Oxygen makes up 21% of the atmosphere. In the vacuum of space there is no atmosphere. Oxygen is created by electrolysis of water, solid fuel oxygen generators and recycling carbon dioxide (CO2).
Electrolysis of Water
In electrolysis an electric current is passed through water (H2O), separating it into its constituent elements: hydrogen (H2) and oxygen (O2). The oxygen is then vented into the spacecraft's atmosphere.
The hydrogen, a highly flammable gas, may be vented overboard. Sometimes it's used to generate more water through chemical reaction with waste carbon dioxide.
On the International Space Station (ISS), water is stored in tanks. Through electrolysis, a kilogram of water can yield about 0.89 kilograms of oxygen. The oxygen is redirected into the cabin for astronauts to breathe.
Solid Fuel Oxygen Generators (SFOGs)
Usually emergency backup systems, like oxygen masks on airplanes. SFOGs use a chemical reaction with a solid compound like lithium perchlorate (LiClO₄), to release oxygen upon heating.
Effective for short-term needs, they are less sustainable than electrolysis as they deplete the solid fuel.
Recycling Carbon Dioxide (CO2 Reduction)
This is a component of a closed-loop life support system. Astronauts exhale carbon dioxide, which can quickly build up to toxic levels inside a spacecraft.
CO₂ reduction systems convert exhaled CO₂ back into usable oxygen. The Sabatier reaction, widely used on the ISS, combines hydrogen (obtained through electrolysis) with CO₂ to produce methane (CH₄) and water.
Water can then be re-split into oxygen and hydrogen through electrolysis, closing the loop.
Chemical Oxygen Generators (COGs)
These also ensure astronauts have enough breathable air. A well-known example uses potassium superoxide (KO₂). When KO₂ reacts with carbon dioxide, it generates oxygen.
When astronauts exhale, they increase CO2 levels in their environment. The COGs replenish oxygen and remove excess CO2. Such systems are necessary in confined spaces like spacecraft.
Astronauts: Breathing in a Carefully Crafted Atmosphere
As the oxygen creates a breathable environment for the astronauts, composition and pressure of the atmosphere must be carefully controlled
Atmospheric Composition: Spacecraft use a higher percentage of oxygen than occurs in Earth's atmosphere. A space vehicle often has lower atmospheric pressure to minimize stress on the hull.
A higher oxygen percentage ensures astronauts get enough oxygen despite the lower pressure. Some spacecraft use a mix of around 30% oxygen and 70% nitrogen.
Atmospheric Pressure: The atmosphere inside a spacecraft is pressurized to a level lower than Earth's sea level, but sufficient to support human life. It reduces stress on the spacecraft's structure and simplifies spacewalks.
The ISS maintains an atmospheric pressure close to Earth's sea level, which makes it more comfortable for long-duration stays. Recycling is essential for maintaining breathable air on the ISS.
Advanced systems continuously monitor and adjust air quality to ensure optimal oxygen levels are maintained. Excess CO2 can accumulate quickly due to human respiration, so carbon dioxide scrubbers are used.
The scrubbers use various reactions to convert CO2 back to oxygen, ensuring air quality is preserved during long missions. Recycling systems reduce the need for regular resupply missions.
During extravehicular activities (EVAs), spacesuits are equipped with life support systems to supply oxygen, regulate temperature and remove carbon dioxide and moisture.
Each suit has a backpack with tanks delivering a steady flow of air. A fan disperses it through the suit, run by electricity generated in the backpack. In a typical EVA, astronauts need up to 3.3 liters of oxygen per hour.
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