Look at the picture of Voyager below. The probe, which was launched in 1978 and just left the magnetic bubble surrounding our solar system earlier this month, is missing a component that most people probably associate with spacecraft. What’s missing? Solar panels.
Even though solar panels can provide enough juice to power missions like the International Space Station, Hubble Space Telescope, and even the Spirit and Opportunity rovers on Mars, they aren’t ideal for missions into deeper space. The further you are from the Sun, the less power solar panels can generate, which affects both the length of a mission and the number of instruments and experiments that a probe can contain.
What powers Voyager if it isn’t the warming rays of our Sun? The answer is plutonium decay. Plutonium-238 is a byproduct of nuclear weapons manufacturing with a half-life of 88 years. That means over a period of 88 years, half of the atoms of Pu-238 in a mass will have decayed to uranium-234 while also emitting an alpha particle. (An alpha particle is a helium molecule that is missing its electrons.) Plutonium-238 decays, releasing alpha particles that contain a great deal of energy, without also releasing much nasty gamma or beta radiation. This makes Pu-238 the preferred fuel for radioisotope thermo-electric generators on space probes. Those thermo-electric generators convert the heat from alpha decay into electricity that the probe can use to power gyroscopes, radios, and experiments for many years longer than batteries, fuel cells, or technologies would work.
So what’s the problem? As reported by Wired, since the fall of the Soviet Union and the decline of nuclear weapons manufacturing (both good things), the United States hasn’t manufactured new Pu-238 in the last 25 years. We have about 36 pounds of the material left, most of which is spoken for in upcoming deep space missions and the second Mars Science Laboratory, a sister rover to Curiosity.
The good news is that NASA has both plans to manufacture more Pu-238 and plans for a potential next-generation replacement for today’s thermo-electric generators that’s more efficient than current designs. As the Wired article details, there are political and budgetary battles that they'll have to fight to insure we’ll be able to power the next generation of deep space explorers.