Last year, Voyager 2 finally broke through into interstellar space after traveling more than 11.2 billion miles. This epic mission was made possible by nuclear power, the technology that has powered spacecraft for decades.
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Spacecraft like the Voyager pair are powered with radioisotope thermoelectric generators, or RTGs. These engines rely on the fact that radioactive substances release heat as they break down. By converting the heat generated by the decay of plutonium-238 (P-238) into electricity, spacecraft keep going long after the sun’s rays are a distant glimmer.
But RTGs are also constraining us. If we want to send spacecraft—or humans—farther, faster, and more often, we can’t keep relying on the same decades-old nuclear technologies. How can we expand our reach?
What’s happening right now
Our supply of plutonium-238 is running dry. The original batch was made in the US as a by-product of creating weapons-grade plutonium-239 during the Cold War. To keep exploring, NASA needs a lot more.
Oak Ridge National Lab took on the task of manufacturing it in 2012. It was a slow manual process to make even a few grams. But last month, researchers at Oak Ridge announced they’d finally developed a way to automate and scale up the production of neptunium and aluminum pellets needed to make P-238. The pellets are transformed into precious P-238 by pressing and enclosing them in aluminum tubing and irradiating them in a reactor.
Creating these pellets was the biggest bottleneck in the process, and taking humans out of the equation took a lot of experimenting. “In a lot of nuclear work, it’s cook and look,” says program manager Bob Wham. “You design it, putting a lot of safety factors on design; take it out; and see if it performs like you expected.” After years of work automating the measuring and making, it did.
The lab now makes 50 grams of P-238 a year but expects to be up to 400 grams a year soon. It predicts it will be able to hit NASA’s annual target of 1.5 kg within two years. The more P-238 we have, the more missions we can send to deep space.
NASA has also investigated making more efficient RTGs called eMMRTGs, or enhanced multi-mission RTGs. But to really take a bigger step forward, we have to look at something new. “Eventually we will need higher-power systems. Only fission can supply that in any type of near-term scenario,” says Los Alamos National Laboratory researcher David Poston.
Poston is the chief reactor designer for Kilopower, a prototype fission reactor that NASA successfully testedlast year. It could provide power over the course of long missions, possibly even for human planetary outposts. “The way we evolved it to being feasible was simplifying things,” says Poston. “We’ve had plenty of space reactor programs over the past 30 years, but they’ve all failed. Mostly because they became too expensive.” Kilopower currently has an output of 4 kilowatts, but researchers hope to reach 10 kW.
There have been some pie-in-the-sky nuclear ideas for a while, including detonating atom bombs out the back of spacecraft in what’s called nuclear pulse propulsion (you might be able to spot a few practical problems with that one). But some people are still working on making some equally crazy ideas a reality.
One of those teams is at Princeton Satellite Systems, which is looking to generate megawatts of power using fusion. Yes, we have gone from watts to kilowatts to megawatts. You’re probably familiar with fusion—it happens in the sky every day courtesy of our sun. Fusion produces several times the amount of energy fission creates, but it is hard to control.
Princeton Satellite Systems is developing a direct fusion drive,which uses magnetic fields to generate current in plasma and heat it up to 1 billion °C. The team says the thrust the minivan-size machine would (theoretically) produce would cut inter-solar-system travel times by more than half (trips to Pluto would take about four years rather than nine), with power to spare.
“If you have power when you get there, you can do a lot of really cool experiments,” says the firm’s physicist Charles Swanson. “One of the coolest things Cassini did is radar images of Saturn’s moon Titan. But radar is power hungry and was limited. Having a megawatt of power frees up options.”
The company has received a boatload of funding from NASA and the US Department of Energy, so it looks as if someone believes this moonshot could work. But let’s be frank: it isn’t going to happen anytime soon—or even in our lifetime. Fusion is still in the earliest stages of research here on Earth.
Even so, it’s still fun to imagine what it might make possible. It could be the leap we need to fast-track our trips to the outer planets and beyond.
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June 11-12, 2019