Clean (Spacecraft) Air

Marianne Dyson, August 2018

With no air in space, lungs empty like popped balloons. Blood boils, turning people into giant bruises. Eyes pop and eardrums burst. Yuck!

People must have air. We need it to breathe, and we need its pressure on us so air and liquids inside us don’t escape. We also need the right mix of gases to stay healthy and avoid fires in space.

Providing clean spacecraft air for a three-year round trip to Mars is quite a challenge, but one we are learning how to meet thanks to the experience gained on the International Space Station. To help others (especially you science fiction writers out there!) understand and appreciate that there is more to the life support system than worrying about the Klingons causing a hull breach, I’m sharing a slightly edited excerpt from my children’s book, Space Station Science (which you can order via Amazon or my website).

Bring Your Own Air

At the beginning of the space program, NASA filled spaceships with pure oxygen—the only gas people need to breathe. But during an Apollo 1 training session, the 100 percent oxygen atmosphere caused a fire to spread so fast that the three-man crew was killed in a matter of seconds. After that tragedy, NASA began mixing the oxygen with nitrogen during ground tests. Nitrogen slows fires, and people are used to breathing nitrogen and oxygen because natural air is four parts nitrogen to one part oxygen.

Station modules are launched with natural air inside. This air quickly grows stale and gradually escapes. It must be replaced. The nitrogen and oxygen for space station air are hauled to space from Earth. In order to fit in smaller tanks, these gases are chilled into liquids. The liquids are warmed to gas again before being released into the modules.

The Russians use a system called Elektron to turn wastewater into oxygen. Water is about 90 percent oxygen by weight. Electricity separates the water into hydrogen and oxygen. The oxygen goes into the cabin. Hydrogen is dangerous. A leak into the cabin could cause an explosion. Therefore the hydrogen is vented overboard.

Oxygen and nitrogen are stored in tanks in the Progress resupply ships or mounted outside the air lock. Tank valves open like little doors, “inflating” the station when the air pressure inside drops below a certain level.

When guests visit, more fresh air is needed. But astronauts can’t open a window to get it. When the space shuttle visited, hoses with air holes were snaked through the tunnels and hatches. The hoses transferred oxygen from the shuttle’s cabin to the station’s modules. Just before a shuttle departed, it “puffed up” the station with an extra shot of air.

The Russian Soyuz, a much smaller vehicle, does not carry extra air like the space shuttles did. When it brings visitors to the station, the Russians use portable solid fuel oxygen generators to provide the extra air needed. These generators were first developed for submarines and were used on the Mir space station. Like portable heaters, each generator sits in the aisle of a module. Cosmonauts insert a chemical candle that “smokes” oxygen for 5 to 20 minutes. These generators get very hot, and twice started fires on Mir. The crew were not hurt either time, but because of the risk, the generators are used only during visits and as a backup system on the station.

Keeping It Clean

Replacing oxygen and nitrogen is not enough. People breathe in oxygen but breathe out carbon dioxide. Carbon dioxide is poisonous. It can cause sickness and death even if there is enough oxygen in the air with it. On Earth, plants absorb it. In space, chemicals do the job.

Space suits use canisters of a chemical called lithium hydroxide to absorb carbon dioxide. The space shuttles also used these canisters. Like a litter box used by many cats, these canisters must be changed often. New ones must be stored and full ones thrown away. Enough canisters to supply the station between cargo supply visits would fill an entire module. So the station has a reusable air-scrubbing system. The Russian system is called Vozdukh, and the American one is called the Carbon Dioxide Removal Assembly (CDRA, pronounced see-drah).

Air Filtration System Diagram: About every 2.5 hours, the valves and heaters in the CDRA change to the opposite position. This forces the air to flow over a fresh zeolite bed. Diagram by Dave Klug from page 27, Space Station Science, © Marianne Dyson.

With no up and down, hot air does not rise. So station fans constantly stir it. Dust and debris collect on fan screens and filters.

After filtering, the fans blow the station’s air across beds of a chemical called zeolite, which is often used in fertilizers. The carbon dioxide in the air sticks to the zeolite while the oxygen and nitrogen sail on through. When a zeolite bed gets “soaked” with carbon dioxide, the airflow to it is shut off. The bed is heated, releasing the carbon dioxide overboard. Once all the carbon dioxide is gone, the zeolite bed is cooled, the airflow is turned back on, and the cycle starts over again.

Note: maintaining the CDRA system has proven quite challenging for space station astronauts as described in Scott Kelly’s book, Endurance, which I highly recommend.

Water vapor from breathing, washing, and sweating also must be removed from the air. Otherwise, it fogs windows and allows mold to grow.

To remove water from the air, the station uses a system that works like a dehumidifier on Earth. Fans blow the humid air over chilled water pipes. The water condenses onto the pipes like it does on glasses of iced tea. In Earth dehumidifiers, these drops naturally slide down into a collector tray. In the free-fall environment of space, spinning is needed to force the water to flow into a collector. This water is not wasted. It is stored in a tank and recycled for drinking and oxygen production. [End edited excerpt of Space Station Science.]

What combination of systems will astronauts headed to Mars use to keep their air fresh and clean? Whatever systems are chosen, they must operate for the entire time that astronauts are away from Earth—about three years for a round trip to Mars. 

Writing about Space

The Callahan Kids: Tales of Life on Mars is going out of print at the end of August. The stories are forever, but the company that sponsored the anthology which has two of my stories (“Martian Mice” and “Dropping the Martian Ball”) has gone out of business. The eBook book targeted at upper elementary and middle-school kids is now only 99 cents on Amazon. The print book is $9.99 on Amazon, but only $9.00 if you use my coupon code via CreateSpace. See my Book Orders page right-hand column for the code. You may also order a signed copy from me through my website.

An excerpt of my memoir, A Passion for Space, describing my experiences as a flight controller during the first space shuttle launch, will be included in the FenCon 2018 Program Book this September. Register to attend to get your copy!

My next book, coauthored with Buzz Aldrin, To the Moon and Back: My Apollo 11 Adventure, a pop-up book from National Geographic with art by Bruce Foster, is available for preorder now from Amazon. Look for it in stores everywhere on October 16.

Speaking about Space

Teachers, librarians, and event organizers, please consider me for Author Visits. Writers and publishers, I offer science consulting, content and technical editing.

September 21-23, Science GOH at FenCon XV in Dallas. See their website for program details. Writer GOH is Larry Niven.

September 29, Attending SCBWI Houston conference. Come and get a special pop-up book mark for To the Moon and Back from artist Bruce Foster.

October 2, Instructor for first class of Women and Space course at Rice University’s Glasscock School of Continuing Studies.

October 12, Featured speaker on Friday 11 to noon at the National Science Teachers Association conference in Reno, Nevada.

See my contact page for a complete appearance schedule and photos from previous events.

Author: Marianne

Marianne Dyson is an award-winning children's author, science fiction writer, and former NASA flight controller. To invite her to speak or order her books, visit her website,