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Helium-3 is a non-radioactive and light isotope of helium. It has two protons but only one neutron in contrast to two neutrons in ordinary helium. Helium-3 is rare on Earth and sought-after for use in fusion. More abundant helium-3 is thought to exist on the Moon (embedded in the upper layer of regolith from solar wind over billions of years) and the gas giants (left over from the original solar nebula) in our solar system.

Helium-3 is used in cryogenics to achieve temperatures as low as a few thousandths of a kelvin.