NASA’s New Horizons spacecraft will fly close to Pluto this week. On the way it will take hundreds of observations before beginning the long, slow process of beaming gigabytes of data back to Earth.
These complex tasks require electricity. New Horizons is too far from the Sun to use solar panels. The deep space probe must instead rely on power cells known as radioisotope thermoelectric generators (RTGs).
RTGs use radioactive materials, but they aren’t miniature nuclear plants. Instead, the heat from their radioactive decay warms a material known as a thermoelectric – a substance that generates an electric current when warmed.
In the case of New Horizons, the source of that heat is plutonium-238: an element that was originally named after Pluto itself. The same stuff powers Cassini, Voyager, the Mars Curiosity rover and numerous other space missions.
But there’s a hitch. Plutonium-238 supplies are running low. So could space exploration be about to end?
In the US, the situation is not yet critical. “We have enough for the current planned missions for NASA for at least the next decade,” says Stephen Johnson at the Department of Energy’s Idaho National Laboratory. His lab is working toward producing new plutonium, using materials left over from the US nuclear weapons program. “We have a fairly decent supply of the starter material, neptunium,” he says, adding that his lab will begin producing plutonium-238 in the next five to eight years. “I think we’ve got enough for the better part of the century.” But eventually even neptunium stores will run out.
Americium is available and relatively cheap. “We’re turning something that would have to be disposed of into something valuable.”
In Europe, the situation is different. Not only are plutonium-238 stores running low, says Tim Tinsley, a chemical engineer and business manager at the UK’s National Nuclear Laboratory, but Europe has neither the neptunium-237 nor the necessary processing facilities to make more of it.
Fortunately RTGs don’t specifically require plutonium-238. They’ll run on any radioisotope that can generate sufficient heat and doesn't decay too quickly. “So we looked for an alternative,” Tinsley says, “and the best one we could find was americium-241”.
Like neptunium, americium is an element few people would have heard of. But it's not uncommon. “There’s a small amount in smoke detectors in everybody’s houses,” says Tinsley.
It’s also a constituent of spent nuclear reactor fuel, so there’s a lot around, unused and unwanted. “In Europe, we have several tonnes,” Tinsley says. “It’s available, relatively cheap and we’re turning something that would have to be disposed of into something valuable.”
That said, americium isn’t a perfect plutonium substitute. Plutonium-238 produces so much heat per gram that New Horizons only needed about 11 kilograms of specially packaged plutonium dioxide pellets to supply the 200 watts of power the spacecraft needed once it arrived at Pluto.
If New Horizons had been powered by americium-241, it would have required about 40 kilograms of the radioactive material to produce the same amount of energy. The difference comes from americium’s half-life of 432 years compared to plutonium’s 88 years. “Both are emitting very similar radiation, but the plutonium does it quicker,” Tinsley says.
But that slower rate of decay can also be an advantage because it means an americium-powered RTG will run far longer than a plutonium-powered one.
While plutonium’s 88-year half-life might seem like a long time, it’s actually short enough to pose problems for deep space exploration. When the two Voyager spacecraft were launched in 1977, for example, their plutonium-powered RTGs were capable of supplying more than 400 watts of power. Now, 38 years later, NASA reports their power generation has dropped to about 250 watts. By 2025 the two spacecraft will no longer be able to operate.
A similar fate awaits New Horizons, says Alan Stern, a planetary scientist from the Southwest Research Institute in Boulder, Colorado, who is the mission’s principle investigator. “If we had power to run the spacecraft, we could operate for over another 50 years.” That time could be used to try and hunt down the distant, undiscovered objects many astronomers now believe lurk at the frigid fringes of our Solar System. Instead, New Horizons will run out of juice in the mid to late 2030s.
And americium’s extra weight is not necessarily a deal breaker. The plutonium in New Horizons’ RTG is only 2.3% of the spacecraft’s total mass. Switching to americium wouldn’t change that enormously.
So as the world waits for a close-up of Pluto, the European Space Agency is funding Tinsley’s lab’s efforts. “We could have these flying in the next decade,” he says.