In 2018 at the Nevada National Security Site, scientists finished tests of the first new US nuclear reactor design in about 40 years. That new device wasn’t a typical reactor. Called Kilopower, it was meant not for use on Earth but for use in space. For a total of 28 hours, the reactor core sustained a controlled chain reaction involving uranium-235. That fission generated approximately 3 to 4 kilowatts of thermal energy, which flowed through heat pipes and into an electricity-producing Stirling engine.
Someday, such a system could power and propel spacecraft or keep the lights on in lunar or Martian habitats. That sort of nuclear electricity source would be a major advance; no one has launched a fission reactor since 1965. A series of federal policy declarations has helped revive research on those devices, and the possibility has caught the eye of the US Space Force (USSF).
Eric Felt, space vehicles director at the Air Force Research Laboratory (which supports both the air and space forces), spoke to Physics Today on behalf of the space force’s interests. The branch, he says, is monitoring developments in space-based nuclear reactors but not yet funding them. The recently established branch has no immediate plans to use off-Earth nuclear reactors and hasn’t determined if or how space fission fits into its portfolio. Felt says both the technological and policy barriers have shrunk; the obstacle that remains is to find a goal that needs nuclear fission.
One place where nuclear technology could fit is cislunar space—the expanse between Earth and the Moon. The region is set to grow more populated in the coming decades. “When there was nothing up there, we were not as concerned about it,” says Felt. But as new satellites, spacecraft, and space debris occupy the region, the space force wants to keep track of it all, deter and respond to threats, and deal with emergencies that may arise, such as astronaut rescue scenarios. “For example, if the SpaceX Starship propulsion were to fail during a return journey from the Moon to the Earth, and Japanese billionaire Yusaku Maezawa needed to be rescued, the USSF would use whatever capabilities we have to assist,” Felt says.
Patrick McClure, a Kilopower developer at Los Alamos National Laboratory, believes his technology could help keep an eye on cislunar goings-on: A Kilopower-powered craft could keep a long-lived watch over a large volume of space without relying on the Sun. “In cislunar space, solar power is pretty good,” says McClure’s colleague David Poston, the chief reactor designer. But a reactor doesn’t need to be oriented toward the Sun, and it’s stealthier. “A solar array is very large and has to shine in the sunlight to work,” says Poston. Radiation from a reactor isn’t easily detectable unless you’re in close proximity. “The radiator is perhaps one-tenth the size of solar panels, not reflecting light,” he says, although it will show up with a comparatively small IR glow.
McClure, Poston, and former Los Alamos associate director Andy Phelps have spun out their innovation into a company called Space Nuclear Power Corp, or SpaceNukes. The team sees its system as space-useful because the reactor self-regulates; it lacks many breakable component parts such as pumps, valves, and filters; and it is designed to be radiologically benign before it’s turned on in space and to stay subcritical in every rocket-launch nightmare imaginable. The 2018 Kilopower test was “a stepping-stone to much more capable, high-power systems that could be used for rapid movements of assets and very high power for applications,” says Poston.
SpaceNukes is not the only group working on nuclear reactors for cislunar “space domain awareness.” Among others, the Defense Advanced Research Projects Agency (DARPA) is pursuing a related project, the Demonstration Rocket for Agile Cislunar Operations, or DRACO. The craft would be propelled by a nuclear-thermal system and capable of surveilling a large area. Keeping track of orbiting objects is difficult near Earth, and the problem expands as the spread increases. “Realistically, there’s a huge tyranny of distance,” says Nathan Greiner, DRACO program manager. He explains that nuclear-pumped spacecraft could better handle such vast surroundings because of their maneuverability. “Nuclear thermal propulsion produces higher specific impulse—aka ‘gas mileage’—than chemical propulsion,” says Greiner. “That allows nuclear thermal propulsion to do large maneuvers quicker than chemical propulsion.”
Felt has met with the DRACO team and agreed to consider partnering with DARPA to transition the technology to the space force once it’s mature. The caveat—as with the Kilopower project—is that the space force must first find a “killer app” for it, says Felt. “Maybe ‘raison d’être’ would be more accurate,” he adds.
Should that application be found in the cislunar domain or elsewhere, recent policy developments will smooth its trajectory. In 2019 the Trump administration published National Security Presidential Memorandum–20, which governs the launch of space nuclear systems. It states that nuclear power doesn’t have to be essential to a mission in order to be greenlighted. “You don’t have to go get special presidential approval and prove there’s no other way to do it,” says Felt. “But I kind of joke with the team, ‘Just because somebody’s going to let you do it doesn’t mean you should.’ ”
The 2019 launch memorandum was followed in 2020 by a space policy directive laying out a national strategy for space nuclear power and propulsion and in January 2021 by an executive order promoting small modular reactors for space.
DARPA plans to take DRACO for a first flight in 2025. The SpaceNukes engineers have their own launch ambitions but are also helping with DARPA’s program. Poston is drawing up reference designs for DRACO, and McClure is developing launch safety protocols.
Nuclear reactors in space are not without hazards. Accidents involving nuclear reactors could put Earthlings at more risk than with conventional spacecraft. And any technology involving uranium could stir international objections, especially if it uses highly enriched uranium (HEU), as Kilopower does. “The problem with HEU is it’s weapons-grade material,” says McClure. “There’s always the concern that, if we lost it on launch, some bad person would recover it and do nefarious things.” Some policymakers and nonprofit groups worry that producing and using more HEU, which the US has worked to minimize for decades, poses proliferation risks.
The bomb-ready fuel isn’t banned completely, but its use would put the Kilopower design in the most stringent launch-approval category. The system can use a less-controversial fuel called high-assay low-enriched uranium, or HALEU, which does not carry the same proliferation risks, but it would add 700 kilograms to the reactor’s mass. “We prefer to use HEU because the system is lighter,” says Poston. But by using HALEU, DRACO’s launch approval would be greatly simplified.
Zhanna Malekos Smith, a senior associate at the Center for Strategic and International Studies, sees this moment as an opportunity for the country to “demonstrate its strategic resolve and intentions in space when it comes to safety, security, and sustainability.” Of course, that example-setting cuts both ways. “If the US were to, let’s say, cut any safety corners for operating these enhanced systems, other states may potentially view that as something to emulate,” she says.
Whether the US will set any kind of example remains to be seen; the new White House hasn’t yet laid out a position on space nuclear power and propulsion. “These are seeds that have been planted,” says Malekos Smith, “and it is up to the rest of the Biden administration to decide whether they wish to cultivate those seeds and grow them, or grow something else entirely, or take them out.”
Felt is not sure the seeds will flower within the space force. But, he says, “I like to have the option on the table.”