The International Space Station (ISS)-a habitable artificial satellite orbiting Earth at an altitude of ~400 kilometers-relies on a sophisticated, closed-loop oxygen system to sustain its crew of 7 astronauts (maximum capacity) for months at a time. Unlike Earth, where oxygen is abundant in the atmosphere, space is a vacuum with no natural oxygen source. This means the ISS must produce, store, distribute, and recycle oxygen entirely on-board, while also managing waste gases like carbon dioxide (CO₂). The system's design prioritizes reliability (to avoid life-threatening failures), efficiency (to minimize resupply missions), and adaptability (to handle crew size changes and equipment malfunctions). Below is a comprehensive breakdown of the ISS oxygen system, including its core components, working principles, challenges, and backup protocols.
1. Sustaining a Habitable Atmosphere
Before delving into technical details, it's critical to understand the ISS oxygen system's primary objective: maintaining an atmosphere that mimics Earth's as closely as possible. For human survival, the ISS requires:
Oxygen Concentration: 21% (the same as Earth's atmosphere), which is the optimal level for respiration and avoiding hypoxia (low oxygen) or oxygen toxicity (high oxygen).
Pressure: 101.3 kilopascals (kPa) or 1 atmosphere (atm)-equivalent to sea-level pressure on Earth. This prevents decompression sickness (a risk when pressure drops too low) and allows astronauts to breathe normally without specialized equipment (except during spacewalks).
Gas Scrubbing: Removal of waste gases like CO₂ (produced by respiration) and trace contaminants (e.g., volatile organic compounds from equipment or food).
To achieve this, the ISS oxygen system operates as a semi-closed loop-it produces new oxygen, recycles oxygen from waste streams, stores excess oxygen for emergencies, and distributes it evenly throughout the station's modules.
2. The Oxygen Generation System (OGS)
The ISS's main source of oxygen is the Oxygen Generation System (OGS), a modular setup developed by NASA and Russia's Roscosmos (with contributions from the European Space Agency, ESA, and Japan Aerospace Exploration Agency, JAXA). The OGS uses electrolysis-the same chemical process used in some Earth-based oxygen generators-to split water (H₂O) into oxygen (O₂) and hydrogen (H₂). Here's a detailed breakdown of its components and operation:
2.1 Components of the OGS
The OGS consists of three key subsystems, each with specialized hardware:
Water Processing Assembly (WPA): Before electrolysis, water must be purified to remove contaminants (e.g., salts, organic matter) that could damage the OGS's electrodes. The WPA collects water from three sources:
Recycled Water: Condensate from the station's air (water vapor from respiration and sweating), treated wastewater (e.g., from sinks, showers), and urine (processed by the Urine Processing Assembly, UPA).
Resupply Water: Water delivered via cargo spacecraft (e.g., SpaceX's Dragon, Northrop Grumman's Cygnus) as a backup for when recycling systems fail.
Fuel Cell Water: A byproduct of the station's former fuel cells (used to generate electricity before the installation of solar arrays). While fuel cells are no longer primary power sources, their residual water is still used if available.
Electrolysis Module (EM): The heart of the OGS, the EM contains two Solid Oxide Electrolysis Cells (SOECs)-advanced devices that use high temperatures (600–800°C) to split water into oxygen and hydrogen. Unlike traditional electrolysis systems (which use liquid electrolytes), SOECs use a solid ceramic electrolyte that is more efficient, compact, and durable in space. Here's how the process works:
Purified water is fed into the SOECs as steam (vaporized to increase efficiency).
An electric current (from the ISS's solar arrays) is applied to the SOECs' electrodes (anode and cathode).
At the anode, steam reacts with the ceramic electrolyte to produce oxygen gas (O₂), electrons, and hydrogen ions (H⁺).
Electrons flow through an external circuit (generating a small amount of additional electricity), while hydrogen ions move through the electrolyte to the cathode.
At the cathode, hydrogen ions combine with electrons to form hydrogen gas (H₂).
Oxygen Handling Subsystem (OHS): After production, oxygen from the EM is processed and distributed:
Cooling: The hot oxygen gas (from the SOECs) is cooled to room temperature using heat exchangers (connected to the ISS's thermal control system).
Drying: Any remaining water vapor is removed using molecular sieves (similar to those in Earth-based oxygen concentrators) to prevent condensation in the station's pipes.
Distribution: The dry, pure oxygen (99.999% purity) is sent to the ISS's atmosphere via a network of valves and pipes, mixing with the existing air to maintain the 21% concentration.
Hydrogen Venting: The hydrogen byproduct is not used by the ISS (since the station runs on solar power, not hydrogen fuel cells) and is vented into space. This is a key difference from early space stations like the Mir, which used hydrogen to generate electricity.
