NASA Needs Plutonium

NASA’s longest running mission is the Voyager 2 space probe. It has been traveling through space for over 40 years now. In that time, it has escaped the gravitational pull of our Sun, and soon enough, it will be — along with its twin, Voyager 1 — one of the first manmade objects to leave the solar system. It now lies over 17 billion kilometers away from Earth, on the edge of interstellar space. But ever since its launch in 1977, the Voyager 2 probe has never ceased transmitting data back to NASA.

And it’s all possible because of a single element: Plutonium.

Plutonium is exceedingly rare. It only exists on Earth in trace amounts. But because of its radioactivity, certain isotopes of it make for good power sources — like plutonium-238. That’s why NASA used it to power the Voyager space probes.

Voyager 2 is equipped with three of these power sources, called radioisotope thermoelectric generators or RTGs. An RTG converts heat from the decay of radioactive materials into electricity. And they’ve been used as power sources in applications as diverse as lighthouses and pacemakers but they’ve been especially crucial to spacecraft.

But now NASA is running out of plutonium-238. Its whole supply could be exhausted within 8 years and the prospect of finding more in time for their next deep space mission is dim.

There are other radioactive isotopes but plutonium-238 is a particularly good heat source for an RTG because of three factors:

  1. It has a long half-life. A little over 87 years. So after 87 years, your RTG will only lose half of its power. That makes an RTG with plutonium-238 ideal for long space missions far from home. It can be relied on to produce energy for very long periods of time.
  2. Plutonium-238 also gives off very low levels of harmful gamma and neutron radiation so it doesn’t fry electronics or the people that use them.
  3. And it can be small. A sliver of plutonium-238 the size of a paperclip can provide over half a watt of electricity. So a relatively small chunk of it can power an entire spacecraft without taking up too much room.

And it never has to be recharged — unlike a battery or a fuel cell.

The radiative heat given off by plutonium-238 can also keep sensitive electronics warm in the cold depths of outer space.

At the distance that the Voyager spacecraft are at now, the sun is barely brighter than any other star you see in the night sky. Solar power is useless. They depend entirely on their RTGs.

The Voyager spacecraft isn’t the only mission using plutonium-238. The Pioneer spacecraft used it. When the Cassini probe burned up in Saturn’s atmosphere (on purpose), it took 50 pounds of this increasingly hard to find isotope with it. Pretty much any NASA mission into the outer reaches of our solar system used plutonium-238.

Most of the plutonium-238 we have today was manufactured during the cold war. The US stopped making it in 1988. After that, NASA was able to get some from Russia — until 2009 when the supply presumably ran out.

NASA has about 77 pounds of plutonium-238 left right now. Because of its half life, the older it gets, the more it decays. So even when they’re not using any of it, their stockpile is slowly being depleted.

In 2011, congress voted to allow the Department of Energy to start manufacturing it again. But it’s going to take a while to make enough plutonium-238 to replace NASA’s dwindling supply. The personnel with the know-how to make plutonium-238 are all gone. They left when the US first closed down the manufacturing process 30 years ago. The facilities to manufacture it are few and far between, spread out across the country. A production slowdown in one spot means the supply pipeline can easily be disrupted or shut down.

And it’s just hard to make.

You need the proper processing facilities with the right people, with the right expertise, and even then a batch of plutonium-238 can come out contaminated if you don’t use the right recipe.

So far they’ve only been able to make about 100 grams of the isotope — just over 3 ounces. Far short of what NASA needs for a full-blown space mission.

The DOE was hoping to be at full production by 2019 but the deadline has kept moving back. Now they don’t expect to be at full capacity until 2026. And if NASA is going to run out in 8 years, their supply of plutonium-238 is not going to be replenished in time.

Right now, NASA has enough plutonium for its 2020 Mars Rover mission and another nuclear powered deep space mission. After that, without a new source for the plutonium, they will run out completely. And all deep space mission will be on hold until a new source of power is found.

The Voyager 2 spacecraft is expected to continue functioning until roughly 2025. After that, the plutonium in its RTG will have decayed too much to provide enough power to keep any of its scientific instruments going. And so it will go dark. It will cease transmitting data back to Earth. All of the scientific instruments that had been operating nonstop for the last 40 years will shut down forever. But if it doesn’t run into anything, the spacecraft itself will continue coasting into interstellar space for eternity.

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