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Plutonium Powers Deep Space Missions

A Plutonium "marshmallow" pellet. Credit: Department of Energy
A Plutonium "marshmallow" pellet. Credit: Department of Energy

Deep space exploration is a lot like camping. You’ve got to bring everything with you.

One thing that’s really hard to bring with you into the depths of the solar system is power. Deep space probes depend on making their own throughout the mission. Solar panels are a great way to generate power in space.

But the downside of solar panels is you need sunlight. NASA's Director of Planetary Science Jim Green says a lot of the places planetary scientists are looking are "in the dark," like deep craters on Mercury and the moon

"That’s important for us," said Green, "because what’s in these permanently shadowed crater’s we’re now sure of is some really early primordial material, cometary material, from the collapsing cloud and from comet impacts from the very beginning of our solar system."

Makin' Power

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A technician measures the radiation output on one of three RTGs used in the Cassini spacecraft. (1997) Photo: NASA[/caption]

Back in the 1960s scientists developed a way to make energy without sunlight. They came up with a Radioisotope thermoelectric generator – or RTG for short.

It’s basically an electric generator the size of a beer keg. It use the heat generated from the natural decay of radioactive material, in this case Plutonuium-238, and turn it into energy using thermocouples that are "hot on one side, cold on the other," said Rebecca Onushak from the Department of Energy. "That temperature difference makes electricity. Just a DC voltage similar to a battery.”

The Department of Energy is responsible for building these deep space power plants.

RTGs are imperative to deep space exploration, according to NASA's Jim Green, because they can last for decades. "We need radio isotope power to go to Pluto, or Uranus, or Neptune, but we also need it in other places too."

The Mars rover Curiosity uses these RTGs. Even though it has solar panels, the RTG system helps the rover charge its batteries and perform tasks overnight without the need for sunlight.

Supplies Thaw

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Technicians work on radioactive material in a protective box. Photo: Department of Energy[/caption]

But here’s the problem. Our supply of Plutonium needed for these RTGs was dwindling. As the Cold War was thawing, the US scaled back on its nuclear weapon production - the DOE’s primary source of Plutonium for energy usage.

"When we had a limited resource," said NASA's Jim Green, "we did indeed think in a limited way in how to conserve it."

So NASA and the DOE started working on a way to manufacture more Plutonium for deep-space exploration.

Bob Wham runs a Department of Energy lab that’s making new units of Plutonium to help NASA explore deep space.
He uses a nuclear reactor the size of an oil drum to radiate neptunium. It turns into Plutonium, then into a powder which can be made into a Plutonium pellet.

"Think of the big marshmallow you roast over a campfire – the final pellet that goes into the RTG looks about like that, looks about that size," says Wham.

Now that Wham figured out how to manufacture Plutonium, he’s hoping to ramp up production for NASA which is about 10 Plutonium marshmallows a year.

More Exploration

Back at NASA, Jim Green says Wham’s efforts are opening up a whole new world of exploration

"We now can be much more visionary about how we can go and use the material to missions in the outer parts of the solar system or even on objects closer to the sun."

With a growing stockpile of Plutonium, he has his eyes set on the moons of Saturn: Titan and Enceladus.


Brendan Byrne is Central Florida Public Media's Assistant News Director, managing the day-to-day operations of the newsroom, editing daily news stories, and managing the organization's internship program. Byrne also hosts Central Florida Public Media's weekly radio show and podcast "Are We There Yet?" which explores human space exploration, and the weekly news roundup podcast "The Wrap."