Since time immemorial, humans gazing up at the moon have asked grand questions. Where did it come from? Why does it wax and wane? Is it made of cheese?

We now have responses to most of these (“a giant impact,” “orbital phases” and “no, sadly,” respectively). But as an international 21st-century lunar race intensifies, one pragmatic query remains: How can you make money on the moon?

The answer, according to several scientists and entrepreneurs, is a resource that’s vanishingly rare on Earth yet may exist in lunar abundance: helium-3.

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Helium-3 is spectacularly useful, and demand for it is soaring. A superlative coolant, helium-3 enables quantum computers to reach their operating temperatures, fractions of a degree above absolute zero. The precious substance is also vital for advanced medical imaging, as well as sniffing out smuggled nuclear material, and holds promise as a clean fuel for future fusion reactors. On terra firma, most of the available supply of helium-3 comes as a by-product of nuclear weaponry via the radioactive decay of tritium, a rare isotope of hydrogen that boosts the power of thermonuclear bombs. This process makes just a few kilograms of helium-3 per year worldwide, and a single kilogram currently costs about $20 million.

But scientists estimate that somewhere on the order of a billion kilograms of helium-3 are lacquered onto the lunar surface. So the moon-based mining of helium-3 could, it seems, someday become a multitrillion-dollar industry.

All this sets helium-3 apart from another much ballyhooed lunar resource: water ice, found in some of the moon’s deepest, darkest craters. Those reservoirs could hydrate crops and astronauts alike on any crewed moon base, and water split into its constituent hydrogen and oxygen can manufacture rocket fuel. But lunar water has little use on Earth. So “helium-3 is where the money is,” says Clive Neal, a lunar geoscientist at the University of Notre Dame.

That’s assuming there’s actually enough of it accessible on the moon to be profitably extracted. “Once you’ve proven you can do it, then you have to scale it, which has its own challenges,” says Paul van Susante, principal investigator of the Planetary Surface Technology Development Lab at Michigan Technological University.

Building a Treasure Map

Helium-3 is an isotope of helium that possesses one fewer neutron than its run-of-the-mill counterpart, helium-4, which is the only other stable helium isotope. Earth has both varieties. Helium-4 is naturally produced in the mantle through the decay of uranium and thorium, so there’s a lot of it. Most of the natural supply of helium-3 formed in the first few minutes after the big bang, and Earth’s stores were laid down billions of years ago, when our planet formed. The rare isotope is mostly locked away deep within our world’s innards, but vanishingly small quantities are belched out in volcanic eruptions and through natural gas pipelines.

Researchers realized the moon was a potential helium-3 treasure trove in the 1970s, after finding it in drill cores gathered by astronauts during some of NASA’s Apollo missions. China’s robotic sample-return program, the Chang’e series, has found it as well on the moon’s near side and far side. Only meager traces of lunar helium-3 are present in these samples, yet these amounts still far exceed Earth’s abundance.

“The moon has an extra source of helium, which is the sun,” says Sara Russell, a planetary scientist at London’s Natural History Museum. The solar wind—the stream of charged particles emanating from the sun’s atmosphere—carries various chemical species, including helium-3, out into space. “Earth is shielded from this solar wind because of our lovely atmosphere and magnetic field. The airless moon doesn’t have this shield, so helium-3 gets spray-painted across the whole of the lunar surface.”

Helium-3 isn’t guaranteed to stick around on anything it strikes, but we’ve lucked out by having a moon that is relatively rich with ilmenite—a mineral made of iron, titanium and oxygen with a physical structure that acts like a trap for the gas. “Ilmenite is like a sponge. It holds onto the solar-wind-implanted species better than any other mineral on the moon,” Neal says. That means prospecting for lunar helium-3 starts by simply making mineralogical maps of the moon’s surface.

First, find all your ilmenite-rich regions (which are typically in lunar mare, the dark patches on the moon that signify frozen seas of ancient lava). Then make sure they’ve got good exposure to the solar wind. Generally, these areas will be “more equatorial regions and often—though not exclusively—[will be located] on the lunar far side,” says David Lawrence, a planetary scientist at the Johns Hopkins Applied Physics Laboratory. Finally, check to see if the surfaces are relatively free of recent meteorite impacts, notionally allowing more opportunity for helium-3 to accumulate.

Such bombardment, however, can both give and take in the accounting of lunar helium-3. “There is a continued ‘gardening’ of the surface by micrometeorite impacts, which churn up the surface,” says Christopher Dreyer, director of engineering at the Center for Space Resources at the Colorado School of Mines. On one hand, the mechanical and thermal effects of impacts can shake and bake stores of helium-3 out of ilmenite-laden lunar soil (technically called “regolith”). On the other, minerals freshly exposed by impact gardening can soak up more helium-3 from the solar wind, and the churning overturn might bury and preserve enriched material to build up a repository several meters deep.