2.2 Efficiency and Capacity of the OGS
The OGS is designed to meet the ISS's daily oxygen demand, which is ~0.84 kilograms (kg) per astronaut (equivalent to ~588 liters of gaseous oxygen at 1 atm). For a crew of 7, this totals ~5.88 kg of oxygen per day. The OGS's key performance metrics include:
Production Rate: Each SOEC can produce ~0.5 kg of oxygen per day, so the two SOECs together generate ~1 kg per day. However, the system is operated in a staggered mode (one SOEC active, one on standby) to reduce wear, resulting in a net production of ~0.5 kg per day. This means the OGS alone cannot meet the full crew's demand-hence the need for additional oxygen sources (see Section 3).
Energy Efficiency: SOECs are highly efficient, converting ~80% of the electrical energy into oxygen (compared to ~60% for traditional electrolysis systems). This is critical because the ISS's solar arrays have limited capacity (~120 kilowatts, kW, of power for all systems).
Reliability: The OGS has a design lifespan of 15 years (extended from the original 10 years) and includes redundant components (e.g., backup SOECs, valves) to prevent failures. Since its installation in 2008 (as part of the ISS's Node 3 module, Tranquility), the OGS has only experienced minor issues (e.g., clogged water filters) that were resolved via remote troubleshooting.
3. Backup and Supplemental Systems
While the OGS is the primary oxygen source, the ISS relies on three secondary systems to ensure a continuous supply-critical for when the OGS malfunctions or during peak demand (e.g., when the crew size increases temporarily).
3.1 Pressurized Oxygen Tanks (Russian Segment)
The ISS's Russian Segment (RS)-which includes modules like Zvezda (Service Module) and Nauka (Multipurpose Laboratory Module)-uses pressurized oxygen tanks as a backup. These tanks are:
Design: Cylindrical tanks made of titanium alloy (to withstand high pressure and space radiation) with a capacity of ~40 liters each. They store oxygen as a high-pressure gas (3,000 psi, or 20.7 MPa)-the same type used in Earth-based scuba tanks but modified for space.
Supply: Tanks are delivered to the ISS via Russian cargo spacecraft (e.g., Progress) and attached to the RS's external ports. Each Progress mission carries 2–3 tanks, providing ~100–150 kg of oxygen per mission (enough to support a crew of 7 for ~20–25 days).
Deployment: When the OGS fails, the RS's life support system opens valves to release oxygen from the tanks into the station's atmosphere. The tanks are also used during spacewalks (EVA, Extravehicular Activity) to supply oxygen to astronauts' spacesuits.
3.2 Oxygen Candles (Chemical Oxygen Generators)
For emergency situations (e.g., a major OGS failure combined with a delay in cargo resupply), the ISS uses oxygen candles-compact, chemical-based generators that produce oxygen via a thermal reaction. These candles are:
Composition: Each candle is a solid block of sodium chlorate (NaClO₃) mixed with a catalyst (e.g., iron powder) and a fuel (e.g., aluminum). When ignited, the sodium chlorate decomposes at high temperatures (500–600°C) to produce oxygen gas and sodium chloride (table salt).
Capacity: A single candle (weighing ~1 kg) produces ~60 liters of oxygen (enough for one astronaut for ~10 hours). The ISS carries ~100 candles, stored in fireproof containers in each module (e.g., Zarya, Unity) for easy access.
Safety: Oxygen candles are designed to be safe in space-they do not produce open flames (only heat) and the sodium chloride byproduct is non-toxic (it is collected in a filter and later removed during cargo missions). However, they are only used as a last resort due to their limited capacity and the need for manual activation.
3.3 Regenerative Life Support: Recycling Oxygen from CO₂
The ISS's Environmental Control and Life Support System (ECLSS) includes a regenerative component that recycles oxygen from CO₂-reducing the need for new oxygen production. This is done via the Carbon Dioxide Removal Assembly (CDRA) (U.S. Segment) and the Vozdukh System (Russian Segment):
CDRA (U.S. Segment): Uses a two-step process called solid amine water desorption to remove CO₂ and produce oxygen:
CO₂ Adsorption: Air from the ISS is pumped through a bed of solid amine (a chemical compound that binds to CO₂). The amine traps CO₂, while clean air (without CO₂) is returned to the station.
Desorption and Oxygen Production: When the amine bed is saturated, it is heated to release the trapped CO₂. The CO₂ is then reacted with hydrogen (from the OGS's electrolysis process) in a Sabatier reactor (another ECLSS component) to produce water (H₂O) and methane (CH₄). The water is then sent to the OGS to be split into oxygen and hydrogen, creating a closed loop.
