A possible future for NASA’s forays into deep space can be found at the Department of Energy’s Oak Ridge National Laboratory in Tennessee, at the bottom of what looks like an indoor swimming pool.

There, bathed in the electric-blue light of the nuclear High Flux Isotope Reactor, aluminum tubes packed with small, silvery cylinders of the radioactive element neptunium-237 are being bombarded with neutrons. It is modern-day alchemy; the neutrons are transmuting the neptunium into something that, at least to NASA’s mission planners, is more precious than gold: plutonium-238 (Pu-238), one of the rarest and most fleeting materials in the universe. Once made, the Pu-238 will glow red-hot for years on end as it gradually decays into uranium. Pu-238 cannot be used to make atomic bombs, nor is it particularly useful for fueling nuclear reactors, which are widely considered too controversial and expensive for practical use in space missions. Instead, Pu-238’s steady supply of heat makes it an ideal power source for long-haul interplanetary voyages where conditions may be too dim and cold for solar power and chemical batteries.

But NASA’s supply is running out, and the Department of Energy’s efforts to make more at Oak Ridge are proceeding too sluggishly for comfort. Alarmed members of Congress have repeatedly demanded that NASA produce studies detailing just how much plutonium it needs, how it plans to acquire the plutonium, and what‘s at stake if the stockpile runs out, but to date, those demands have not passed into law. The latest push came in late July, when Senator Rob Portman and Representative Steve Stivers, both from Ohio, each introduced their own version of the Efficient Space Exploration Act, which mandates such reports. Both bills remain in committee and have not been brought to a formal vote.

Without sustained support and clear direction, NASA’s nascent efforts to shore up more Pu-238 could falter, and further missions to the darkest depths and corners of the solar system could become impossible. Unless, as some nuclear-shy mission planners advocate, NASA manages to use solar power farther out from the Sun than previously thought possible.

More heat than light

Over the past half-century, NASA has used a total of 140 kilograms of Pu-238 to push the frontiers of exploration. Coupled to one of the agency’s “thermoelectric” generators that convert heat into electricity, four kilograms of the stuff can power a spacecraft for decades. Pu-238 was used in Apollo-era science experiments on the Moon, in the Galileo mission to Jupiter and in the Pioneer and Voyager space probes now exiting our solar system. Hefty hunks of Pu-238 power the Mars Curiosity rover, the Cassini orbiter at Saturn and the New Horizons spacecraft now roaming beyond Pluto. In the future, Pu-238 could power robotic probes to burrow beneath the ice of ocean-bearing moons, planes to fly in the alien atmospheres of other worlds, ships to sail the liquid ethane seas of Saturn’s moon Titan and much, much more.

Those future missions can only occur if there’s enough plutonium to go around. Practically all of NASA’s Pu-238 stockpile was made as a byproduct of building nuclear weapons during the Cold War. As the Cold War wound down, so too did the Department of Energy’s Pu-238 production; it made its last batch in 1988, shutting off NASA’s supply save for occasional deliveries of small, lower-quality batches from Russia that ceased in 2010. At present, only about 35 kilograms of Pu-238 are left for the space agency, and radioactive decay has rendered all but 17 kilograms too weak to be readily used in NASA’s thermoelectric generators. NASA and DOE officials estimate there is only enough for four more generators, one of which is already committed to NASA’s upcoming Mars 2020 rover.

In 2013, after a quarter-century hiatus, the DOE began making Pu-238 again, deep in the reactor pool at Oak Ridge. But ramping up to full capacity is proving to be a slow, frustrating process. The project is behind schedule and unlikely to reach its target of producing 1.5 kilograms of generator-ready plutonium per year by 2021—instead, DOE officials say, the pipeline might be annually producing less than a third of that amount by 2019.

