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Content: Plutonium-238 fuel is used to power a radioisotope thermal generator (RTG), which is the probe's long-life battery. An RTG converts heat from naturally decaying plutonium into electricity. The RTG uses plutonium dioxide ceramic pellets as a heat source and solid-state thermocouples that convert the plutonium's heat energy to electricity. Half of the plutonium for New Horizons was on hand when the U.S. Department of Energy (DoE) stopped work at the nuclear weapons plant in July 2004. A total of 36 of the 72 fuel units ordered had been left over from a spare RTG built earlier for NASA's Galileo and Cassini missions. When the lab shut down, it had 18 more units in the works. The launch went ahead with as few as 61 fuel units. More plutonium dioxide ceramic could be made at Los Alamos scientists by converting plutonium bought from Russia into pellets packaged in hockey-puck-sized containers. The Argonne National Laboratory at Idaho Falls could put the pellet containers into an RTG. An RTG with a full load of 72 fuel units can deliver 200 watts of electricity.