NASA engineers install MOXIE on the Perseverance rover, March 20, 2019.
A little more than a month ago, NASA’s Perseverance rover made a daring landing on the Martian surface that’s now been watched (and rewatched) by millions. But now, the real work begins. Tucked deep inside Percy is an instrument designed to inhale Mars' carbon dioxide-rich atmosphere and exhale oxygen. Essentially, it's a mechanical tree—one that could reshape humanity’s future on the Red Planet.
Mars’ atmosphere is roughly 1 percent the density of Earth’s. If we have any dreams of living and working on the Red Planet, we’ll need to generate and store oxygen.
“What breathes the most on a mission to Mars? Not the people,” Michael Hecht, the Associate Director for Research Management at MIT’s Haystack Observatory and the principle investigator of NASA's MOXIE project, tells Popular Mechanics. “It's the rocket that is going to take you home from Mars, that is going to get you off the planet.”
According to NASA’s estimates, a four-person crew will need a lot of propellant—approximately 15,000 pounds of fuel and roughly 55,000 pounds of oxygen—to generate the thrust needed to leave the Martian surface and return home. Lugging all of that oxygen from Earth is a hassle.
That’s where the Mars Oxygen In-Situ Resource Utilization Experiment, or MOXIE, comes in.
Roughly the size of a car battery, MOXIE is one of NASA's many in-situ resource utilization (ISRU) experiments. In essence, ISRU is the agency’s version of homesteading, and the experiments explore ways for future spacefarers to produce things from resources available on other worlds.
“If we really want to get off-planet and do something besides a science mission, you really need to start thinking about living off the land,” Jerry Sanders, who leads the ISRU Capability Leadership Team at NASA’s Johnson Space Center in Houston, tells Popular Mechanics.
NASA is investing valuable time and a lot of money—about 50 million in the case of MOXIE—to develop strategies to create self-sustaining settlements on the moon and Mars.
After years of work, we’re about to find out if MOXIE—and other experiments like it—can really work.
How It Works
MOXIE uses a method called solid oxide electrolysis. First, a tube filters and pumps Martian carbon dioxide into a scroll compressor which then squeezes it to pressures similar to what we might experience at sea-level here on Earth. That compressed carbon dioxide is then sent to the 10-cell solid oxide electrolysis stack.
Principal Investigator Michael Hecht is in the MOXIE development laboratory at NASA’s Jet Propulsion Laboratory, Pasadena, California.
“This electrolysis system is really the heart of MOXIE,” Asad Aboobaker, a MOXIE collaborator and instrument systems engineer at NASA’s Jet Propulsion Laboratory in Pasadena, tells Popular Mechanics.
The stack is composed of layers of metal and specialized ceramic cells that use oxygen ions to conduct electricity when heated to high temperatures. “If you have an applied voltage, you can selectively drive the oxygen ions through that ceramic membrane and separate them out from everything else,” says Aboobaker.
The result? Oxygen.
MOXIE is a fine-tuned system. Carbon dioxide goes in. Oxygen and carbon monoxide—a harmless byproduct, in this case—come out. If it gets too much electricity, Hecht says the system could generate carbon, or soot, as a byproduct instead of carbon monoxide. On the other hand, if too low a voltage is applied, too much carbon dioxide could flood the system and start to oxidize the instrument.
“We need to stay right in the sweet spot between the two,” he explains.
Making oxygen is a hot process. MOXIE can reach temperatures of up to 1,500 degrees Fahrenheit. To make sure the system doesn’t melt any of its surrounding components, it is encased in a box made of gold, a poor conductor of heat.
The largest white tube on the top surface takes in filtered carbon dioxide-rich Martian atmosphere, which is then pressurized and passed through the Solid Oxide Electrolysis unit, where it is split into carbon monoxide and oxygen. The smallest tube sends the oxygen produced by MOXIE through a composition sensor to measure purity before venting oxygen out to the Martian atmosphere.
For now, MOXIE is just a technology demonstration. Hecht estimates that MOXIE will run for a total of maybe 10 hours in the next few years. Each of the instrument’s two hour-long experiments will generate only about six to ten grams of oxygen, or enough to sustain a small dog.
