Oxygen Atmosphere in Spacecraft

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Oxygen Atmosphere in Spacecraft

Astronauts sealed in a spacecraft or space station need a continuous supply of oxygen. When they inhale, the oxygen in the air is absorbed by the blood and used by the body. When they exhale, nitrogen, water vapor, and carbon dioxide (CO₂) are expelled. During a flight, oxygen must be added to the air, while water vapor, CO₂, and other impurities must be removed.

Earth's atmosphere at sea level consists of 21 percent oxygen, 78 percent nitrogen, and 1 percent CO₂, water vapor, argon, methane, and traces of other gases, at a pressure of 14.7 pounds per square inch (psi). Pure oxygen is highly corrosive and reacts with most substances, sometimes violently, as in a fire or an explosion. Nitrogen in Earth's atmosphere dilutes the oxygen so that such violent reactions do not usually occur spontaneously.

In January 1967 three astronauts died while testing and practicing procedures on the launch pad in the Apollo 1 capsule, which had been supplied with a pure oxygen atmosphere at 16 psi pressure. A fire started, spread extremely rapidly, burned out in less than a minute, and the astronauts did not have time to escape. Later Apollo flights used a mixture of 60 percent oxygen and 40 percent nitrogen at 16 psi on the launch pad, then switched to pure oxygen at only 5 psi in space. This proved to be much safer.

The Skylab space station also had a pure oxygen atmosphere at 5 psi. Russian Salyut and Mir space stations all maintained atmospheres similar in composition and pressure to Earth's atmosphere, as do the space shuttle and the International Space Station.

On Earth, gravity keeps the air moving continuously as warm air rises and cool air sinks. In a weightless spaceship blowers must force the cabin air to circulate. As it is drawn through the ducts of the circulation system the air is cleansed of impurities. A bed of charcoal removes noxious gases and odors. Filters with very small holes trap floating particles down to the size of bacteria. Moisture condenses onto cold plates similar to refrigerator coils, and the water is collected in a tank.

Excessive CO₂ can be deadly and must be removed. The simplest way is to blow the air through a canister of lithium hydroxide, which absorbs CO₂. However, the canisters must be replaced when they become saturated with CO₂. This is not practical for long voyages because many heavy canisters would have to be carried along. The International Space Station uses better absorbing materials that can be recycled while in orbit. To drive out the CO₂ some of these materials are heated while others are just exposed to the vacuum of space.

The space shuttle carries tanks of liquid oxygen to replenish the air. For the Mir space station, Russia developed an electrolysis system called Elektron, which split the water molecules (H₂) into hydrogen and oxygen. The oxygen was used in the cabin and the hydrogen was vented outside to space. This type of system will be used throughout the International Space Station. In this case, Elektron's water supply will come from the space shuttle and from recycling moisture in the air, urine, and wash water. In the future, the CO₂ removed from the air could be chemically combined with the hydrogen from Elektron to produce methane and water. The methane would be vented overboard and the water would be reused in Elektron to produce more oxygen—an exceptional recycling system.

As a backup, lithium perchlorate generators can be used to produce oxygen when they are ignited. They must be used with care. In February 1997 one of them burned out of control for fourteen minutes on Mir with a blow-torch-like flame at about 480°C (900°F). Mir was damaged, but no one was injured.

see also Apollo (volume 3); Closed Ecosystems (volume 3); Emergencies (volume 3); Living in Space (volume 3).

Thomas Damon