Plutonium-238

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The isotope that powers deep-space missions that cannot use sunlight. It is an alpha emitter with a half-life of about 87.7 years, which is the property that matters: long enough that output falls slowly over a working lifetime, short enough that the decay is energetic enough to be useful as heat. Alpha particles are stopped by the fuel cladding, so the shielding burden is modest compared with harder emitters. It is not a reactor fuel and it does not fission in the generator; the heat is simply the kinetic energy of decay products thermalised in the fuel. Its supply has been the constraining resource for outer-planet exploration for decades, since United States production ceased in the late 1980s and restarted only recently at a low rate. Not a place.

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