Vozdukh System (Russian Segment): Uses a similar process but with a different chemical (lithium hydroxide, LiOH) to absorb CO₂. Unlike the CDRA, the Vozdukh System does not recycle CO₂ into oxygen-instead, the LiOH is discarded after it becomes saturated (it is replaced via cargo missions). However, it is simpler and more reliable than the CDRA, making it a valuable backup.
The regenerative system reduces the ISS's oxygen demand by ~40%-a critical efficiency gain that minimizes the need for resupply missions. For example, without recycling, the station would need ~9.8 kg of oxygen per day for 7 astronauts; with recycling, this drops to ~5.88 kg.
4. Ensuring Resilience for Emergencies
In addition to the secondary sources, the ISS has dedicated oxygen storage systems to handle peak demand and emergencies. These systems are designed to store oxygen in two forms: high-pressure gas and liquid.
4.1 High-Pressure Gas Storage (U.S. Segment)
The U.S. Segment's High-Pressure Gas Tanks are located in the Node 1 (Unity) and Node 3 (Tranquility) modules. These tanks:
Design: Spherical tanks made of Inconel (a nickel-chromium alloy resistant to corrosion and high temperatures) with a capacity of ~150 liters each. They store oxygen at 6,000 psi (41.4 MPa)-twice the pressure of the Russian Segment's tanks-allowing for more oxygen to be stored in a smaller space.
Capacity: Each tank holds ~100 kg of oxygen (enough for 7 astronauts for ~17 days). The U.S. Segment has 4 such tanks, providing a total backup of ~400 kg (enough for ~68 days).
Use Case: These tanks are used to supplement the OGS during peak demand (e.g., when two astronauts are on a spacewalk, increasing oxygen consumption by ~50%) and as a backup if the OGS fails. They are also used to repressurize the station after a spacewalk (since some air is lost during EVA).
4.2 Liquid Oxygen (LOX) Storage (Emergency Only)
For long-term emergencies (e.g., a months-long OGS failure), the ISS can store liquid oxygen (LOX)-the same form used in rocket fuel. LOX is stored in:
Design: Double-walled tanks with a vacuum insulation layer to keep the LOX at -183°C (its boiling point at 1 atm). The tanks are small (~50 liters each) due to the limited space on the station.
Capacity: A 50-liter LOX tank holds ~60 kg of oxygen (since LOX has a density of 1.141 kg/L), enough for 7 astronauts for ~10 days. The ISS has 2 such tanks, providing a total of ~120 kg (enough for ~20 days).
Challenges: Storing LOX in space is difficult because the station's temperature fluctuates (from -120°C in shadow to 120°C in sunlight), causing some LOX to boil off (vaporize). To minimize boil-off, the tanks are equipped with heaters that regulate temperature and a pressure relief valve that vents excess gas (which is then captured and used in the station's atmosphere).
5. Ensuring Uniform Supply Across Modules
The ISS is a complex network of 16 modules (as of 2024), including living quarters (e.g., Crew Quarters), laboratories (e.g., Columbus, Kibo), and service modules (e.g., Zvezda, Nauka). To ensure every module has a consistent 21% oxygen concentration, the station uses a centralized distribution system with the following components:
5.1 Air Circulation Fans
Each module has 4–6 air circulation fans that move air at a rate of ~1 cubic meter per minute. These fans:
Prevent stagnant air pockets (which could lead to low oxygen levels in corners of the module).
Mix the newly produced oxygen with the existing air to maintain the 21% concentration.
Push air through the CDRA/Vozdukh systems to remove CO₂ and contaminants.
The fans are critical because, in microgravity (weightlessness), air does not circulate naturally (as it does on Earth due to convection). Without fans, astronauts could experience hypoxia in areas far from the oxygen source.
5.2 Valves and Pipes
A network of stainless steel pipes (2–4 inches in diameter) connects the OGS, storage tanks, and modules. Each pipe is equipped with:
Solenoid Valves: Electrically controlled valves that open and close to regulate oxygen flow. These valves are redundant (each pipe has two valves) to prevent leaks.
Pressure Sensors: Monitor the pressure in the pipes to ensure it matches the station's atmospheric pressure (101.3 kPa). If pressure drops (e.g., due to a leak), the sensors trigger an alarm and close the affected valves.
Filters: Remove dust and debris from the oxygen to prevent damage to the fans and life support systems.
5.3 Module-Specific Regulators
Each module has a pressure regulator that adjusts the oxygen flow into the module based on its size and occupancy. For example:
Small modules (e.g., the Crew Quarters, which are ~10 cubic meters) require a lower flow rate (~0.1 kg of oxygen per day) than large modules (e.g., the Columbus Laboratory, which is ~75 cubic meters, requiring ~0.5 kg per day).
The regulators also ensure that the module's pressure remains at 101.3 kPa, even if other modules are being repressurized (e.g., after a spacewalk).
