“Plutonium-238 is a unique isotope of plutonium that principally decays by alpha radiation, and because of that, it generates a lot of heat,” says Robert Wham, the plutonium supply program manager at Oak Ridge National Laboratory, which is now responsible for making the stuff for NASA. “For a small spacecraft like Perseverance, you don’t want fission power. You just want thermal decay.”Perseverance is only the second Mars rover to use nuclear power as its main source of electrical energy. The agency’s first three rovers—Sojourner, Spirit, and Opportunity—all used solar power, but this meant they ran the risk of losing power completely when enough dust accumulated on the panels . Starting with Curiosity, which arrived on the Red Planet in 2012, NASA engineers switched to nuclear power as the rover’s main source of energy. It was a bold choice considering that, at the time, the US stockpile of nuclear fuel for space missions was dwindling and there wasn’t a single facility in the US capable of making more.
Plutonium-238 isn’t used in nuclear weapons (that’s its sister isotope, plutonium-239). But as the Cold War wound down in the late 1980s, the US stopped manufacturing all flavors of plutonium to comply with disarmament protocols. “Most of the plutonium-238 was from the Savannah River Site, which at the time was a defense facility rather than a national lab,” says Wham, referring to the Georgia site that formerly produced most of the materials for US nuclear weapons. Today, the Savannah River Site is one of the most contaminated places on the planet due to the nuclear waste buried on the premises from these activities.