Spacecraft are linked to electricity more fundamentally than nearly anything else. They need power to run their scientific instruments, to communicate with Earth, to change orientation, and just to keep warm. With no interplanetary gas stations, they need to be self reliant. So how do we power them? For trips in the inner solar system, the choice is easy - we use the Sun. Solar panels power virtually all spacecraft which visit Mercury, Venus, and Mars. Even the giant International Space Station gets its power from the Sun. But the solar system is a big place, and the Sun doesn't shine so brightly everywhere. From its perch in the Saturn system, the Cassini spacecraft receives only 1% of the light we get here on Earth. At Neptune it's only one tenth of one percent. Even Jupiter, the closest of the gas giants, barely receives enough sunlight to keep a spacecraft alive.
For missions to these far-out places, we need a more powerful source of energy. Instead of solar panels, these probes use plutonium-238. A radioactive element, plutonium continuously decays to other, more stable, elements. In the process, it releases energy in the form of heat. A lot of heat: one gram of the stuff releases about half a watt of power. Put another way, just one kilogram would be enough to power several laptops (which are extremely inefficient by space standards). The heat released can be directly used to heat the spacecraft, while a device called a radioisotope thermoelectric generator (RTG) can be used to convert the heat into electricity to power everything the spacecraft does. One RTG can provide decades of power anywhere in the solar system.
So why don't we put these in all spacecraft? For one, they're rather expensive when compared to solar panels. If the Sun can provide the needed energy, it's a much more economical option. Recently, though, a more pressing problem has arisen. NASA's sole source of plutonium-238, the Department of Energy's Savannah River reactor, was shut down more than a quarter century ago. Once the agency's stocks ran out, NASA was forced to buy extra plutonium from Russia to power missions like Curiosity and New Horizons. But the Russians, too, ran out of material in 2010. Without plutonium to power the RTGs, there can be no new missions to deep space. That means no missions to Saturn, Uranus, or Neptune, and only limited exploration of Jupiter. Future Mars rovers would be forced to revert to the smaller design of Spirit and Opportunity instead of the more powerful Curiosity.
Fortunately, it looks as if the Department of Energy has managed to restart production of plutonium-238 at Oak Ridge National Laboratory. Once the program comes online, it should be capable of producing about 1.5 kg of fuel a year. That's not a lot, but it's enough to revive hope for future exploration of the outer solar system.
Source: Discovery News