Control and Life Support System (ECLSS) water electrolysis systems, and forms liquid water (H2O) and methane gas (CH4). The methane gets vented into outer space.

In the future, NASA scientists hope to create oxygen and eliminate carbon dioxide aboard spacecraft and space colonies naturally by growing plants. The plants would not only supply breathable air, but also food for the astronauts. For more space-related info, see the links on the following page.


Lots More Information

Related HowStuffWorks Articles

  • American Chemistry Council, "Sodium Chlorate: Providing Emergency Oxygen" January 2007.
  • Carbon Dioxide Control: Molecular Sieves
  • Knox, J, Howard, D, "Clearing the Air: Life Support for Space Exploration"
  • Launius, RD, "Space Stations: Base Camps to the Stars" Smithsonian Books, Washington, DC, 2003
  • Reference Guide to ISS
  • NASA Science News, "Breathing Easy on the Space Station"