The International Space Station (ISS) functions as a permanent human habitat orbiting Earth, providing a unique environment for scientific research and long-duration spaceflight. Maintaining a continuous supply of breathable air is paramount for the astronauts. Unlike Earth, the ISS cannot simply draw from a vast atmosphere, necessitating complex, self-sustaining life support systems to ensure crew survival. These technologies recycle and regenerate vital resources, making long-term human presence in space possible.
Generating Oxygen Onboard
The primary method for producing oxygen on the International Space Station is electrolysis, which uses electricity to split water molecules. The Oxygen Generation System (OGS), a major component of the station’s Environmental Control and Life Support System (ECLSS), performs this task. Located within the U.S. segment, the Destiny laboratory module, the OGS breaks down water (H2O) into hydrogen (H2) and oxygen (O2) gases.
This process relies on an electric current passing through water, with potassium hydroxide added as an electrolyte to improve conductivity. The oxygen produced is circulated into the station’s cabin air for the crew to breathe. The hydrogen, a byproduct of this reaction, is either vented into space or directed to a Sabatier reactor, where it can be combined with carbon dioxide to generate more water and methane.
The water used for electrolysis comes from various sources, including recycled wastewater, condensate collected from the cabin air, and purified urine. Although the ISS recycles over 90% of its water, some is resupplied from Earth by cargo vehicles, ensuring continuous oxygen production. The electricity for the OGS is primarily supplied by the station’s solar panels.
Removing Carbon Dioxide
Removing carbon dioxide (CO2) is as important as oxygen generation for maintaining a breathable atmosphere on the ISS. Human respiration continuously produces CO2, and its accumulation can lead to health issues such as headaches, nausea, and impaired cognitive function. Efficient CO2 removal systems are integral to the station’s life support.
The U.S. segment utilizes the Carbon Dioxide Removal Assembly (CDRA), located in the Destiny Laboratory module and Node 3. The CDRA operates using adsorbent beds, made of zeolite, to capture CO2 from the cabin air. This regenerative system involves two alternating beds: one absorbs CO2 while the other regenerates by venting the captured CO2 into space through a vacuum.
The Russian segment employs the Vozdukh system, situated in the Zvezda Service Module. Both CDRA and Vozdukh use a desiccant bed to remove moisture from the air before it enters the CO2 adsorbent bed, as dry air improves CO2 removal efficiency. The Vozdukh system can scrub air for a six-person crew and uses less power than the CDRA. These systems work in tandem, especially with more than three crew members, to keep CO2 levels below 0.5%.
Backup and Auxiliary Oxygen Sources
Beyond the primary electrolysis method, the ISS has several backup oxygen sources to ensure crew safety and atmospheric stability. One method involves Solid Fuel Oxygen Generators (SFOGs), also known as “oxygen candles” or Vika. These canisters contain a chemical mixture, primarily lithium perchlorate, which releases oxygen when ignited. Each SFOG cartridge provides oxygen for one crew member for approximately 24 hours, used for short-term needs or emergencies.
Pressurized oxygen tanks serve as a supplementary source. These tanks are delivered to the station by resupply vehicles, such as the Russian Progress and American Cygnus cargo spacecraft. The oxygen from these tanks replenishes the atmosphere as needed, providing a rapid infusion to maintain pressure and composition.
Maintaining the overall atmospheric pressure and composition requires more than just oxygen. Nitrogen, which makes up about 78% of Earth’s atmosphere, prevents oxygen toxicity and fire hazards. Nitrogen is supplied to the ISS in high-pressure tanks, delivered by resupply missions or through dedicated Nitrogen Oxygen Resupply System (NORS) tanks. These nitrogen reserves account for atmospheric losses due to leakage and operational needs, ensuring a stable, Earth-like environment.
Maintaining the Internal Atmosphere
Beyond the direct generation and removal of gases, the overall management of the ISS’s internal atmosphere encompasses several other systems. Air circulation is continuously maintained throughout the modules by intermodule ventilation systems, which prevent the buildup of contaminants and ensure a consistent atmosphere. This constant movement of air is essential for crew comfort and the efficient operation of life support systems.
Humidity control is another important aspect, as human activities and certain experiments produce significant moisture. Systems like the Condensate Water Recovery System collect this moisture, which is then processed to produce potable water, conserving the station’s limited water supply. The removal of trace contaminants, such as volatile organic compounds and ammonia, prevents long-term health effects on the crew. The Trace Contaminant Control Subassembly (TCCS) uses activated charcoal filters and catalytic oxidizers to purify the air.
The ISS environment is monitored by various sensors and analyzers. The Major Constituent Analyzer (MCA) continuously tracks the levels of oxygen, nitrogen, carbon dioxide, and other gases, providing real-time data on atmospheric composition. Temperature and pressure are also closely regulated to maintain Earth-like conditions, enhancing crew comfort and ensuring equipment functions correctly. This integrated approach to environmental control and life support allows astronauts to live and work safely for extended periods in the unique microgravity environment.