Helium-3

Helium-3 (³He^[1][2] see also helion) is a light, stable isotope of helium with two protons and one neutron (in contrast, the most common isotope, helium-4 has two protons and two neutrons). Other than protium (ordinary hydrogen), helium-3 is the only stable isotope of any element with more protons than neutrons. Helium-3 was discovered in 1939.

Helium-3 occurs as a primordial nuclide, escaping from Earth's crust into its atmosphere and into outer space over millions of years. Helium-3 is also thought to be a natural nucleogenic and cosmogenic nuclide, one produced when lithium is bombarded by natural neutrons, which can be released by spontaneous fission and by nuclear reactions with cosmic rays. Some of the helium-3 found in the terrestrial atmosphere is also an artifact of atmospheric and underwater nuclear weapons testing.

Nuclear fusion using helium-3 has long been viewed as a desirable future energy source. The fusion of two helium-3 atoms is aneutronic and does not release dangerous radiation, unlike most other fusion reactions. Helium-3 fusion requires much higher temperatures than traditional fusion reactions, however,^[3] and the process may unavoidably create other reactions that themselves would cause the surrounding material to become radioactive.^[4]

The abundance of helium-3 is thought to be greater on the Moon than on Earth, having been created in the upper layer of regolith by the solar wind over billions of years,^[5] though still lower in abundance than in the Solar System's gas giants.^[6][7]