Deep inside some of NASA's most venerable space probes lie plutonium-filled hearts beating to warm and power the robots, which include the twin Voyager spacecraft, Cassini before its daring plunge through Saturn's rings and New Horizons trekking through the rubble of the Kuiper Belt.
But in the wake of the Cold War, the U.S. stopped producing its own plutonium. For a while, NASA could run its missions solely off existing or imported plutonium. But thanks to a change in the space agency's partnership with the Department of Energy, last summer, fresh American plutonium once again left Earth inside NASA's Mars-bound Perseverance rover — and more missions will do so in years to come. And for scientists who focus on the outer solar system, that's vital.
"Our voyage to discovery requires us to be able to untether ourselves from our own solar system," Abigail Rymer, a space physicist at the Johns Hopkins University Applied Physics Laboratory in Maryland and a member of the Outer Planets Assessment Group that advises NASA, told Space.com. "We need to be able to take our home with us; our power supply needs to not be reliant on our own star. The further we go out, the more true that becomes."
There are only a few familiar, reliable ways to power a spacecraft; plutonium and sunlight are the most common choices. But as a probe moves farther out into the solar system and farther away from the sun, sunlight quickly loses its power: NASA's Juno spacecraft currently orbiting Jupiter, for example, required advances in solar power technology to survive without nuclear technology, Rymer said.
So, if you want to send a spacecraft to our solar system's giant planets and beyond, or to other dark places like the permanently dark regions deep in craters near the moon's poles, you'll likely want nuclear power. That preference isn't just about sunlight either; nuclear power also helps spacecraft confront threats like low temperatures and high radiation.
"It allows us to explore where the sunlight doesn't get, but it also allows us to explore harsh environments, and that's because we can take our heat with us," June Zakrajsek, the Radioisotope Power Systems (RPS) program manager for NASA at the Glenn Research Center in Ohio, told Space.com. "It's the reliability and those kinds of factors that are really important for our missions, and we couldn't do some of the missions without it."
NASA's next plutonium-powered spacecraft will be the Dragonfly rotorcraft mission, launching in 2027 to Saturn's strange moon Titan, which NASA says receives about 1% of the sunlight that Earth does. Because of Dragonfly's nuclear power source, the spacecraft will probably freeze to death in the landscape of liquid methane and towering water-ice cliffs long before it runs out of power, Zakrajsek said.
Dragonfly belongs to a class of NASA missions dubbed New Frontiers, the agency's more ambitious tier of planetary science expedition proposals that NASA accepts from scientists beyond its centers. The fact that NASA even considered Dragonfly — much less selected it — speaks to the agency's progress working with the Department of Energy to increase the supply of plutonium available to mission designers.
In the selection campaign for the previous New Frontiers mission, Zakrajsek said, NASA stipulated that spacecraft must operate without nuclear power, since the agency wasn't sure that the partnership would have enough plutonium to supply a new mission. (The OSIRIS-REx mission to sample near-Earth asteroid Bennu was selected during that round.)
Zakrajsek called the fuel's availability for the more recent selection "a big deal." "The fact that we're not making mission-limiting decisions anymore based on RPS [radioisotope power systems] is important," she said. "It seems to be making the scientists a little bit more happy."
The transition is due, in part, to a NASA decision to annually evaluate its plutonium needs for the next decade, she said, giving the partnership more preparation to ensure the necessary supply. It's also due to the Department of Energy's decision to produce spacecraft plutonium at a steady rate — a stark change from its previous process.
"NASA would approach the department and let us know, 'Hey, we've got a mission coming up,'" Tracey Bishop, the deputy assistant secretary for nuclear infrastructure programs at the Department of Energy, told Space.com. "We would pull the equipment out of standby, go and hire new staff, requalify equipment and processes, manufacture fuel, support the development of the RPSs — and once the mission was over, we would stand down the capabilities and put them in cold standby until the next addition."
That system was designed in part because NASA spacecraft are the Department of Energy's only use for this particular material, plutonium oxide also dubbed plutonium-238. And spacecraft may not be what anyone thinks of first when asked about plutonium. "Our use is by far the least famous of the things that plutonium gets used for," Rymer said. (The plutonium used in nuclear weapons and reactors includes an extra neutron compared to the spaceflight variety.)
But in 2017, NASA and the Department of Energy decided that the stop-and-go process was too risky for the spacecraft that can't launch without plutonium-238. Bishop said that with the new steady-production system, the agency hopes to shave as much as two years off the production timeline, which could last up to a decade.
The plutonium aboard the Perseverance rover speaks to the impact of this new approach. The Department of Energy hadn't planned to supply that particular spacecraft with fuel. But the first plutonium from the new production process was ready and needed to be evaluated anyway, so once the agency determined it met NASA's requirements, program officials decided to go ahead and finish preparing the material a couple years ahead of schedule to test the systems, Bishop said. She noted that the project's success increased the Department of Energy's confidence that it can meet NASA's fuel needs into the future.
"It's really easy to turn the dial a little bit if a mission projection changes, versus the dial being off and now you've got to turn it on and wait for it to warm up and build up into the process," she said. "Now it's more of a fine-tuning."
While the Department of Energy ramps up plutonium production, NASA is working on developing the next generation of power systems that will hold that plutonium, Zakrajsek said, with work focused on two different approaches.
One, called a dynamic radioisotope power source, can be three or four times more efficient than the current standard, the multi-mission radioisotope thermoelectric generator (MMRTG). However, the dynamic system is tricky because, as the name suggests, it incorporates moving parts.
"Space is hard, and it's really hard on systems that move," Zakrajsek said. NASA is currently working on the design for such a system, which could potentially be ready for a test flight on the moon near the end of the decade, she noted.
The second approach builds on NASA's own legacy, grounded in the very first spacecraft nuclear power systems. This system would be a honed, more efficient version of the General Purpose Heat Source (GPHS) RTG units that flew on Galileo, Cassini and New Horizons. Zakrajsek said this sort of power source would be particularly appealing for larger missions heading to Neptune or Uranus.
Coincidentally, Rymer led a team exploring how just such a hypothetical mission might study Neptune and its largest moon, Triton. She described the challenging process of trying to "shave every watt and gram that we can" off the instruments to meet the constraints of launch opportunities and power supplies without sacrificing scientific goals.
"It's an enormous effort because it's one of the things that we can control," she said. "Physics tells you how much power you need to survive to get to your target, but we have scientists and engineers who can actually optimize how much power you need to use when you get there, so we do focus on that quite a lot."
Based on current production schedules, if the only preceding spacecraft to use nuclear power were the Dragonfly mission, there would be — just barely — enough plutonium to power the hypothetical Neptune/Triton mission.
However, there's a chance that another mission will join the queue. NASA will soon announce which of four finalists it has selected in the smaller Discovery class of missions. (That program includes spacecraft like the Lunar Reconnaissance Orbiter and the Mars geology lander InSight, as well as future asteroid missions Lucy and Psyche.)
One of the four finalists, Trident, would explore Triton — and would rely on two nuclear power units. Chances are, if that mission is selected, NASA would skip a larger Neptune/Triton idea to avoid duplicated science work, but there are plenty of other distant worlds worth exploring. And the continuing specter of a shortage points to the constraints that scientists still must have in the back of their minds when considering future spacecraft.
It's just that constraint that NASA hopes the retooled partnership with the Department of Energy will eliminate.
"If the plutonium had been being produced consistently throughout, then we would have a nice big stockpile of it," Rymer said. "I hope that that's going to be the situation that we're going to be in in not too many years."
Email Meghan Bartels at firstname.lastname@example.org or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.