Chris Baraniuk
Technology journalist
Sierra Space
Sierra Space is working on a device designed to produce oxygen in lunar conditions
Inside a giant sphere, engineers bent over their equipment. Before them was a silver metal craft wrapped in colorful wires – a box that they hope will one day produce oxygen on the moon.
Once the team left the sphere, the experiment began. The box-shaped machine was now ingesting small amounts of a dusty regolith – a mixture of dust and sharp gravel with a chemical composition mimicking real lunar soil.
Soon this regolith became gloop. A layer heated to temperatures above 1650°C. And, with the addition of certain reagents, oxygen-containing molecules began to bubble up.
“We’ve now tested everything we can on Earth,” says Brant White, program manager at Sierra Space, a private company. “The next step is to go to the Moon.”
The Sierra Space experiment took place at NASA’s Johnson Space Center this summer. It’s far from the only technology researchers are working on, developing systems that could supply astronauts living on a future lunar base.
These astronauts will need oxygen to breathe, but also to make rocket fuel for spacecraft that could take off from the Moon and head to more distant destinations, including Mars.
The inhabitants of the lunar base might also need metal and could even harvest it from the dusty gray debris that litters the lunar surface.
It all depends on whether or not we can build reactors capable of extracting these resources efficiently.
“This could save billions of dollars in mission costs,” says White, explaining that the alternative – bringing lots of spare oxygen and metal from Earth to the Moon – would be arduous and expensive .
Sierra Space
The chamber recreates the pressures and temperatures of the moon
Fortunately, lunar regolith is full of metal oxides. But while the science of extracting oxygen from metal oxides, for example, is well mastered on Earth, it is much more difficult on the Moon. Particularly because of the conditions.
The huge spherical chamber that hosted Sierra Space’s tests in July and August of this year induced a vacuum and also simulated lunar temperatures and pressures.
The company says it has had to improve the machine’s operation over time to better handle the extremely irregular and abrasive texture of the regolith itself. “It spreads everywhere and uses all kinds of mechanisms,” says Mr. White.
And the one crucial thing you can’t test on Earth or even in orbit around our planet is lunar gravity – which is about one-sixth that of Earth. It may not be until 2028 or later that Sierra Space will be able to test its system on the Moon, using real regolith in low-gravity conditions.
NASA
NASA’s Artemis mission plans to send astronauts to the Moon in 2027
The Moon’s gravity could pose a real problem for some oxygen extraction technologies unless engineers engineer it, says Paul Burke of Johns Hopkins University.
In April, he and his colleagues published a paper detailing the results of computer simulations showing how a different process of oxygen extraction might be hampered by the Moon’s relatively weak gravitational pull. The process studied here was electrolysis of molten regolith, which involves using electricity to split oxygen-containing lunar minerals to directly extract the oxygen.
The problem is that such technology works by forming oxygen bubbles on the surface of electrodes deep within the molten regolith itself. “It’s the consistency of, say, honey. It’s very, very viscous,” says Dr. Burke.
“These bubbles won’t rise as quickly – and may actually be delayed before breaking away from the electrodes.”
There might be ways around this. One of them could be to vibrate the device of the oxygen production machine, which could release the bubbles.
And extra-smooth electrodes could make it easier to release oxygen bubbles. Dr. Burke and his colleagues are currently working on ideas like this.
Sierra Space’s technology, a carbothermal process, is different. In their case, when oxygen-containing bubbles form in regolith, they do so freely rather than on the surface of an electrode. This means there’s less chance of them getting stuck, White says.
Highlighting the value of oxygen for future lunar expeditions, Dr Burke estimates that, per day, an astronaut would need the amount of oxygen contained in around two or three kilograms of regolith, depending on their physical fitness and their activity level.
However, a lunar base’s life support systems would likely recycle the oxygen exhaled by astronauts. If that’s the case, it wouldn’t be necessary to process so much regolith just to keep lunar residents alive.
The real use case for oxygen extraction technologies, Dr Burke adds, is to provide an oxidizer for rocket fuels, which could enable ambitious space exploration.
MIT and Shaan Jagani
Palak Patel worked on ways to extract oxygen and metal from lunar dust
Obviously, the more resources that can be produced on the Moon, the better.
Sierra Space’s system requires adding a certain amount of carbon, although the company says it can recycle most of it after each oxygen production cycle.
With his colleagues, Palak Patel, a doctoral student at the Massachusetts Institute of Technology, developed an experimental system for electrolysis of molten regolith, making it possible to extract oxygen and metal from lunar soil.
“We’re really looking at it from the perspective of, ‘Let’s try to minimize the number of resupply missions,'” she says.
When designing their system, Ms. Patel and her colleagues addressed the problem described by Dr. Burke: low gravity could prevent the detachment of the oxygen bubbles that form on the electrodes. To counter this, they used a “sonicator”, which blasts the bubbles with sound waves in order to dislodge them.
Ms Patel says future resource mining machines on the Moon could extract iron, titanium or lithium from regolith, for example. These materials could help lunar astronauts make 3D printed replacement parts for their lunar base or replacement components for a damaged spacecraft.
The usefulness of lunar regolith doesn’t stop there. Patel notes that in separate experiments she melted simulated regolith into a tough, dark glass-like material.
She and her colleagues discovered how to turn this substance into solid, hollow bricks, which could be useful for building structures on the Moon – a towering black monolith, for example. Why not?
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