Deep Space Missions Get Boost As Production Of Plutonium-238 Restarts

Pioneer 11 needed plutonium-238 to explore the distant Solar System. NASA

For the first time in nearly 30 years, plutonium-238 has been produced in the U.S., returning a key capability to spacecraft. This isotope of plutonium is essential for powering missions into deep space, and with the world’s stockpile running low, proposals for future NASA missions had been left in the lurch.

Now, mission planners can rest easy, as 50 grams (0.1 pounds) of plutonium oxide were produced by the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) in Tennessee just before Christmas 2015. It’s a small amount, but the beginnings of steady production of the isotope. ORNL is aiming for up to 400 grams (0.9 pounds) per year at first, rising to 1.5 kilograms (3.3 pounds) in the near future.

“Once we automate and scale up the process, the nation will have a long-range capability to produce radioisotope power systems such as those used by NASA for deep space exploration,” said Bob Wham, project lead at the lab’s Nuclear Security and Isotope Technology Division, in a statement.

With a half-life of 87.7 years, plutonium-238 steadily decays into uranium-234. Each gram that decays produces about 0.5 watts of thermal power, which can be used by spacecraft to power their various instruments and systems with a radioisotope thermonuclear generator (RTG). The Curiosity rover on Mars, for example, uses plutonium-238 as its energy needs are too high for solar power alone.

Plutonium-238 can power spaceraft for decades. NASA

Such was the state of affairs that some missions had to try different methods, though. The last plutonium-238 to be produced in the U.S. was at the Savannah River Plant in South Carolina, ending in 1988. This left NASA relying on their existing stockpile, and also buying from Russia, limiting the capabilities of some space missions

NASA’s Juno spacecraft, for example, which is arriving at Jupiter this year, runs solely on solar power, in part due to the shortage of plutonium-238 during its design. This will be the furthest spacecraft from Earth ever to operate on just solar power.

ESA’s Rosetta spacecraft in orbit around comet 67P Churyumov/Gerasimenko, and the Philae lander on the surface, would also have benefitted from plutonium-238. Instead, they had to remain in hibernation – with their systems powered down – for several years while they drifted towards the comet.

To create plutonium-238, as part of a program costing $15 million a year, ORNL mixes neptunium-237 with aluminium, and presses the mixture into high-density pellets. The so-called High Flux Isotope Reactor at ORNL then irradiates the pellets, creating neptunium-238, which decays quickly into plutonium-238.

Shown is part of the process the lab is using. ORNL

ORNL said that NASA only had about 35 kilograms (77 pounds) left, only half of which met power specifications for spacecraft, enough for two or three more missions into the 2020s. The newly produced material can be mixed with the existing supply, giving NASA an increased capability much longer into the future, although some have suggested it may not be enough.

NASA’s next mission to use plutonium-238 will be the as-yet unnamed Mars 2020 rover, which is similar in design to Curiosity. And who knows what other future missions will benefit from this new production line.

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