Plutonium-238 is an artifact of the Cold War, a byproduct of the process used to make nuclear weapons. Since nuclear non-proliferation became popular, the flow of plutonium-238 has ceased and left limited stockpiles of this incredibly useful and relatively long-lived fuel. Plutonium-238 continues to power deep-space missions such as Voyager, Curiosity, and New Horizons, but according to a new report by Space News, there's only enough left to make three more nuclear batteries.