Those deep space missions use plutonium 238 to power radioisotope thermoelectric generators: https://en.wikipedia.org/wiki/Plutonium-238#Use_in_radioisotope_the

Those deep space missions use plutonium 238 to power radioisotope thermoelectric generators: https://en.wikipedia.org/wiki/Plutonium-238#Use_in_radioisotope_thermoelectric_generators https://en.wikipedia.org/wiki/Plutonium-238#Use_in_radioisot... Surplus weapons plutonium is plutonium 239: https://en.wikipedia.org/wiki/Plutonium-239 https://en.wikipedia.org/wiki/Plutonium-239 Since plutonium 239 decays 275 times slower than plutonium 238, it also generates 275 times less decay heat and is therefore not useful in RTGs.