If MOXIE can successfully demonstrate an ability to generate oxygen on this mission, the next step is to go big. That means building a larger compressor and scaling up the electrolysis stacks by a factor of ten, according to Hecht. The system works in such a way that increasing the size and number of stacks increases oxygen production.
A scaled-up MOXIE—designed to produce enough oxygen to support a four-person crewed mission—will need to run for an estimated 10,000 hours at a rate of roughly 2-3 kilograms per hour.
But beefing up the current MOXIE design so that it can produce enough oxygen to support a small colony is just one small step along the path to a sustainable future on Mars. There are a few other key issues that need to be worked out, like the Martian weather for instance.
On any given day, the Martian surface can experience temperature swings of more than 150 degrees Fahrenheit. Colossal dust storms can swallow the entire globe for months at a time, blotting out the sun and causing air pressure to jump by as much as 12 percent.
“Weather affects how MOXIE operates,” Hecht says. Understanding how violent storms and, specifically, dramatic swings in air pressure impact the instrument’s machinery could inform the design of full-scale systems down the line. For example, if a future full-scale MOXIE system were to encounter a high pressure ridge, Hecht says it might have to run its compressor a little slower to ease up on the carbon dioxide intake.
The mean atmospheric pressure on the Martian surface hovers around 4.5 Torr. At the summit of the Red Planet’s largest volcano, Olympus Mons, atmospheric pressure drops to around 0.2 Torr; in the depths of the Hellas Planitia impact crater, it jumps to about 8.7 Torr. For comparison, Earth’s surface has an atmospheric pressure of about 760 Torr.
“We designed the system to be robust enough and flexible enough to operate over a range of conditions in the atmosphere,” Aboobaker explains. MOXIE can operate in an atmospheric pressure range between two and 12 Torr.
It’ll be tested during the day as well as at night, when the air cools and becomes more dense. And because air pressure can vary by up to 30 percent between the summer and winter months, testing will occur throughout the year. Onboard sensors will check MOXIE’s progress as it runs through each experiment and report back if anything is amiss.
The data returned from these sensors will ultimately inform the design and development of future larger-scale systems, all of which will need to generate oxygen around the clock, regardless of local weather conditions.
Artist concept of the Kilopower Reactor Using Stirling Technology (KRUSTY) experiment in use on the moon. NASA and the Department of Energy’s National Nuclear Security Administration’s nuclear reactor power system could be vital for crewed missions to Mars.
NASA estimates that the first crew that ventures to Mars will need roughly 30 kilowatts each day for general life support. A full-scale MOXIE, will use roughly the same amount of energy. While solar panels might seem like the obvious choice to power a Martian settlement, they come with a significant number of drawbacks.
First, it would take a lot of solar panels to generate that kind of energy needed to power a crewed mission. And thanks to Mars' day and night cycle and its oppressive sun-blotting dust storms, any solar-powered settlement will require a substantial energy storage system.
The most robust solution, Aboobaker argues, will probably be a small nuclear power plant. “It’s kind of exactly the right scale of reactor for powering something like a human-scale MOXIE,” he says.
Nuclear engineer Dave Poston of the Los Alamos National Laboratory (LANL) agrees. It’s an efficient and safe alternative to solar: a single nuclear reactor could replace a football field-sized solar array. You get “more power per kilogram from the reactor than the solar power system,” he says.
This technology isn’t new. Between November 2017 and March 2018, NASA, the U.S. Department of Department of Energy's National Nuclear Security Administration (NNSA) laboratory, and Los Alamos National Laboratory, among other partners, tested a nuclear fission reactor called the Kilopower Reactor Using Stirling Technology, or KRUSTY.
Tucked away in the Nevada desert, the nuclear reactor successfully generated five kilowatts of electrical power—about half the energy needed to power the average home. Last year, the Los Alamos National Laboratory agreed to license plans for the reactor to Poston and fellow LANL nuclear engineer Patrick McClure's New Mexico-based company Space Nuclear Power Corporation, also known as SpaceNukes.
An artist conception of a fission power system on the surface of Mars using four 10-kilowatt units.
According to McClure, the best way to test this technology in situ would be to send a lander equipped with four 10 kilowatt-generating reactors to the Martian surface. This would be about enough to sustain a six-person crew for the duration of their Martian stay.
