When the US got out of the plutonium business, it left NASA with a cache of a few dozen kilograms of plutonium-238 to ration for all future missions. It wasn’t much; the Perseverance rover alone uses nearly 5 kilograms of plutonium. At some point, this stockpile was bound to run out; a 2009 report by the National Academy of Sciences predicted that the US had only enough plutonium for a few more deep-space missions. That left the US with a few unpalatable options: Abandon exploration of the outer solar system, purchase plutonium from abroad, or start making it again domestically.

When Curiosity was launched in 2011, its nuclear battery contained plutonium that had been sourced from Russia. It wasn’t a great look—using Russian fuel on a marquee American space mission—but, more importantly, it also exposed NASA to the vicissitudes of geopolitics. A few years earlier, the Kremlin had reneged on an agreement to deliver plutonium to NASA until the purchase deal was renegotiated. Meanwhile, the Department of Energy, which oversees the fabrication of all nuclear fuel in the US, had been lobbying Congress to allocate funds to restart domestic plutonium production for years. The idea was to split the cost equally between NASA and the DOE, but each time legislators denied the request.

With concerns about a plutonium shortage mounting—Russia was also running low—NASA policymakers decided the agency would foot the bill on its own. And since 2011, NASA has borne almost the entire cost of producing plutonium at the Department of Energy’s Oak Ridge National Laboratory in Tennessee. The investment soon paid off. By 2015, chemists at Oak Ridge produced the first sample of plutonium-238 in the US in nearly 30 years. At the same time, the lab invested heavily in automated production systems that would allow it to produce enough plutonium to meet NASA’s future needs. But even with robots involved, producing plutonium-238 is laborious and involves two other national labs, in addition to Oak Ridge.

The process starts when researchers at Idaho National Lab send neptunium-237, itself a radioactive metallic oxide, to Tennessee, where automated machines press it into pellets the size of pencil erasers. Next, 52 of these pellets are stacked into metal rods called targets and placed in a nuclear reactor at either Oak Ridge or Idaho National Lab, where they are bombarded with neutrons to produce plutonium. After it’s left to cool for a few months, the plutonium is shipped to Los Alamos National Laboratory in New Mexico, where another machine presses the small plutonium pellets to form larger ones the size of marshmallows. Then they’re ensconced in a casing made out of iridium, a virtually indestructible metal that would prevent radioactive contamination in case of an accident when the rover is launched. Finally, the armored plutonium is shipped to Idaho National Lab, where 32 pellets are loaded into the rover’s nuclear battery before it’s installed on the vehicle.