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

Every human body needs roughly 0.9 kilograms of oxygen every day — a need that doesn’t pause just because you’re traveling through the vacuum of space. For the first decades of human spaceflight, this problem was solved quite simply: pack enough pressurized oxygen tanks to last the trip. On Apollo 11, that meant about 50 kilograms of stored oxygen for eight days in space (manageable, if not elegant). But as NASA and its international partners began planning for space stations where crews would live and work for months at a stretch, the math became impossible. Launching thousands of kilograms of oxygen on every resupply flight, at enormous cost and risk, was not a sustainable strategy. The answer was water electrolysis: splitting water molecules into their hydrogen and oxygen components using electricity, producing breathable air directly onboard.

The chemistry behind electrolysis is elegantly simple. Water (H₂O) is fed into a cell containing two electrodes — an anode (positive) and a cathode (negative) — and an electrolyte that allows ions to flow. When a potential difference is applied, water molecules are broken apart: oxygen gas (O₂) forms at the anode, while hydrogen gas (H₂) forms at the cathode. The overall reaction is 2H₂O → 2H₂ + O₂. NASA’s Oxygen Generation Assembly (OGA) on the International Space Station uses a proton exchange membrane (PEM) cell, in which a thin polymer membrane serves as the electrolyte and physically separates the two gas products. This design allows high current densities and produces very pure oxygen which is critical in the tightly controlled atmosphere of a crewed spacecraft.

The oxygen and water management hardware are just parts of the ISS’s broader Environmental Control and Life Support System (ECLSS), one of the most sophisticated engineering achievements in human spaceflight. ECLSS tracks and regulates cabin air pressure, temperature, humidity, carbon dioxide levels, and trace contaminants, all while recovering and recycling water from every possible source: exhaled breath, perspiration, wash water, and even urine. The water recovery system purifies this reclaimed moisture into potable water that is then fed back into the electrolyzer. This closed-loop approach means that the ISS now recycles roughly 98% of all moisture onboard, critically reducing the amount of water that must be shipped from Earth.

The hardest challenge in building an electrolyzer for space is one that almost never comes up in a terrestrial chemistry class: what do you do with the bubbles? On Earth, the answer is trivial: oxygen and hydrogen bubbles are less dense than water, so they float to the surface under gravity and escape. In orbit, where everything aboard a spacecraft is in continuous free fall, there is effectively no buoyancy. Gas bubbles generated at the electrodes have no preferred direction to travel. Instead, they cling to the electrode surfaces, blanketing the active area and blocking the electrochemical reaction. Microgravity experiments have confirmed that this effect is severe enough to significantly reduce the efficiency of an electrolyzer, increasing the power needed to maintain a given rate of oxygen production. The OGA addresses this with a rotating separator: water is pumped through the cell, sweeping bubbles away from the electrodes, and the resulting gas-liquid mixture is then spun at high speed in a centrifuge-like device (think of a washing machine) to force the liquid outward and collect the gas in the center. It works, but it relies on pumps, motors, seals, and rotating assemblies that are inherently complex, subject to degradation, and difficult to repair in orbit.

This is where the Lorentz force enters the picture. Named after the Dutch physicist Hendrik Lorentz, this force acts on any electrically charged particle moving through a magnetic field, pushing it perpendicular to both its direction of motion and the magnetic field lines. The relationship is captured in the equation F = J × B, where J is the electric current density flowing through the fluid, and B is the applied magnetic flux density. The resulting force, commonly known as the magnetohydrodynamic (MHD) force, can set a conducting fluid in motion without any pumps, impellers, rotating seals, or moving parts of any kind. This principle underlies technologies from MHD pumps used in liquid metal cooling loops in experimental nuclear reactors to the fictional silent submarine drive in The Hunt for Red October. In 1992, Japan even built a real MHD-propelled vessel, the Yamato-1, though its top speed of 8 knots revealed just how much engineering remained before the technology could challenge a conventional propeller.

The Magnetohydrodynamic Oxygen Generation Assembly, or MOGA, applies this same principle inside an alkaline electrolyzer in microgravity. Permanent magnets surrounding a cylindrical cell generate a radial magnetic field; the current flowing vertically between two disc-shaped electrodes then experiences a magnetohydrodynamic force in the circumferential direction, driving the electrolyte into a swirling vortical flow. This centrifugal-like spinning pushes the denser liquid to the outer wall of the cell and draws the lighter gas bubbles toward the central axis, where they are collected and routed to the cabin or to a hydrogen management system. This is done without any moving components! A NASA Innovative Advanced Concepts (NIAC) Phase I feasibility study found that MOGA could reduce total system mass (including spare parts) by up to 33% and cut astronaut maintenance time by more than 20% compared to the current ISS OGA architecture, significant gains for a mission where every kilogram launched and every crew-hour spent on repairs carries an enormous cost.

The stakes become clearest when you consider the Mars mission scenario. A 375-day crewed transit to Mars and back would require more than 1,300 kilograms of oxygen for a crew of four. Systems analyses have shown that the ISS OGA, with the spare components needed to achieve 99% reliability, would likely weigh more than all of the oxygen it produces. This is the engineering motivation for MOGA: by eliminating the moving-part architecture and critically reducing the number of components that can fail, a magnetohydrodynamic electrolyzer could be the technology that makes truly long-duration human spaceflight viable.

For more on the engineering challenges of keeping humans alive in deep space, explore NASA’s Human Research Program and the agency’s ongoing work on life support.

Magnetism and electricity are deeply intertwined, and the Lorentz force is just one expression of that connection. Khan Academy’s unit on magnetic forces is an excellent starting point for foundational physics, while The Action Lab offers a pretty cool MHD drive demo on YouTube. For the chemistry of electrolysis, the U.S. Department of Energy’s hydrogen production overview explains how the same electrochemical process used on the ISS is also a leading candidate for clean hydrogen fuel production here on Earth. And if you want to understand just how hostile and strange the microgravity environment is for fluids and materials, NASA’s Physical Sciences Informatics has published decades of fascinating results from experiments aboard the ISS.