A trip to Mars will be difficult to say the least. Although human spaceflight has become a regular occurrence in near-Earth space in recent decades, leaving our gravitational pull requires a lot of rocket power. And then leaving a planet like Mars to return to Earth would also take a long time.
But NASA, other spaceflight agencies and private companies have set their sights on sending humans to Mars and returning them safely to Earth. So engineers and scientists are working to figure out how to make enough fuel to make such a trip possible.
Oxygen, a key component of rocket propulsion, is hard to find on the Red Planet. But results from a prototype machine on Mars suggest the element could be extracted from the air — hinting at future production to power rocket launches, but not yet enough for humans to breathe Martian air directly.
“It’s really difficult, if not impossible, to design a human mission to Mars that doesn’t use in situ resources,” said Carol Stocker, a planetary scientist at NASA Ames Research Center who was not involved in the project, using the science term for “in place.”
Now, a lunchbox-sized device attached to the Perseverance rover has opened the door to producing fuel from resources found on Mars. The Mars Oxygen Resource Exploitation Experiment (MOXIE) has successfully produced oxygen on the Red Planet.
From the time Perseverance landed in February 2021 until the end of that year, MOXIE produced about 50 grams of oxygen in seven launches, according to a report published Wednesday in the journal Scientific progress. MOXIE continued to conduct experiments under various conditions in 2022, said MOXIE Deputy Principal Investigator Jeffrey Hoffman, a professor of aeronautics and astronautics at MIT.
The device can produce 6 to 10 grams per hour, depending on atmospheric conditions. It found this maximum production rate in late August, Hoffman says, when the Martian atmosphere was at its thickest.
The goal of MOXIE, Hoffman says, “is to test whether the process actually works on Mars. And that, I would say, we’re about to do.”
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MOXIE uses the molecules that make up the Martian atmosphere to create oxygen. But it’s not just extraction. Mars’ atmosphere is 95 percent carbon dioxide (Earth’s atmosphere is mostly nitrogen with a lot of oxygen). MOXIE must split the CO2 molecules into carbon monoxide and oxygen.
First, MOXIE draws air through a HEPA filter that keeps Martian dust out of the process. The Martian air is then passed through a compressor because, as Hoffman explains, it is not dense enough for the oxygen production process. The device compresses Martian air, greatly increasing its density: from 100 times thinner than Earth’s atmosphere to about half as thin.
The carbon dioxide is then heated to about 1500°F (800°C). Once it’s heated up, it’s time for the main event: going through the electrolysis unit, which uses electricity to drive a chemical reaction. There, the carbon dioxide collides with catalysts, such as nickel, which cause the CO2 molecule to dissociate into carbon monoxide (CO) and an oxygen ion. Electricity is then used to draw oxygen ions through a filter into another chamber where they combine into oxygen molecules. The result is pure oxygen that can be used for breathing or for rockets.
“The good thing about MOXIE is that on the oxygen side, all you need is the atmosphere,” says Hoffman. “So it doesn’t matter where you are, you can go anywhere you want and you have an atmosphere.”
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MOXIE has produced oxygen at night on Mars, during the day and in many seasons – even in winter. During the coldest months at the poles of Mars, the density of the atmosphere decreases because carbon dioxide is deposited on the polar surface as ice. That means there’s less CO2 available to MOXIE every six months, explains Margaret Landis, a research associate at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder. However, it produces about 6 grams per hour during those times when the atmosphere thins.
“MOXIE can operate at any time on Mars,” says Hoffman. “If we get a few more tests, we’ll try to run it at dawn and dusk, when conditions change rapidly, and we can show that MOXIE can adapt to those changing conditions.”
However, a rate of 6 to 10 grams per hour would not produce nearly enough oxygen to be useful for a human mission to Mars. The average person breathes a little less than 1 kilogram of oxygen each day, Hoffman says, and rockets are even hungrier for O2. It would take tens or even hundreds of tons of oxygen to propel a rocket capable of launching humans from the surface of Mars. But this oxygen can build up over time. A full-scale version of a MOXIE-like system would need to produce about 2 to 3 kilograms of oxygen per hour, Hoffman says, to have any chance of accumulating enough liquid oxygen to be used in a rocket launch system.
Engineers already have a prototype of such a larger device, he says. Because MOXIE had to hitchhike with the Perseverance rover, it was small, but a future mission could send a larger MOXIE-like device to Mars on its own. Hoffman says such a device could have more functions, such as perhaps the ability to turn the product carbon monoxide into something useful.
The ability to produce oxygen does not mean that Mars launch vehicles are ready to launch. Oxygen is only one part of the rocket launch equation, says NASA’s Stocker. It provides half of the combustion reaction—the oxidizer—but a rocket still needs other ingredients for fuel. But, she adds, oxygen can provide more than three-quarters of the mass needed to propel a rocket, and that greatly reduces the amount of stuff that needs to be carried to the Red Planet from Earth.
As MOXIE-like technology expands, Landis says it’s worth considering the environmental impact this process could have on Mars. “It’s something to think about because CO2 is a major component of the Martian atmosphere and plays a really important role in its seasonal cycle,” she says. “There’s still a lot to learn about the exact consequences of what happens if you start to change that balance between surface and atmospheric CO2” and the types of gases.
“Sometimes it feels like you’re living in a science fiction future,” Landis says. “It’s a testament to how much we’ve been able to do on Mars.”