DOE officials don’t portray this slow start to plutonium production as a problem, because NASA has yet to identify exactly how the new plutonium would be used. “The project is planning to produce new Pu-238 at a rate that supports currently projected NASA missions,” says Rebecca Onuschak, program director for the DOE’s plutonium infrastructure. “There is no shortfall of material projected to meeting those needs, and so no remedy or action plan has been implemented.”

But outside experts familiar with the situation say this explanation is a classic example of circular reasoning. “There are tons of things NASA could use the new plutonium for, but there’s nothing on the books,” says Casey Dreier, director of advocacy at the Planetary Society. “Why is that? Because NASA isn’t sure when or if it will have the plutonium it needs!”

Part of the problem is that plutonium is very expensive to make, and no one is particularly eager to pay for it: The production pipeline is scattered between national labs in Idaho, Tennessee and New Mexico, and maintaining it all costs upwards of $50 million per year. Historically, the DOE covered those costs, but that all changed when production restarted in 2013, because Congress and the White House shifted the financial burden of restarting and maintaining production solely to NASA.

NASA’s leadership passed the buck to the agency’s Planetary Science division, because interplanetary missions are NASA’s primary plutonium consumers. Perversely, because the funding for plutonium production comes from the division’s technology development budget, this channels money away from projects exploring how to sustainably use the scarce plutonium supply. In November of 2013, the agency drastically scaled back its Advanced Stirling Radioisotope Generator program, which had spent more than a decade developing high-efficiency piston-based heat exchangers that could provide just as much power as NASA’s thermoelectric generators using three-quarters less plutonium. In its stead, the agency is exploring how to retrofit its thermoelectric generators with new components to give them more modest efficiency boosts.

“This money is supposed to be used for things like developing robotic ascent vehicles to return samples from Mars, or the technologies you need to do landings on icy moons in the outer solar system, stuff like that,” Dreier says. “But now a lot of it is instead going straight out the door to the DOE. This part of the budget really isn’t for technology development anymore—it’s for maintaining the infrastructure for making this type of plutonium.”

Barring major policy changes, plutonium production will continue to drain NASA’s planetary science budget to the tune of at least $50 million per year for as long as the agency exists, potentially closing as many doors as it opens for future interplanetary exploration.

A solar-powered solution?

As the stockpile dwindles and the costs of rejuvenating it continue to rise, NASA’s planetary science community has experienced what seems to be a chilling effect. Missions calling for large amounts of plutonium have become, for lack of a better term, radioactive. No one wants to get too close to such missions, because they have a disconcerting tendency to mutate, weaken and die. In some cases, mission planners now work overtime to justify not using plutonium on missions long thought to require it.

Consider, for instance, NASA’s on-again, off-again plans for exploring Jupiter’s moon Europa, which harbors a vast and possibly life-friendly ocean buried beneath its surface ice. For decades planetary scientists have dreamed of sending a nuclear-powered spacecraft to orbit Europa, armed with a suite of sophisticated instruments to seek out signs of habitability and life. But those dreams have been deferred again and again because of the large amounts of plutonium they would require. One recent well-regarded proposal, dubbed the Jupiter Europa Orbiter, would have used some 17.6 kilograms of Pu-238 when it launched in the 2020s—more than NASA’s entire present viable stockpile.

Instead of pursuing a nuclear-powered Europa orbiter, NASA announced earlier this year that it plans to fly a smaller, cheaper mission in the 2020s that will loop around the icy moon in a complicated and carefully orchestrated series of flybys. Despite the downgrade, the mission in some ways might be considered more ambitious than its plutonium-hungry predecessors, because it will be solar powered.

According to Barry Goldstein, the mission’s project manager at NASA’s Jet Propulsion Laboratory, solar simply proved to be the superior choice for practical rather than political reasons. Solar arrays can be “grown by the watt,” flexibly sized in precise accordance with a spacecraft’s power needs, whereas plutonium-filled thermoelectric generators only come in large, modular chunks. The Europa mission will likely require solar arrays some 50 square meters in size that use specialized “low intensity, low temperature” solar cells optimized for efficient operations in deep space. This solar-cell technology will undergo its most crucial field test beginning in July 2016, when it arrives at Jupiter on another solar-powered spacecraft, NASA’s Juno mission.

