Life Support in Space: How Astronauts Breathe, Drink, and Survive

Atmospheric Management

The ISS maintains a cabin atmosphere similar to Earth's surface: roughly 21 percent oxygen and 78 percent nitrogen at a total pressure of one atmosphere. The Oxygen Generation System produces breathable oxygen by electrolyzing water, splitting H2O molecules into hydrogen and oxygen using electrical current from the station's solar arrays. A backup system uses solid fuel oxygen generators, chemical cartridges that release oxygen when ignited, as emergency reserves in case the primary system fails.

Carbon dioxide exhaled by the crew, roughly one kilogram per person per day, must be continuously removed from the cabin air. The Carbon Dioxide Removal Assembly uses beds of zeolite, a mineral that adsorbs CO2 at cabin temperature and releases it when heated or exposed to vacuum. The captured CO2 is vented overboard or routed to the Sabatier reactor, where it combines with hydrogen from the electrolysis system to produce water and methane. The water is recovered for reuse while the methane is currently vented, though future systems may crack the methane to recover the hydrogen for additional Sabatier cycles.

Trace contaminant control assemblies scrub the atmosphere of volatile organic compounds, ammonia, carbon monoxide, and other potentially harmful gases that accumulate from equipment outgassing, cleaning products, and human metabolism. Activated charcoal beds adsorb many contaminants, while catalytic oxidizers convert carbon monoxide and other combustible gases into less harmful forms. Air quality is continuously monitored with mass spectrometers and infrared sensors, with ground teams reviewing data and recommending filter replacements or system adjustments.

Water Recovery

Water is the most critical consumable in space because it serves triple duty: drinking and food preparation, oxygen generation via electrolysis, and hygiene. Launching water from Earth costs thousands of dollars per kilogram, making recycling essential for economic sustainability. The ISS Water Recovery System processes two main streams: humidity condensate collected from the cabin air and pretreated urine from the crew.

Humidity condensate is relatively clean, requiring filtration and catalytic treatment to remove trace contaminants. Urine processing is more complex, involving vacuum distillation in a rotating drum that separates water vapor from concentrated brine using centrifugal force. The recovered water is combined with humidity condensate and passed through additional filtration, including iodine treatment for microbial control. The final product meets drinking water standards that are actually stricter than most municipal supplies on Earth, with total recovery rates exceeding 90 percent. Future deep-space missions will need to push recovery even higher, potentially to 98 percent or more, to minimize the mass of water that must be launched.

Thermal Control

In space, the absence of convective heat transfer means that thermal management depends entirely on radiation and conduction. The ISS generates substantial heat from electronics, experiments, and crew metabolism, roughly 75 kilowatts that must be rejected to space. An internal thermal loop circulates water through cold plates mounted to heat-generating equipment, collecting thermal energy throughout the station. This heat transfers to an external ammonia loop through heat exchangers and is radiated into space via large white panels mounted on the station's truss structure.

Individual spacecraft like Crew Dragon and Soyuz use similar but smaller-scale systems, with flash evaporators and radiator panels managing the thermal load during relatively short missions. For planetary surfaces, thermal design must account for the local environment: Mars's thin atmosphere provides minimal convective cooling but significant dust deposition on radiator surfaces, requiring different engineering solutions than the pure vacuum environment of orbital operations.

Food, Waste, and Hygiene

Food in space consists primarily of shelf-stable pouches and freeze-dried items that crew members rehydrate with hot or cold water. The menu includes hundreds of items rotated on a roughly eight-day cycle to reduce monotony. Caloric requirements in microgravity are similar to those on Earth, roughly 2,000 to 3,000 calories per day depending on body size and activity level. Fresh produce arrives periodically on resupply vehicles and experiments in growing lettuce, radishes, and chili peppers aboard the station are developing techniques for in-flight agriculture.

Waste management in microgravity requires specialized equipment. The ISS toilet uses airflow rather than gravity to direct waste into collection containers. Solid waste is stored in sealed containers and removed with departing cargo vehicles that burn up during re-entry. Liquid waste enters the urine processing system described above. Personal hygiene uses no-rinse shampoo, wet wipes, and a small amount of water from a squeeze bottle, as conventional showers are impractical in microgravity where water forms floating globules rather than flowing downward.

Future Life Support for Deep Space

Missions to Mars and beyond will require life support systems that operate for years without the regular resupply that sustains the ISS. Fully closed-loop systems that approach 100 percent recycling of water, oxygen, and even solid waste are under development. Bioregenerative life support concepts incorporate plants, algae, and microbial communities that simultaneously produce food and oxygen while processing waste, mimicking Earth's natural biogeochemical cycles in a compact, engineered system. NASA's Veggie and Advanced Plant Habitat experiments on the ISS, along with ESA's MELiSSA project, are testing components of these future systems in actual microgravity conditions.

Carbon Dioxide Removal and Oxygen Generation

Removing carbon dioxide from the cabin atmosphere is a continuous and critical process, since CO2 concentrations above 1 percent cause headaches, impaired cognition, and eventually loss of consciousness. The ISS uses the Carbon Dioxide Removal Assembly, which passes cabin air over beds of zeolite mineral that adsorb CO2 molecules. When one bed is saturated, the system switches to a fresh bed and exposes the saturated bed to vacuum, venting the captured CO2 into space. Newer systems like the Sabatier reactor go further by combining captured CO2 with hydrogen (produced by the electrolysis system) to generate water and methane, recovering hydrogen that would otherwise be lost.

Oxygen generation on the ISS relies primarily on the Oxygen Generation System, which uses electrolysis to split water molecules into hydrogen and oxygen gas. The oxygen is vented into the cabin atmosphere while the hydrogen is either vented overboard or fed to the Sabatier reactor. Backup oxygen comes from pressurized tanks and solid fuel oxygen generators, which produce oxygen through a chemical reaction when ignited. These backup systems provide a safety margin in case the primary electrolysis unit requires maintenance.

Thermal Control

In space, managing heat is paradoxically one of the most challenging engineering problems. Spacecraft generate substantial internal heat from electronics, crew metabolism, and solar heating on sun-facing surfaces. On Earth, heat dissipates through convection and conduction to the surrounding atmosphere, but in the vacuum of space, the only mechanism for rejecting heat is radiation. The ISS uses a two-loop thermal control system where ammonia circulates through external radiator panels, rejecting heat to space by infrared radiation. These radiator panels span roughly 1,600 square meters and are among the most visible features of the station.

The internal thermal control system uses water loops to collect heat from equipment racks and crew areas, transferring it through heat exchangers to the external ammonia loops. Each module has its own temperature control capability, and crew members can adjust conditions to maintain comfort. The system must also prevent components from getting too cold, since parts of the station in shadow can drop to temperatures below minus 150 degrees Celsius. Heaters maintain critical equipment within operational temperature ranges during orbital night, which occurs roughly every 90 minutes as the station orbits Earth.