But, of course, you need to make the fuel there in the first place. The obvious choice for that is water, which can be split to produce hydrogen and oxygen. We know there is water on the Moon, but we don’t yet know how much, and whether it’s concentrated into large deposits. Given that uncertainty, people have also looked at other materials that we know are present in abundance on the Moon’s surface.
And there’s probably nothing more abundant on that surface than regolith, the dust left over from constant tiny impacts that have, over time, eroded lunar rocks. The regolith is composed of a variety of minerals, many of which contain oxygen, typically the heavier component of rocket fuel. And a variety of people have figured out the chemistry involved in separating oxygen from these minerals on the scale needed for rocket fuel production.
But knowing the chemistry is different from knowing what sort of infrastructure is needed to get that chemistry done at a meaningful scale. To get a sense of this, the researchers decided to focus on isolating oxygen from a mineral called ilmenite, or FeTiO3. It’s not the easiest way to get oxygen—iron oxides win out there—but it’s well understood. Someone actually patented oxygen production from ilmenite back in the 1970s, and two hardware prototypes have been developed, one of which may be sent to the Moon on a future NASA mission.
The researchers propose a system that would harvest regolith, partly purify the ilmenite, then combine it with hydrogen at high temperatures, which would strip the oxygen out as water, leaving behind purified iron and titanium (both of which may be useful to have). The resulting water would then be split to feed the hydrogen back into the system, while the oxygen can be sent off for use in rockets.