Plutonium-238

For decades, NASA has been using plutonium-238, an isotope of plutonium, to power spacecraft on extended, deep space missions. No, it's not a matter of nuclear reactions. Rather, plutonium-238 gives off heat, and that heat is used in radioisotope thermoelectric generators (RTGs) to turn that heat into power that runs the spacecraft, often over decades.

The problem is the United States stopped producing plutonium-238 in the 1980s, and now the supply of the stuff is running low, which puts future NASA exploration of the outer planets, for example, in jeopardy. The Department of Energy, however, is restarting production, and plans to have a usable supply by 2017.

Of course, because of federal government budget woes, NASA has cut way back on its outer planets exploration prograam, but if money ever flows back into that area, there should be enough plutonium-238 to power the probes all the way out.