Glenn Fleishman reports that most spacecraft, including satellites and some Mars landers, rely on solar panels, but for rugged and complex outings like the Mars Curiosity rover mission and extremely long-term flights like the one that just took New Horizons by Pluto and its moons, only power extracted from radioactive sources can provide enough amperage and wattage.
NASA uses radioisotope thermoelectric generators (RTGs), a clever combination of material once extracted from the by-products of nuclear weapons and a 200-year-old technique for converting differences in temperature into electricity. RTGs began powering NASA missions in 1969.
An RTG harnesses heat generated by the radioactive decay of plutonium-238 dioxide, which can be refined from the by-products of the enrichment process required to produce a heavier, more fissile isotope, plutonium-239, for nuclear weapons. The lighter isotope produces very little gamma radiation, which means it requires relatively little shielding in a spacecraft.
As the Pu-238 decays over its nearly 88-year half-life after 88 years, about half the original mass remains, then half again in each successive 88-year period. Its heat is converted to electricity through thermoelectric couples (or thermocouples), which rely on the Seebeck effect. This was first described in 1821 by German physicist Thomas Johann Seebeck, although he did not quite realize what he was seeing - that an electric current flows between two metals maintained at different temperatures.
In the type of RTG used in New Horizons and several previous missions, the hot side of the connection averages 1,894 degrees F at launch and the cold side about 559 F. But enough power remains after years in space to energize the systems. However, the Pu supply is being depleted and more will be needed.
RTGs and other power sources are discussed in Chapter 9 of the EMD Memoir 101:
http://i2mconsultants.com/downloads/corp/Memoir101-CHAPTER09Rev.pdf
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