More than a month ago, NASA’s Perseverance rover landed on the surface of Mars. But only now has its real work begun. Deep within Percy is a device designed to suck in Mars’ carbon dioxide-rich atmosphere and produce oxygen. Essentially, it acts like a mechanical tree – something that will shape the future of humanity on the Red Planet.
The atmosphere of Mars has a density of only about 1% of Earth’s atmosphere. If we want to live and work on the Red Planet, we certainly need to produce and store much more oxygen.
“What breathes the most during a mission to Mars? It’s not humans” – Michael Hecht, deputy director for research management at MIT’s Haystack Laboratory, and investigator for NASA’s MOXIE project, stated. “It’s the rocket that will take you home from Mars, the one that will get you off that planet“.
According to NASA estimates, a crew of four will need a lot of propellant – approximately 6.8 tons of fuel and nearly 25 tons of oxygen – to generate enough thrust to lift off from the surface of Mars and return home. Clearly, bringing that much oxygen from Earth to prepare for the return journey is unrealistic.
That’s why NASA is conducting the Mars Oxygen In-Situ Resource Utilization Experiment, abbreviated as MOXIE.
About the size of a car battery, MOXIE is one of many in-situ resource utilization (ISRU) experiments by NASA and the first to be tested in space. Essentially, ISRU refers to experiments conducted to find ways for future astronauts to produce necessary items from resources available on other worlds.
“If we really want to leave the planet and do things other than scientific experiments, you have to think about resource utilization” – according to Jerry Sanders, head of the ISRU Capability Leadership team at NASA’s Johnson Space Center in Houston.
NASA is investing time and a lot of money – about $50 million in the case of MOXIE – to develop multiple strategies that will allow them to establish self-sustaining settlements on the Moon and Mars.
After years of research, we are about to find out if MOXIE – and similar experiments – are truly effective.
How MOXIE Works
MOXIE uses a method called solid oxide electrolysis. First, a filter and pump draw in carbon dioxide from Mars into a screw compressor, where it is compressed to a pressure equivalent to what we experience at sea level on Earth. This compressed carbon dioxide is then fed into a solid oxide electrolysis stack consisting of 10 cells.
“This electrolysis system is the heart of MOXIE” – says Asad Aboobaker, a collaborator on the MOXIE project and a systems engineer at NASA’s Jet Propulsion Laboratory in Pasadena.
The electrolysis stack is made up of several layers of metal and specialized ceramic cells that can use oxygen ions to generate electricity when heated to high temperatures. “If you have voltage, you can selectively push oxygen ions through the ceramic membrane and separate them from everything else” – Aboobaker explains.
The result? Oxygen gas.
MOXIE is a sophisticated system. Carbon dioxide goes in. Oxygen and carbon monoxide – a harmless byproduct – come out. If it receives too much electricity, the system can produce a byproduct of carbon, or soot, instead of carbon monoxide. On the other hand, if the voltage is too low, too much carbon monoxide could flood the system and begin oxidizing this device.
“We need to calculate the sweet spot between the two scenarios” – he explains.
Scaling Up
Currently, MOXIE is just a demonstration technology. Hecht estimates that MOXIE will run for a total of 10 hours over the next few years. Each 2-hour test will produce about 6-10 grams of oxygen, just enough to sustain a small dog.
If MOXIE succeeds in producing oxygen for the Mars mission, the next step is to scale it up. This means building a larger compressor and expanding the electrolysis stacks to 100 stacks – according to Hecht. The way the system operates means that if you increase the size and number of electrolysis stacks, the oxygen yield will also increase.
A larger MOXIE – designed to produce enough oxygen for a mission of 4 astronauts – would need to run for about 10,000 hours at an average output of about 2-3 kg of oxygen per hour.
But “scaling up” the current MOXIE design to produce enough oxygen to support a small crew is only a small step toward a sustainable future on Mars. There are several critical issues that need to be addressed, such as the weather on Mars.
On any given day, the surface of Mars can experience temperature changes of more than 65 degrees Celsius. Massive dust storms can sweep across the entire Red Planet for months, obscuring the sun and raising atmospheric pressure by up to 12%.
“Weather affects how MOXIE operates” – Hecht says. Understanding the severity of dust storms, and specifically how significant changes in air pressure will impact the mechanical systems of the machine, could help determine the right design for larger scale systems. For example, if a large future MOXIE system encounters high pressure conditions, Hecht says it could slow down the compressor slightly to reduce the amount of carbon dioxide being fed in.
Atmospheric pressure on the surface of Mars is around 4.5 Torr. At the peak of the largest volcano on the Red Planet, Olympus Mons, atmospheric pressure drops to about 0.2 Torr; at the deepest part of the Hellas Planitia basin, the pressure rises to about 8.7 Torr. For comparison, Earth’s surface has an atmospheric pressure of around 760 Torr.
“We designed this system to be robust and flexible enough to operate in a range of different atmospheric conditions” – Aboobaker explains. MOXIE can operate in atmospheric pressures ranging from 2-12 Torr.
It will be tested day and night, as the air cools and becomes denser. And since atmospheric pressure can fluctuate by up to 30% between summer and winter months, testing will continue throughout the year. Integrated sensors will track MOXIE’s progress as it runs each test and report back if any issues arise.
The data returned from these sensors will be used to design and develop future large-scale systems capable of producing oxygen continuously, regardless of the weather conditions in that area.
NASA estimates that the first crew to set foot on Mars will need about 30 kilowatts per day to support overall life. A large-scale MOXIE would use an equivalent amount of energy. While solar panels are seen as the obvious choice to power a Martian outpost, they come with a host of drawbacks.
