Sometime around midday on April 20, 2021, a microwave-sized device on Mars finished its first hour of work and reported back to its operators at the Massachusetts Institute of Technology. It had produced 5.4 grams of breathable oxygen, enough for an astronaut to breathe for about ten minutes. The machine, called MOXIE, was the first device ever to manufacture air on the surface of another planet. By the time it shut down for the final time on August 7, 2023, it had run sixteen times and produced a cumulative 122 grams of oxygen — roughly what a small dog breathes in ten hours.
This is a very small amount of oxygen. It is also, depending on how you look at it, one of the most consequential 122 grams in the history of space exploration. The reason is that MOXIE was never meant to keep anyone alive. It was a proof of concept for a much larger machine that does not yet exist, and the question its operators were trying to answer was not whether a human could breathe its output, but whether the principle worked at all under Martian conditions. The principle worked. What follows from that is genuinely large, and considerably more uncertain than the popular framing of the story tends to suggest.
What MOXIE is and what it did
MOXIE, formally the Mars Oxygen In-Situ Resource Utilization Experiment, was a payload aboard NASA’s Perseverance rover, which landed on Mars on February 18, 2021. It was built by a team led by Michael Hecht at MIT’s Haystack Observatory, with Jeffrey Hoffman of MIT’s Department of Aeronautics and Astronautics as deputy principal investigator, in partnership with NASA’s Jet Propulsion Laboratory. The instrument was described in detail in a 2021 pre-flight paper by Michael Hecht, Jeffrey Hoffman, Donald Rapp and colleagues in Space Science Reviews. The device measured about 24 by 24 by 31 centimeters, weighed 15 kilograms on Earth, and drew roughly 300 watts during operation. NASA officially describes it as the size of a microwave oven, although the popular shorthand has sometimes called it toaster-sized. Its design specifications, set out in the 2021 paper, called for producing at least 6 grams of oxygen per hour at greater than 98 percent purity, sustained across at least ten operational cycles.
The principle MOXIE was designed to test is called in-situ resource utilization, or ISRU: the idea that future Mars missions could manufacture some of what they need from materials already present on the planet, rather than carrying everything from Earth. The Martian atmosphere is about 95 percent carbon dioxide, which is, by mass, mostly oxygen. The chemistry to separate the two is well understood on Earth. What had never been demonstrated was whether the process would work reliably on the surface of Mars, at the relevant atmospheric pressures and temperatures, with the kind of equipment that could survive launch, landing, and the operational environment of a rover.
The first peer-reviewed report on the results, Jeffrey Hoffman, Michael Hecht, Donald Rapp and colleagues’ 2022 paper in Science Advances, reported on the first seven runs through the end of 2021. MOXIE produced oxygen at six grams per hour, hitting its design target, across daytime and nighttime operations and through different Martian seasons. The instrument was then pushed harder in subsequent runs. According to NASA’s final-mission announcement, MOXIE eventually reached a peak production rate of 12 grams per hour at a purity of 98 percent or better, twice the original design specification.
How it worked
The process is called solid oxide electrolysis. MOXIE drew Martian atmosphere through a dust-trapping filter, compressed it with a scroll pump, and heated it to roughly 800 degrees Celsius. The heated gas was then passed through a ceramic cell containing a scandia-stabilized zirconia electrolyte, where an applied electric current split carbon dioxide molecules into carbon monoxide and oxygen ions. The oxygen ions passed through the ceramic and recombined as molecular oxygen on the other side, where the instrument measured the purity and quantity before releasing the gas back into the Martian atmosphere. The carbon monoxide byproduct was also released.
None of the chemistry involved is novel. Solid oxide electrolysis cells have been operated on Earth for decades. What MOXIE proved was that the process could be made compact enough, robust enough, and energy-efficient enough to operate on Mars without breaking, while drawing power from a small rover and surviving the cold, the dust, and the temperature swings of a full Martian year.
The gap between proof of concept and a Mars return mission
The popular framing of MOXIE’s success often skips lightly over the scale of what would be required for the technology to actually bring astronauts home from Mars. The numbers are worth being specific about.