The next step is to move from orbital imagery to “ground truthing.” The isotope can only be directly detected with equipment such as a mass spectrometer, which uses absorbed or emitted radiation to determine the chemical makeup of a target sample. Robotic moon rovers that are equipped with spectrometers and drills will be key to such studies, investigating helium-3 reserves both at and just beneath the lunar surface. Expected to launch by next year, NASA’s robotic rover VIPER (Volatiles Investigating Polar Exploration Rover) will use spectrometers and an onboard drill to scout the lunar south pole for signs of water ice and helium. Lunar Polar Exploration (LUPEX), a joint effort between Japan’s and India’s space agencies that is planned for launch in 2028, will do much the same.

Thick deposits of the isotope will be music to the ears of helium-3 hopefuls. But they are also keen to know how quickly the surface helium-3 regenerates via the solar wind. If it takes many centuries or more, that suggests the moon’s bounty won’t long sustain the surging, more immediate needs of quantum computers and other technologies. But a far quicker refresh rate would be game-changer, raising the possibility of making lunar helium-3 a sort of renewable resource.

“There’s a question mark here, but it’s tantalizing,” Neal says. “If helium-3 is a renewable resource, then you’ve got long-term prosperity.”

Lengthy in situ surveys of extensive swaths of the moon’s surface should eventually answer this question.

Harvesting Solar-Wind “Spray Paint”

As straightforward as finding helium-3 might be, extracting it could prove much harder. “It’s like trying to mine spray paint from a wall,” Russell says.

Similar to how cosmic impacts can agitate and heat lunar regolith to liberate trapped particles from the solar wind, machines can do much the same. “You then have to separate the helium-3 from the other stuff, which is nontrivial,” van Susante says. And then you must send it safely back to Earth.

As of yet, though, no one has demonstrated (or even attempted) helium-3 extraction on the moon. That’s the top priority of several space resource companies, including Seattle-based Interlune, founded in 2020.

Last year, in partnership with industrial equipment manufacturer Vermeer Corporation, Interlune revealed a prototype extractor designed to process 100 metric tons of lunar regolith every hour. The company is also setting up a lab to manufacture simulated lunar regolith—powdery, volcanic material that’s almost identical to the real thing. After suffusing some of this lunar simulant with helium-3, Interlune will use those samples to test its extraction methods. And early this month, NASA awarded Interlune a $6.9-million contract to further develop its hydrogen- and helium-capturing technology.

The company’s efforts are planned to culminate with its robotic Prospect Moon mission, launching as early as 2028. “We will have a robotic arm and a mass spectrometer, a camera and three different devices onboard, where we’ll demonstrate different methods of extracting solar wind gases, including helium-3,” says Interlune’s co-founder and CEO Rob Meyerson. “That’s what we need to demonstrate our business case for full-scale operations on the moon.” Besides proving out helium-3 extraction methods, another hurdle for Interlune’s Prospect Moon will be enduring the moon’s corrosive, adhesive lunar dust.

The company has identified “a small number of [landing] sites” for the mission, Meyerson says, without divulging further specifics. It seems safe to say, however, that Interlune would target ilmenite-rich parts of the lunar near side close to the equator, where landing, surface operations and communications with Earth are easiest.

Whether via Interlune or some other aspirant, tapping the moon’s significant stores of helium-3 could lead to an unprecedented lunar “gold rush.”

Many critics of such wanton cosmic acquisitiveness balk at the idea of scarcely regulated private-sector lunar strip-mining. “The moon belongs to everybody, surely,” says Russell, who worries about the cumulative environmental impacts of multiple helium-3 companies working on Earth’s orbital companion. The thought of intensive mechanical harvesting leaving widespread gouges across the moon’s surface—scar tissue potentially visible from Earth—doesn’t sit well.

“What we’re doing comes with a respect for the moon,” Meyerson counters. Unlike in an open-pit mining operation on Earth, he says, Interlune aims to dig down to a depth of around three meters, extract the helium-3, and leave behind no mechanical waste or pollutants. (This is an optimistic vision for helium-3 mining, to say the least; no one can yet say whether any version actually deployed on the lunar surface could manage to be quite so tidy.) “We’ve talked about leaving the site looking like a tilled agricultural field.”

Perhaps the most important factor working in helium-3’s favor is the U.S.’s lunar return, which is driven by the goal of establishing a sustained presence there; companies wishing to mine the moon are taking advantage of this push to test out their technologies for extract not just helium-3 but invaluable water-ice, too. “Helium-3 mining isn’t happening by itself,” Dreyer says. But even NASA administrator Jared Isaacman is a little skeptical of the moon’s economic promise. Although he hopes that a lunar economy can be incentivized, he recently opined that mining asteroids for various resources may offer a greater return than mining the moon for helium-3.

Perhaps the helium-3 industry will be a bust. Perhaps there isn’t as much on the moon as everyone hopes. But, just maybe, “we’re going to hit the mother lode,” Neal says. “If it’s proven, it could change everything.”