Helium-3 is a stable isotope of Helium with only 1 neutron (and 2 protons), vs. the Helium-4 here on Earth which usually has 2 neutrons (and 2 protons).
Since Helium-3 has fewer neutrons, it requires less radiation shielding for nuclear fusion than if using Helium-4. Less shielding means less mass, which is great if you are building a space ship. Fusion can be used to propel a spacecraft in several ways, possibly the most reliable would be to produce electricity to power an ion drive. Helium-3 fusion would still require some shielding.
Ironically, although Helium-3 is rare here on Earth, it is believed to be rather common under the surface of the Moon, which of course would require spaceships to get equipment there to mine it. I hope when the US returns to the Moon, we will look to confirm the presence of Helium-3.
TODO: find the article that explains why the Moon has more Helium-3, I believe it had something to do with cosmic ray bombardment.
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