How does the space station never run out of oxygen? - Alvaro Romero-Calvo and Theo St Francis

Scientific concepts, discoveries, and nature/space phenomena

Life support requirements in space

To sustain crew health, spacecraft must:

• Supply oxygen
• Remove exhaled carbon dioxide (CO₂) before it accumulates

Oxygen demand: astronauts need on average ~0.8 kg O₂ per person per day.

Early oxygen strategies: onboard storage + chemical CO₂ scrubbing

• Pressurized oxygen tanks were used on early missions.
• Example: Apollo 11 (8 days) carried about ~50 kg of oxygen for 3 astronauts.
• Lithium hydroxide canisters chemically react with CO₂ to scrub CO₂ from cabin air.

ISS shift to long-duration habitation

The ISS enabled missions lasting months rather than days. Long expeditions (e.g., 6–8 months with ~7 crew) require over a thousand kilograms of oxygen, making frequent resupply impractical.

Electrolysis-based in-orbit oxygen generation (ISS method)

The ISS uses recovered water as feedstock.

Water recovery system

Moisture is captured and purified from:

• sweat
• exhalation
• wash water
• urine

This produces potable/fresh water.

Electrolysis process

An electrolyzer uses two electrodes (positive and negative). Applying electric current splits water (H₂O) into:

• oxygen (O₂) near the positive electrode
• hydrogen (H₂) bubbles near the negative electrode

Microgravity challenge: bubble handling

• On Earth, bubbles rise due to gravity.
• In microgravity/freefall, bubbles cling to electrodes, reducing gas separation efficiency.

Gas separation workaround on ISS

• A system pumps water through the electrolyzer to carry bubbles away.
• A separator uses centrifugal spinning (“washing-machine” style):
  • liquid is forced outward
  • gas collects toward the center

Hydrogen management

Hydrogen is either:

• vented to space, or
• sent to a reactor where it combines with captured exhaled CO₂ to form methane (CH₄) and water (H₂O).

Limitations for future missions

The ISS approach depends on complex moving parts (pumps/separators), which can fail or require frequent maintenance. This becomes an increasing problem for longer, deeper-space missions (e.g., Mars-class durations).

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New concept: Magnetohydrodynamic Oxygen Generation Assembly (MOGA)

A proposed alternative gas separation method is the Magnetohydrodynamic (MHD) swirling of a gas-liquid mixture inside the electrolyzer.

Physics principle: Lorentz force

• Lorentz force arises when charged/negatively charged species move perpendicularly through a magnetic field.
• The force acts at a right angle to both the electric and magnetic fields, effectively swirling the liquid.

Goal

Separate O₂ and H₂ without requiring:

• pumps
• mechanical separators

Claimed benefit

• No moving parts → lower maintenance
• fewer replacements
• more reliability and available onboard space

Status

• Still in research, but promising for future space stations and deep-space missions.

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Researchers or sources featured

• Alvaro Romero-Calvo
• Theo St Francis