The newest Mars rover, Perseverance, launched from Cape Canaveral Air Force Station Thursday at approximately 8 a.m. EDT. Perseverance, though, is unlike any Mars rover that went before—it is the first to be powered entirely by American-made plutonium.
The United States stopped producing plutonium-238 in 1988. In the years since, the U.S. used its stockpile and supplemented by purchasing from Russia. However, a 2009 report revealed the stockpile was not enough to continue NASA’s deep space missions beyond 2020. At roughly the same time, Russia said they would not fulfill the U.S’s next order of the radioactive isotope, set for 2010 delivery, and was considering canceling a 2011 order, as well.
As a result, in 2012, the Obama Administration and NASA came to an agreement with the Department of Energy to resume production of plutonium-238 using NASA funds. The successful launch of Perseverance today was the biggest milestone yet in that $15 million per year program, which has been successfully meeting goals since it started.
On Dec. 22, 2015, Oak Ridge National Laboratory successfully produced 50 grams of plutonium-238.
“With this initial production of plutonium-238 oxide, we have demonstrated that our process works and we are ready to move on to the next phase of the mission,” said Bob Wham, Program Manager for the Pu-238 Supply Program at ORNL. “Once we automate and scale up the process, the nation will have a long-range capability to produce radioisotope power systems.”
That automation came four years later. In January 2019, ORNL scientists announced they had automated the production of neptunium oxide-aluminum pellets, eliminating a key bottleneck in the creation of plutonium-238.
Production of plutonium-238 starts with neptunium-237 feedstock, which engineers mix with aluminum and press into high-density pellets. They then use ORNL’s High Flux Isotope Reactor to irradiate the pellets, creating neptunium-238, which quickly decays and becomes plutonium-238. The irradiated pellets are then dissolved and ORNL researchers use a chemical process to separate the plutonium from remaining neptunium. The plutonium product is converted to an oxide, while remaining neptunium is recycled into new targets to produce more plutonium-238.
“Automating part of the Pu-238 production process is helping push annual production from 50 grams to 400 grams, moving closer to NASA’s goal of 1.5 kilograms per year by 2025,” said Wham. “The automation replaces a function our team did by hand and is expected to increase the output of pressed pellets from 80 to 275 per week.”
And the ORNL team is still working to improve the process where possible. For example, they are currently developing new equipment to improve fabrication of the targets that are irradiated in the High Flux Isotope Reactor. The national laboratory is staffing operations to ensure the process can continue 24 hours a day.
Like other rovers on deep space missions, Perseverance’s traveling power on Mars comes from thermoelectric generators that create electricity from heat generated from the decay of plutonium-238 in the form of oxide ceramic pellets. Pu-238 produces heat as it decays, and the rover’s multi-mission radioisotope thermoelectric generator converts that heat into electricity to charge the lithium-ion batteries that move the rover and power the instruments it will use on the surface of the Red Planet as it searches for signs of ancient life and collects rock and soil samples for possible return to Earth. Perseverance is set to land on Mars on Feb. 18, 2021.
Photo: Pu-238, in pellet form, produces heat that the rover’s multi-mission radioisotope thermoelectric generator converts to electricity. Its long half-life makes it ideal for deep-space travel. Credit: Jaimee Janiga/ORNL, U.S. Dept. of Energy