Glenn Fleishman reports that an energy technology that NASA began using in the 1960s made the recent Pluto fly-by possible. Most spacecraft, including satellites and some Mars landers, rely on solar panels, but for rugged and complex outings requiring more power 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 nuclear power extracted from radioactive sources can provide enough oomph.
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 .... 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 1,308 Kelvin (1,894 degrees F) at launch and the cold side about 566 Kelvin (559 degrees F). The maximum temperature decreases over a long time as the fuel decays, contributing to a gradual reduction in electrical output.
For more on RTGs, see http://i2mconsultants.com/downloads/corp/Memoir101-CHAPTER09Rev.pdf
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