First, you would need a lot of solar panels to generate enough energy for a crewed mission. And due to Mars’ day-night cycle, as well as dust storms obscuring the sun, any outpost that relies on solar power will require an additional sustainable energy storage system.
The best solution, according to Aboobaker, would be a small nuclear power plant. “It’s a reasonably scaled reactor to provide power for some things like a human-sized MOXIE” – he adds.
Nuclear engineer Dave Poston from Los Alamos National Laboratory agrees. It’s an efficient and safe alternative to solar power: a single nuclear reactor could replace a solar panel the size of a football field. You would get “more energy per kilogram than traditional solar power systems” – he states.
This technology is not entirely new. Between November 2017 and March 2018, NASA, the National Nuclear Security Administration (NNSA), and Los Alamos National Laboratory, along with several other partners, tested a nuclear fission reactor called the Kilopower Reactor Using Stirling Technology, or KRUSTY.
Located in the Nevada desert, this nuclear reactor successfully generated 5 kilowatts of electricity – about half the energy needed to power a household. Last year, Los Alamos National Laboratory agreed to license the reactor to Poston and LANL nuclear engineer Patrick McClure.
According to McClure, the best way to test the technology in practice is to send a probe equipped with 10-kilowatt reactors to the surface of Mars. This number is calculated to be just enough to support life for a 6-person crew for a day on Mars.
Future Kilopower systems, with larger scales to support larger communities, could produce several megawatts of energy. Instead of continuing to be mounted on landers, Poston says these reactors could be buried beneath the surface of Mars or placed half a mile from the Martian settlement. This way, they would be less likely to be damaged during the launch of the lander.
Poston believes Kilopower could be ready to take off in the next decade. “The problem isn’t with us – we can build a reactor pretty quickly” – McClure says. “The problem is finding someone with a launch vehicle and the right equipment to land it“.
The Storage Challenge
Next is the issue of storage. “Storage methods are not nonexistent, but like any engineering task on other planets, it can be daunting” – Hecht says. “Knowing how to do it and actually doing it are two different things“.
Liquid oxygen created for use in rocket propellant is particularly difficult to store on the surface of Mars. It must be cooled to around -182 degrees Celsius – a process, according to Sanders, that is extremely energy-intensive and requires 10 times the energy compared to simply storing it.
Keeping storage tanks adequately cold so oxygen does not warm up and evaporate into the atmosphere is critical. Designing an insulated tank for Mars is a completely different task than designing a similar tank for use in a microgravity environment.
“In space, because you have a vacuum environment, those insulating layers work very effectively” – Sanders says. “However, Mars has an atmosphere, so all the technologies we’ve developed up to this point for space applications don’t really work“.
A temporary solution is to send a steel tank covered in a vacuum jacket, which is often used to keep cryogenic liquids cold on Earth. “You basically have to put one tank inside another tank, and between them, you suck the air out to create a vacuum” – Sanders explains. “That vacuum environment reduces the amount of heat entering the inner tank that holds the cryogenic liquid“. Nevertheless, these options are bulky and costly, consuming fuel to transport them to the surface of Mars. Aerogels, a super-lightweight silica material, could also be used to insulate a metal tank and help reduce the cargo’s weight.
“Mars has an atmosphere, so all the technologies we’ve developed up to this point for space applications don’t really work”
Sanders adds that NASA is also exploring the use of pumpable tanks, which would be compacted during the journey to Mars and then inflated upon arrival. While these tanks save fuel, space, and costs, he notes they are less effective in terms of thermal retention. “That’s probably a trade-off we have to consider” – Sanders says.
And next is the dust problem. “When you have a surface covered in dust, its thermal properties will change” – Sanders states. Just like how a dust layer on an ice cap absorbs heat and causes it to melt faster, a layer of Martian dust on the surface of a cryogenic tank could cause its temperature to rise.
A research group at NASA’s Kennedy Space Center is currently developing electrostatic propulsion technology designed to push lunar or Martian dust off surfaces. Additionally, periodically blowing compressed air onto surfaces could help clear dust away. A simpler solution? Just create a tank with a conical shape. We’ll design a tank that’s conical, leveraging gravity to push dust off the surface – Sanders says.
Initially, the size of these tanks will be designed to fit the size of the landers carrying them. However, NASA is now considering building larger stations – much like building a gas station on Mars – where future settlers can refuel their rockets.
Continuing the Work
While MOXIE is busy “breathing” out oxygen on the Red Planet, engineering teams from Earth will continue to research a human-sized system.
Hecht and his team are collaborating with a Colorado-based company called Air Squared to develop a larger compressor. Another company, OxEon Energy in Salt Lake City, just received a significant grant from NASA to develop a large-scale stacked oxide electrolysis structure capable of producing nearly 1 kg of oxygen per hour. At MIT, researchers are developing smaller, lighter filters to keep dust out.
Hecht believes a complete MOXIE system could be present on Mars within the next two decades. That is, unless the government reconsiders and reallocates funding to other areas in the solar system. “If you ask when we will do it, the answer depends more on politics than science” – he says. “I believe we will do it around 2030 – if we are ambitious and serious about it“.
The key to a successful settlement future lies in establishing everything within a single cycle – about 26 months – before humans set foot on the Red Planet. “That’s the timeframe we need to fill this oxygen tank” – Hecht explains. “You start doing that when the system arrives, and you want to finish in time to notify everyone back on Earth that the tank is full”
As a small investment project, about the size of an electric car battery, scientists hope the results will be entirely worthwhile and ensure success for future settlement efforts.
Source: PopularMechanics