By Irene Klotz - Reuters - The U.S. space agency turned to buying radioactive plutonium-238 from Russia after safety issues prompted the Department of Energy to close its Savannah River Site in South Carolina in the late 1980s.
The Russian supply line ended in 2010, leaving NASA with a small and aged supply of plutonium for space probes flying missions that are ill-suited for solar power.
Plutonium naturally radiates heat, which can be converted into electricity by a device called a radioisotope thermoelectric generator.
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