Every spacecraft ever sent to operate beyond Jupiter has depended on plutonium-238—a reactor-made isotope that cannot be mined. The United States stopped producing it in 1988, then spent nearly three decades drawing down old reserves and supplementing them with material purchased from Russia. In 2015, American production finally restarted with a batch of just 50 grams.

Every spacecraft ever sent to operate beyond Jupiter has depended on plutonium-238—a reactor-made isotope that cannot be mined. The United States stopped producing it in 1988, then spent nearly three decades drawing down old reserves and supplementing them with material purchased from Russia. In 2015, American production finally restarted with a batch of just 50 grams.

Every spacecraft that has gone on operating beyond Jupiter has carried the same answer to a basic problem: plutonium-238. Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Cassini and New Horizons all depended on the heat released by this isotope. Yet plutonium-238 is not something a mining company can extract from the ground. It has to be manufactured in reactors, chemically separated, turned into a ceramic and sealed into heat-source modu…