Plutonium, here in pellet form, is the power source that just keeps on giving (Image: NASA)

The future is looking brighter for missions to the solar system’s dark corners. Plutonium needed to power the spacecraft that cannot rely on solar power has been created in the US for the first time in 25 years – albeit in small quantities.

Some destinations, such as the outer solar system or the polar regions of Mars, receive too little sunlight for ambitious missions to use solar panels there. Instead, heat from the decay of the radioactive isotope plutonium-238 is needed to generate electricity.

Unfortunately, stores of the stuff are running out. The US stopped producing the isotope – which had been generated as a by-product of cold-war nuclear weapons programmes – in 1988. Its remaining stocks have been estimated at just 16.8 kilograms.


“Without a restart of plutonium-238 production, it will be impossible for the United States, or any other country, to conduct certain important types of planetary missions after this decade,” concluded a US National Research Council report in 2011.

New beginning

Now, however, the US has taken the first steps towards restarting its plutonium-238 production line. A few grams of the isotope have been created in tests at the Department of Energy’s Oak Ridge National Laboratory in Tennessee in the past year, and the results suggest the lab could scale up to 1.5 kg annually, in line to meet many, if not all, of NASA’s goals.

In the tests inside a reactor at Oak Ridge, neutrons were fired at an aluminium target filled with radioactive neptunium-237. Later, the resulting plutonium-238 was chemically separated from the neptunium target, which was then reused.

The tests will continue for the rest of this year, with the aim of finding the best shape and density of the target for the reactor and improving ways of extracting the plutonium produced, says Tim Powers, director of the lab’s non-reactor nuclear facilities division.

Glove box

“It’s a very well-defined problem, but it’s an engineering challenge to optimise all these things,” says Jeff Binder, the lab’s associate director for nuclear science and engineering.

Performing such tests and upgrading facilities at both Oak Ridge and Idaho National Laboratory in Idaho Falls to produce useful amounts of plutonium-238 could cost upwards of $100 million, though the US Department of Energy has yet to finalise its cost estimates. Powers says the main upgrade needed at Oak Ridge is the installation of several stainless steel “glove boxes” that would allow workers to safely make the neptunium targets. “This is probably the most significant thing that needs to happen to go to 1.5 kilograms per year,” he says.

The US Department of Energy aims to ramp up to that production level by late 2017.