NASA’s Mars Rover Will Be Powered by US-Made PlutoniumFeedzy

On Thursday, NASA is expected to launch its new Mars rover, Perseverance, on a mission to search for signs of ancient life on the Red Planet. It’s the agency’s largest and most autonomous Martian explorer yet. It’s also the first to be powered entirely with American plutonium.

At the heart of Perseverance is a small “nuclear battery” the size of a beer keg called a radioisotope thermoelectric generator, or RTG. Unlike the nuclear reactors that create electricity on Earth, RTGs don’t have to initiate or sustain a fission reaction to generate power. They don’t even have any moving parts. Instead, they passively harvest the natural heat produced by the decay of plutonium-238 and convert it into electricity. They can reliably provide energy and heat to a spacecraft for decades—the two plutonium-powered Voyager probes launched in the late 1970s are still transmitting from interstellar space—and have been NASA’s go-to power source for more than two dozen deep-space missions.

“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 South Carolina 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.

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.

Today, Oak Ridge is only producing about half of its target of 3.5 pounds of plutonium a year, a milestone Wham and his colleagues plan to hit by the mid-2020s. “All we’re doing is just making sure that there’s sufficient material to power whatever NASA has coming down the road in the next 10 to 20 years,” says Wham.

The Perseverance rover is the first NASA mission to use new plutonium-238 produced at the national labs, but it won’t be the last. Future nuclear-powered deep-space missions, like the Dragonfly mission to hunt for life on the surface of Titan, Saturn’s largest moon, will also pull from this new production line. And as NASA works to spin up small reactors for nuclear-powered rockets and lunar power plants, the launch of Perseverance could very well mark the beginning of an American nuclear renaissance in space.

Updated 7-30-20: Savannah River Site is in South Carolina, not Georgia

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