“I don’t want to discount the fact that we effectively have a canary on the way to the Jupiter coal mines for us,” Goldstein says. “But we have been tracking the performance of Juno’s arrays against our modeling and our tests at lower-intensity radiation doses, and they’re tracking right on.” When Juno arrives at Jupiter next year, Goldstein says, he fully expects its performance to help calibrate and validate the Europa mission’s solar-power plans.

There is, however, more to the choice of solar at Europa than first meets the eye. The National Space Policy of the United States mandates that NASA can use nuclear power sources if and only if they “significantly enhance” any given space mission. “The rules say that we can only use a nuclear power system if other options can’t get the job done,” says Curt Niebur, NASA’s Europa program scientist. “And for a multiple flyby mission of Europa a solar power system can get the job done. So we chose solar.”

A careful reading reveals a troubling flaw in this logic, because no consensus exists on how far solar power can be pushed. Lacking a clearly defined boundary beyond which solar becomes unreasonable, and relying in part on relatively unproven technology, mission planners could soon find themselves on a slippery slope, sliding into uncertainties that inadvertently preclude the use of nuclear power for practically all space missions.

Already, some planetary scientists are earnestly discussing more solar-powered missions that could fly beyond Jupiter to Saturn and other far-flung targets in the outer solar system. Solar energy diminishes drastically the farther one travels from the Sun—every doubling of distance decreases the amount of sunlight received by a factor of four. Saturn is almost twice as far from the Sun as Jupiter, meaning that a Saturnian equivalent of NASA’s planned Europa spacecraft would probably require solar panels exceeding 200 square meters in size.

While certainly unwieldy, such large solar arrays are not impossible, and could even be seen as more practical than using politically charged and vanishingly scarce plutonium for the foreseeable future. And the prospect of more technological development makes the prospect of solar power at the outer planets seem less daunting: Additional improvements in the efficiency of solar cells or the use of sunlight-concentrating lenses or mirrors could further shrink the required area or mass of a solar array. Failing that, missions could always be downsized to accommodate solar, sacrificing science objectives to simply get off the ground. But beyond some ill-defined point, presumably diminishing science returns should make solar more trouble than it is worth.

“There are some missions that just cannot be done on solar power,” says Ralph McNutt, an expert in space-based nuclear power systems at Johns Hopkins University’s Applied Physics Laboratory. “Without [Pu-238] neither the Voyagers, nor Cassini, nor New Horizons would currently be bringing back cutting-edge science data. Consider also that both the Huygens probe on Titan and the Philae lander [on Comet 67P/Churyumov–Gerasimenko] would still be operating if they had nuclear rather than chemical power sources.”

Even so, McNutt acknowledges, nuclear “would likely have been cost prohibitive for both” Huygens and Philae. In other words, launching missions severely handicapped by relying on non-nuclear power is preferable to launching none at all. A future era of solar-dominated spaceflight might allow more deep-space missions to take place, but those missions would be less capable and shorter-lived than they might otherwise have been with nuclear power. Whether this would be a worthy trade-off is not a question most planners for NASA missions are presently eager to answer.

Still, for those who believe nuclear power in space is worth pursuing, there is some reason for optimism: At the bottom of the electric-blue pool in Tennessee, the nuclear alchemy goes on.

“We’re in really good shape compared to where we were five, ten years ago,” Dreier says. “Back then Russia was about to stop selling us plutonium, and we had no plan to create new plutonium to restore our rapidly diminishing national supply. NASA was in a bad situation. It’s taken a long time because of the delicacy of the subject. But in Oak Ridge right now they’re bombarding neptunium to make plutonium. And they’re not doing that to make bombs—they’re doing it to send little machines to worlds billions of miles away, because who knows what’s out there? Isn’t that a beautiful thing?”