Helium-3 is an isotope of helium with two protons and one neutron instead of the usual two. On Earth it's vanishingly rare, a byproduct of tritium decay in nuclear warheads, produced in quantities measured in tens of kilograms per year. On the Moon, the solar wind has been depositing it directly into the regolith for approximately four billion years, unimpeded by the magnetic field that keeps it away from us. Estimates of the total He-3 content in the lunar regolith run to roughly one million metric tonnes. That number is cited frequently in discussions of lunar resource potential, so it's worth noting what it means in context.
He-3 matters because it's a theoretically cleaner fusion fuel than the deuterium-tritium mix that current fusion research uses. D-T fusion produces energetic neutrons, which activate surrounding materials and produce radioactive waste. D-He3 fusion, in theory, produces mostly protons, which are charged and can be converted directly to electricity, with significantly fewer neutrons. Fewer neutrons means less activation, less waste, and a reactor that's considerably easier to manage.
The problem is plasma temperature. D-T fusion requires plasma heated to roughly 100 million degrees Celsius, about six times hotter than the sun's core. D-He3 fusion requires plasma temperatures approximately ten times higher than that. Building a containment vessel for plasma at one billion degrees is, technically speaking, harder.
ITER, the international fusion research project in southern France, is designed for D-T fusion and has been under construction since 2010. As of early 2026, its first plasma experiments remain pending after years of delays. ITER isn't a power plant; it's a research device designed to demonstrate that fusion reactions can produce more energy than they consume, a milestone called Q greater than 1. The US National Ignition Facility achieved this milestone in December 2022 for approximately 3.15 megajoules of output against 2.05 megajoules of input, a result that required ignoring the 300 megajoules of grid power used to charge the lasers. Commercial fusion power plants based on D-T fusion aren't imminent. Commercial fusion power plants based on D-He3 fusion are further away than that.
Commonwealth Fusion Systems, Helion Energy, TAE Technologies, and others are making genuine progress toward commercial fusion on various technical approaches. Commonwealth Fusion achieved a 20-tesla high-temperature superconducting magnet milestone in 2021 and is targeting pilot plant operation in the early 2030s. These are real milestones. They're also milestones on a road to D-T or alternative fuel fusion, not specifically to the D-He3 scenario that makes lunar He-3 economically compelling.
He-3 is a fuel source for a reactor that hasn't been built, to be extracted from a mine 384,000 kilometers away, by companies whose investors presumably receive quarterly updates. This isn't an argument against pursuing either fusion or lunar resource development... it's an argument for keeping those two investment theses clearly separated from each other, which the promotional materials often do not.
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