This, as he found out, isn’t something you should do lightly, as the one he used ended up containing an interesting mix of radioactive materials, including small amounts of plutonium-239 ...

Beginning in the late 1950s, the U.S. met this challenge by developing nuclear batteries known as radioisotope thermoelectric generators (RTGs) and producing plutonium-238 (Pu-238) as their fuel, ...