The vacuum of space is hostile to human life, requiring a sealed and artificial atmosphere for survival. Astronauts need a continuous supply of breathable air, maintained at a safe pressure and temperature, to live and work in orbit. This challenge is met by two distinct systems: the large-scale habitat, such as the International Space Station (ISS), and the individual spacesuit. These systems must supply oxygen, manage exhaled carbon dioxide, control humidity, and regulate atmospheric pressure.
Creating a Habitable Atmosphere
The atmosphere inside the International Space Station (ISS) closely replicates the air found at Earth’s sea level. The total atmospheric pressure is maintained at approximately 14.7 pounds per square inch (psi), equivalent to one full Earth atmosphere. This pressure ensures the physiological safety of the crew and the proper function of onboard equipment.
The gas composition within the ISS is a mixture of approximately 78% nitrogen and 21% oxygen, with trace gases making up the remainder. This nitrogen buffer is a safety measure, preventing the risk of fire associated with a pure oxygen environment at standard pressure. Nitrogen also helps prevent decompression sickness, or “the bends,” which occurs when the body is rapidly exposed to lower pressures.
The station’s Environmental Control and Life Support System (ECLSS) continuously monitors the air pressure and composition. Minor air leaks are inevitable and must be compensated for by periodically adding oxygen and nitrogen. Nitrogen is resupplied by visiting cargo spacecraft to maintain the atmospheric balance.
The Mechanics of Air Renewal
Sustaining a breathable atmosphere requires constantly recycling the air, as resupply from Earth is costly and infrequent. Oxygen is generated through electrolysis, where an electrical current splits water into hydrogen and oxygen. The Oxygen Generation System (OGS) releases the oxygen into the cabin, while the hydrogen is sent to a carbon dioxide reduction system.
Removing exhaled carbon dioxide is accomplished using specialized machinery like the Carbon Dioxide Removal Assembly (CDRA). The CDRA uses beds of zeolite, a mineral material that acts as a molecular sieve, to trap carbon dioxide molecules from the cabin air. The system is regenerative: the zeolite beds are periodically heated to release the captured carbon dioxide, which is then vented overboard or sent to a processing unit.
Waste gases are processed further to reclaim resources, closing the oxygen and water loops as much as possible. The Sabatier reactor, for example, combines waste hydrogen from electrolysis with removed carbon dioxide to produce water and methane. The reclaimed water is purified and fed back into the OGS to create more oxygen, while the methane is vented into space. In addition to oxygen and carbon dioxide management, the air revitalization system also controls humidity and removes trace contaminants.
Breathing During Spacewalks
When astronauts leave the station for a spacewalk, known as Extravehicular Activity (EVA), they rely on the self-contained Extravehicular Mobility Unit (EMU) spacesuit. The atmosphere inside the EMU is dramatically different from the cabin, utilizing 100% pure oxygen at a lower pressure of about 4.3 psi. This low pressure allows the suit to be flexible enough for the astronaut to move and work.
Life support functions are contained in the Primary Life Support System (PLSS), a backpack unit worn by the astronaut. The PLSS supplies pure oxygen, removes exhaled carbon dioxide using chemical canisters, and controls the suit’s temperature and humidity. Fans circulate the oxygen, ensuring a continuous flow of breathable gas and directing contaminants to the removal systems.
The transition from the ISS’s cabin pressure of 14.7 psi to the suit’s lower 4.3 psi presents a physiological hazard. The pressure difference could cause nitrogen dissolved in the astronaut’s tissues to rapidly bubble out, leading to decompression sickness. To prevent this, astronauts undergo a pre-breathing protocol before an EVA.
This protocol requires the astronaut to breathe pure oxygen for several hours to flush the inert nitrogen gas from their bloodstream and tissues. The process of denitrogenation is accelerated by incorporating light exercise while breathing the oxygen. Once nitrogen levels are safely reduced, the astronaut can proceed with the pressure change and the spacewalk, ensuring safety in the low-pressure environment of the spacesuit.