Most of what humanity knows about the outer planets came back to Earth on plutonium power. Cassini’s ongoing exploration of Saturn, Galileo’s trip to Jupiter, Curiosity’s exploration of the surface of Mars, and the 2015 flyby of Pluto by the New Horizons spacecraft are all fueled by plutonium-238. The characteristics of this metal’s radioactive decay make it a super-fuel. More importantly, there is no other viable option. But there’s a problem: We’ve almost run out.