Future Kilopower systems, scaled up to support larger communities, could generate up to a few megawatts worth of energy. Instead of remaining attached to a lander, Poston says, these reactors would either need to be buried beneath the Martian surface or erected about a half a mile away from the Martian colony they power. This way, there’s no chance they could be damaged during the launch of an ascent vehicle.
Poston believes Kilopower could be ready to fly within the next decade. “The problem is not us—we could build a reactor pretty fast,” says McClure. “The problem would be finding someone with a launch vehicle and the right equipment to get it landed.”
Then there’s the issue of storing it. “There’s no mysteries about how to do this, but like any other engineering job for another planet, it’s daunting,” Hecht says. “Knowing how to do it and doing it are two different things.”
Liquid oxygen generated for rocket propellant is particularly difficult to store on the Martian surface. It has to be cooled to roughly 90 kelvin or about -297 degrees Fahrenheit—a process that, according to Sanders, is incredibly power-intensive and takes about ten times more energy than the act of simply storing it.
Keeping these tanks cool so that the oxygen doesn’t heat up and boil off into the atmosphere is critical. Designing an insulated cryogenic tank for the Martian surface is an entirely different animal than designing one to be used in the vacuum of space.
“In space, because you have a vacuum, these insulation layers work really well,” Sanders says. “However, Mars does have an atmosphere, so all of the technologies that we’ve developed up to this point for space applications really don’t work.”
One work-around may be sending a steel vacuum-jacketed tank, commonly used to chill cryogenic liquids on Earth. “You literally have a tank within a tank, and between those two, you pull a vacuum,” Sanders says. “That vacuum reduces the amount of heat that goes into that inner tank holding the cryogenic fluid.” Still, these options are heavy and will cost money and fuel to send to the Martian surface. Aerogels, an ultra lightweight silica material, could also be used to insulate a metal tank and may serve to lighten the load.
“Mars does have an atmosphere, so all of the technologies that we’ve developed up to this point for space applications really don’t work.”
Sanders says that the agency is also exploring the use of inflatable tanks that are packed tightly for the journey to Mars and then inflated upon arrival. While these tanks save fuel, space and cost, he explains, they are less efficient with regards to heat loss. “That might be a trade we consider,” Sanders says.
And then there’s dust. “When you coat a surface with dust, it changes the thermal properties,” Sanders says. In the same way that a layer of dirt on top of a glacier absorbs heat and helps to melt it faster, a layer of Martian dust atop a cryogenic cooling tank may start to heat it up.
One team at NASA’s Kennedy Space Center is developing electrostatic repulsion technology designed to repel lunar or Martian dust off of surfaces. Alternatively, periodically blowing compressed gas onto the surface may also keep the surfaces dust free. Another blindingly simple solution? Just tip the thing. Building a sloped storage tank that uses gravity to slough dust off of a surface could work, too, Sanders says.
At first, the size of these tanks will be regulated by the size of the landers that deliver them. But the agency is already starting to think about building larger depots—a kind of Martian gas station—where future settlers can top off their ascent rockets.
While MOXIE is busy churning out oxygen on the Red Planet, teams of Earth-bound engineers will be tinkering away at a human-scale system.
Hecht and his team are working with a Colorado-based company called Air Squared to develop a larger compressor. Another company, the Salt Lake City-based OxEon Energy, received a grant from NASA to develop a larger solid oxide electrolysis stack capable of producing roughly one kilogram of oxygen per hour. At MIT, researchers are developing smaller, lighter filters to keep dust at bay.
Tales From the Red Planet
Hecht believes a full-scale MOXIE system could be established on Mars in the next two decades. That's if shifting political winds don't pull funds toward other regions of the solar system. “If you asked me to say ‘When will we?' That's more politics than it is science,” he says. “I believe we'd be able to in the 2030s—if we're ambitious and serious about it.”
The key to a future settlement’s success will lie in setting up everything at least one cycle—roughly 26 months—before humans arrive on the Red Planet. “That's the time that we set aside for filling up this oxygen tank,” Hecht explains. “You start it when that system gets there and you want to finish in plenty of time to give the thumbs up to Earth that the tank is full.”
For such a small, car battery-sized investment, scientists hope for a big pay off for future explorers.