Helium-3 is an isotope of helium that is used in nuclear fusion reactors, and as fuel for 'fusion torches', which are the main commercial form of starship thruster. It is rare on Earth, but more common on the Moon. Its importance lies in the fact that it could be an efficient fuel for nuclear fusion reactors. Helium-3 is produced during the decay of tritium; downsizing the nuclear weapons arsenal led to halting US tritium production in 1988. Since then, reserves of He-3 have been rapidly declining just when the applications for He-3 have dramatically increased, especially in detecting smuggled fissile materials. Helium-3 has uses in nuclear medicine, ultracold refrigeration, neutron detection, and potentially, nuclear fusion. Additionally, it has the potential to be used as a fuel in future nuclear fusion power plants. Since its electron shell also has two electrons, helium-3 consists of an odd number of particles, which makes it what is termed a fermion.