Putting boots on Mars isn’t easy, but it’s a lot easier than bringing them back.
This week, NASA launches its Perseverance rover on a one-way trip to the surface of Mars. Among many other tools, the craft carries an experimental instrument that could help astronauts in the future make roundtrip voyages to the planet. The Mars Oxygen In-Situ Resource Utilization Experiment, or MOXIE, is small, about the size of a car battery. It’s designed to demonstrate a technology that converts carbon dioxide into oxygen with a process called electrolysis. Mars’ thin atmosphere is 95 percent carbon dioxide, but sending anything back into space requires fuel, and burning that fuel requires oxygen. NASA could ship liquid oxygen to the planet, but the volume needed takes up a good deal of space.
MOXIE could show the way to a solution. If successful, a larger-scale version of MOXIE’s oxygen production technology could then be used to launch a rocket home. “NASA definitely doesn’t want to just leave people on Mars,” says Asad Aboobaker, an engineer at NASA’s Jet Propulsion Laboratory.
Michael Hecht, an associate director at MIT in charge of MOXIE, says that since the 1990s, discussions about Mars exploration always came back to a list of four open questions. Two revolved around proving Mars is safe enough for human missions, since the planet’s bare atmosphere menaces anything on its surface with dust storms and radiation. Those questions have been resolved. The 2001 Mars Odyssey measured radiation on and around the planet, informing how shielding for astronauts would have to be constructed. After 2007, the Phoenix lander profiled Mars’ dust and soil chemistry, and found nutrients that could support Earth’s plants. Another question asked how large vehicles could land on the planet. That concern has been tackled as four NASA rovers landed safely on the planet between 1996 and 2011. The fourth question, still unanswered, wondered how we might bring all necessary resources onto Mars.
The biggest resource left outstanding, according to Hecht, is a propellant needed for a flight home. “All you have to do to convince yourself of that is to look at a rocket as it launches from Earth,” says Hecht. “There’s an enormous oxygen tank to lift up a little tiny capsule on top.”
MOXIE is just one tool on the $2 billion Perseverance rover; the craft also includes a Mars helicopter and SHERLOC, an ultraviolet laser that will scan for signs of ancient life. Despite MOXIE’s gold-plating, it looks humble for its ambitious mission. The experiment’s casing is a roughly cubic box, measuring about a foot in every dimension.
Within that tidy casing is a compressor, a filter and a rigorously tested ceramic stack which carries out the electrolysis reaction. Each element must perform remotely, precisely, and in conditions that choke up moving parts and wear down precious hardware.
The atmosphere on Mars is 170 times thinner than on Earth. Even though that air is so rich in carbon dioxide, the low pressure means that ambient amounts of air floating into MOXIE’s reactive core won’t produce much oxygen. MOXIE’s compressor, however, sucks in nearby carbon dioxide and feeds it to the electrolysis unit at an Earth-like pressure. There, a chemical catalyst operating at 800 degrees Celsius rips away an oxygen atom from each incoming CO2. Pairs of oxygen atoms rapidly combine to form the stable diatomic oxygen, which exits with carbon monoxide.
Since MOXIE runs so hot, keeping the equipment safe from itself and the rest of the Perservance tools requires high-tech insulation. Complicating the matter even further is that Mars itself is a cold planet. Its sparse atmosphere struggles to retain the day’s heat, so Perseverance can expect enormous temperature swings, from possible highs in the 60s Fahrenheit to lows as cold as 130 below zero. “We want to show we can run [MOXIE] in the daytime, and the nighttime, in the winter, and in the summer, and when it’s dusty out...in all of the different environments,” says Hecht.
Insulating a high temperature experiment to operate on such was tough, according to Aboobaker, who has been involved with testing MOXIE at the NASA Jet Propulsion Laboratory since 2014. “How do you build it? And how do you package it in a way that survives launch?,” he says. “There was a lot of iteration.”
NASA engineers used aerogels developed for an entirely different project as a lightweight and durable insulator. Each solution like this one raised the likelihood of success, and even uncovered new science—the team recently published a study about how filters perform in a simulated Martian atmosphere. “We’ve learned so much already without even going to Mars,” says Hecht.
According to Hecht, Perseverance’s busy schedule also presents a huge technical challenge for MOXIE’s operation. The instrument must cycle on and off to share precious battery power with other instruments onboard. All of that stopping and starting—freezing and sweltering—is rough on the system. MOXIE’s tests revealed that this cycling degrades the experiment in many ways. The team used fixes—such as recirculating some carbon monoxide to prevent carbon dioxide from degrading the reaction site—to solve the cycling issue, which Hecht says was “the single hardest problem.”
NASA finalized its hardware work in March 2019. Since Perseverance won’t land until next February, MOXIE’s hardware will have sat unused for nearly two years. “To begin with, just sitting on a shelf for two years is a challenge,” says Hecht. “Not to mention the extreme abuse it goes through between leaving that shelf and turning on on Mars.”
Asked if that’s a major concern at this point, Aboobaker answers “No—because we designed for it.”
One lingering uncertainty, according to Hecht, is letting MOXIE run on its own without someone nearby to smack an off-button. If a glitchy sensor drives the instrument’s voltage too high, elemental carbon could form and ruin the experiment.
“I fully expect MOXIE to do what it says it’s going to do,” says Julie Kleinhenz, a NASA resource expert not involved with developing MOXIE. But Kleinhenz says MOXIE faces what she calls “unknown unknowns.” Mars’ unique environment could hold surprises, such as unfamiliar effects from dust. “The whole idea here is to try something that’s a good bet, see what it does, and get the data back.”
On Mars, success will be somewhat unceremonious. “If you were standing next to the rover—in a space suit obviously—you probably wouldn’t be able to tell much was going on actually,” says Aboobaker.
MOXIE won’t be filling any tanks with its oxygen. Instead, scientists will track success with three independent measures: a pressure sensor in the gas’s path; a fluorescent molecule that dims after encountering oxygen; and an electric current from the reaction. Perseverance is also bringing microphones—the first on any Mars mission. Scientists listening to the audio feed will look out for the hum of a healthy compressor.
MOXIE is long awaited, but only makes about six grams of oxygen per hour, approximately the breathing rate of a corgi. Humans need several times that amount to breathe, and a rocket needs about 200 times that oxygen to return home. For scientists, a Martian year's worth of success—687 earth days—would be a green light to scale up MOXIE beyond corgi-like scales. Since a fully capable unit would also need to run 1000 times longer than MOXIE will, engineers can use MOXIE’s results to keep designs as reliable—and compact—as possible. And NASA may commission other resource experiments, such as methane production, to make rocket fuel. But since making methane involves finding, excavating, and melting ice on Mars, Hecht says it’s more likely “once we have boots on the ground.” Whatever the future goals, the timing of Mars' orbit means the next mission won't happen for another 26 or 52 months, at least.
Asked if he'd ever accept an opportunity to visit Mars himself, Hecht laughs, saying "Hey, I’m not comfortable on top of a ladder." After more thought, he concedes that he couldn’t pass on the opportunity. NASA engineer Aboobaker has one caveat. "Mars seems like a nice place to visit," he says. "It would be awesome...if I could make sure I could come back."