Hecht, in interviews around the time of the first run in 2021, gave the following figures. Getting four astronauts off the Martian surface aboard a return vehicle would require approximately seven metric tons of methane rocket fuel and roughly 25 metric tons of liquid oxygen to burn it with. Sustaining the same four astronauts for a year on the surface would require approximately one additional metric ton of breathable oxygen. MOXIE, at its peak rate of 12 grams per hour, would need to run continuously for approximately 2 million hours, or more than 230 years, to produce the rocket-fuel oxygen alone. The breathing oxygen would take roughly another nine and a half years at the same rate.
The actual plan, were such a mission to proceed, would involve sending a substantially larger ISRU plant to Mars ahead of the crew. A 2023 paper by Hoffman, Eric Hinterman, Hecht, Rapp and Joseph Hartvigsen in Acta Astronautica, summarizing eighteen months of MOXIE operations alongside an optimization study for a human-scale system, sets out the engineering parameters for what the MOXIE team calls “Big MOXIE.” The study targets the oxygen requirements for a six-person Mars Ascent Vehicle and finds that a viable human-scale system would need to produce oxygen at roughly two to three kilograms per hour and would draw on the order of 25 to 30 kilowatts of continuous power, about two orders of magnitude more demanding than the prototype on Perseverance. The system would need to begin operating more than a year before the astronauts arrived, producing oxygen continuously throughout that period. No such system has yet been built, and no firm timeline exists for building one. What MOXIE established is that the chemistry works in the conditions it would have to work in. The engineering of a system at this scale, capable of operating unattended for more than a year on the Martian surface, is a separate problem, and a large one.
What this proves, narrowly
What MOXIE proves is narrow and important. It proves that solid oxide electrolysis works reliably on Mars, across the range of conditions a future ISRU plant would encounter. It proves that the technology degrades gracefully rather than catastrophically over a Martian year, with the operators observing only a small increase in the internal resistance of the cell across all sixteen runs. It proves that the engineering challenges of operating sensitive electrochemistry inside a rover, with limited power and through extreme thermal cycling, are tractable. And it proves the broader principle that an interplanetary mission can manufacture a critical consumable from local materials rather than carrying it from Earth, which is the foundational assumption of every serious proposal for sustained human presence on Mars.
It does not prove that astronauts will reach Mars, or return from it. It does not prove that a full-scale ISRU plant can be built, launched, landed, deployed, and operated reliably for the year-plus that would be required before a crew arrived. It does not solve the much larger problems of radiation exposure during transit, of long-duration life support, of the political and budgetary commitments that a crewed Mars mission would require, or of the medical and psychological feasibility of keeping humans alive on the planet for the length of stay a return-window mission would impose. The popular framing in which MOXIE has “opened the door” to a Mars return is true in the technical sense that one specific technical obstacle, previously theoretical, has now been demonstrated in operational conditions. The door is large. MOXIE has unlocked one of the locks on it.
What is worth taking from the result
For all the careful narrowing above, the result is still genuinely significant, and worth saying so plainly. Until April 20, 2021, no human-made device had ever produced a useful consumable from materials found on another planet. The principle that interplanetary missions could partly support themselves, rather than carrying everything from Earth, had been a design assumption in mission proposals for decades and a demonstrated capability in zero cases. It is now a demonstrated capability in one. The case for in-situ resource utilization as a serious component of future Mars planning is much stronger after MOXIE than it was before, because the alternative scenario, in which the underlying chemistry would have proven uncooperative under Martian conditions, has been ruled out.
The 122 grams of oxygen MOXIE produced over its lifetime will not bring anyone home. The technology it tested, scaled up by two orders of magnitude and operated continuously for years, plausibly could, if the larger system is built and the larger mission is funded. Both of those remain open questions. What is no longer an open question is whether the underlying chemistry works on Mars. It does, and the first proof of that was 5.4 grams of breathable oxygen, produced in the spring of 2021, by a microwave-sized device on a rover most of the people on Earth will never see.
