The International Space Station (ISS) operates as a completely contained environment, sustaining human life without continuous replenishment from Earth’s atmosphere. The station is a closed system that must constantly regenerate its air, water, and other consumables. This necessity requires a sophisticated life support infrastructure capable of generating fresh oxygen and actively managing the accumulation of waste gases.
This breathable atmosphere is achieved through a complex, regenerative process designed to minimize dependence on costly resupply missions. The engineered ecosystem maintains a specific atmospheric pressure and composition to ensure astronaut health and the proper functioning of sensitive onboard equipment. For long-duration spaceflight, the life support system operates in a highly efficient, closed-loop cycle where every component is recovered and reused.
The Primary Method: Water Electrolysis
The primary method for generating breathable oxygen on the International Space Station is electrolysis, which uses electricity to split water molecules. Running a direct electrical current through water (\(\text{H}_2\text{O}\)) separates it into oxygen (\(\text{O}_2\)) and hydrogen (\(\text{H}_2\)). The oxygen is released directly into the cabin air for the crew to breathe.
The ISS employs two main electrolysis systems: the Russian Elektron system, located in the Zvezda Service Module, and the American Oxygen Generation System (OGS), situated in the Destiny Laboratory Module. Both systems perform the same chemical function, though they use different technologies. The OGS, for example, utilizes a Solid Polymer Electrolyte (SPE) cell stack for efficiency.
The OGS produces oxygen sufficient to support the metabolic needs of up to seven crew members, in addition to compensating for slow atmosphere leakage. Generating this oxygen requires a constant input of power, which the OGS draws from the station’s electrical bus.
The hydrogen (\(\text{H}_2\)) byproduct from the electrolysis process is highly flammable and is routed to other systems for further recycling. This dual production of oxygen and waste hydrogen is a deliberate part of the overall strategy to close the life support loop. The efficiency of electrolysis is tied directly to the availability of the station’s solar-generated electricity and a clean, continuous supply of water.
The Water Supply Chain: Recycling and Recovery
Electrolysis is entirely dependent on a steady input of water, which is a resource that is far too heavy and expensive to continuously resupply from Earth. Therefore, the International Space Station’s Environmental Control and Life Support System (ECLSS) focuses on an extensive water recovery and purification process. This system ensures that the same water used for drinking and hygiene can also be fed into the oxygen generators.
Water vapor in the cabin air, largely from crew respiration and perspiration, is collected by advanced dehumidifiers and is a primary source of reclaimed water. This condensate is channeled into the purification system alongside other sources of wastewater. The process is designed to be highly aggressive in its reclamation efforts, which includes recycling crew urine.
The Urine Processor Assembly (UPA) uses a vacuum distillation process to evaporate the water content from the urine, leaving behind a highly concentrated brine. This distilled vapor is then combined with the humidity condensate and purified in the Water Processor Assembly (WPA). The WPA uses a series of filters and a catalytic reactor to break down contaminants, with purity sensors checking the quality before iodine is added to prevent microbial growth.
Through this multi-stage recycling process, the ISS has demonstrated the ability to recover up to 98% of the water it uses. This highly purified water is then distributed throughout the station, serving as the source for crew consumption, hygiene, and the feed water for the Oxygen Generation System. This closed-loop water cycle allows the crew to maintain a continuous, self-sufficient oxygen supply without relying on external deliveries.
Backup and Emergency Oxygen Sources
While the water electrolysis systems provide the primary means of oxygen generation, the ISS maintains multiple redundant systems and stored reserves for times when the primary units are offline or in the event of an emergency. These backup sources are to ensure that the crew always has a breathable atmosphere, even during maintenance or a sudden pressure loss.
One key backup is the use of Solid Fuel Oxygen Generators (SFOGs), often referred to as “oxygen candles.” These chemical generators contain a compound, typically lithium perchlorate, that releases oxygen when ignited. SFOGs are a chemically stable, compact, and reliable method for quickly producing oxygen without requiring an external power source.
The station also utilizes stored, pressurized oxygen, which is delivered via uncrewed resupply vehicles like the Russian Progress spacecraft. These tanks provide bulk oxygen for routine atmosphere maintenance and can be used to replenish the high-pressure tanks stored in the Quest Airlock. The pressurized tanks serve as a readily available reserve for both cabin air revitalization and for recharging the life support systems of spacesuits used during Extra-Vehicular Activities (EVAs).
Maintaining the Atmosphere: Carbon Dioxide Removal
Generating oxygen is only one half of the atmospheric management challenge; the other is the removal of the crew’s metabolic waste product, carbon dioxide (\(\text{CO}_2\)). Without effective scrubbing, \(\text{CO}_2\) levels would quickly rise to dangerous concentrations, leading to health issues for the crew. The primary system for this function is the Carbon Dioxide Removal Assembly (CDRA).
The CDRA works by using specialized absorbent materials, called molecular sieves, to capture \(\text{CO}_2\) from the cabin air. The assembly operates in a cyclic manner, where one bed of material absorbs \(\text{CO}_2\) while the other is heated to release the captured gas into a vacuum vent line. This temperature-swing adsorption process effectively scrubs the air while regenerating the absorbent material for the next cycle.
The released \(\text{CO}_2\) can then be used in a chemical process to further close the life support loop, which is the function of the Sabatier system. This system combines the waste \(\text{CO}_2\) from the CDRA with the waste hydrogen (\(\text{H}_2\)) from the OGS, reacting them to produce methane (\(\text{CH}_4\)) and, critically, water (\(\text{H}_2\text{O}\)). This reclaimed water is then fed back into the WPA for purification and eventually back to the OGS.
The Sabatier system represents the ultimate goal of a fully regenerative life support architecture. By creating water from waste gases, the system reduces the need to launch water supplies and brings the ISS closer to a truly self-sustaining environment for future deep-space missions.