In 2018, at the Nevada National Security Site, scientists completed testing of America’s first new nuclear reactor in about 40 years. This new device was not a typical reactor. Called Kilopower, it was not intended for use on Earth but for use in space. For a total of 28 hours, the reactor core underwent a controlled chain reaction involving uranium 235. This fission generated approximately 3 to 4 kilowatts of thermal energy, which circulated through heat pipes and into a Stirling engine producing electricity.

Someday, such a system could power and propel spacecraft or keep lights on in lunar or Martian habitats. This type of nuclear power source would constitute a major advance; no one has launched a fission reactor since 1965. A series of federal policy statements have helped revive research on these devices, and the possibility has caught the attention of the US Space Force (USSF).

Eric Felt, director of space vehicles at the Air Force Research Laboratory (which supports both air and space forces), spoke with Physics today in the name of the interests of the space force. The branch, he says, is monitoring developments in space nuclear reactors but is not yet funding them. The newly formed branch has no plans to use off-Earth nuclear reactors immediately and has not determined whether or how space fission fits within its portfolio. Felt says technological and political barriers have been reduced; the remaining obstacle is to find a target which requires nuclear fission.

One place where nuclear technology could fit in is cislunar space – the expanse between the Earth and the Moon. The region is set to become more populated in the decades to come. “When there was nothing up there, we weren’t so worried,” Felt says. But as new satellites, spacecraft and space debris occupy the region, the space force wants to track everything, deter and respond to threats, and deal with emergencies that may arise, such as astronaut rescue scenarios. “For example, if SpaceX Spatialship propulsion was to fail on a return trip from the Moon to Earth, and Japanese billionaire Yusaku Maezawa was to be rescued, the USSF would use whatever capabilities we have to help, “says Felt.

Patrick McClure, a Kilopower developer at Los Alamos National Laboratory, believes his technology could help keep tabs on cislunar events: a Kilopower-powered craft could keep long-term surveillance over a large volume of space without relying on the Sun. “In cislunar space, solar power is pretty good,” says David Poston, colleague of McClure, chief designer of the reactor. But a reactor doesn’t need to be aimed at the Sun, and it’s more stealthy. “A solar panel is very large and has to glow in sunlight to work,” Poston explains. Radiation from a reactor is not easily detectable unless you are nearby. “The heater is maybe a tenth the size of solar panels, not reflecting light,” he says, although it will appear with a relatively small infrared glow.

McClure, Poston, and former Los Alamos associate director Andy Phelps turned their innovation into a company called Space Nuclear Power Corp, or SpaceNukes. The team considers their system useful for space because the reactor is self-regulating; it lacks many breakable components such as pumps, valves and filters; and it’s designed to be radiologically benign before it’s ignited in space and to remain subcritical in every rocket launch nightmare imaginable. The 2018 Kilopower test was “a stepping stone to much better performing, 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 realm 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 machine would be powered by a thermo-nuclear system and capable of monitoring a large area. Objects orbiting near Earth are difficult to track, and the problem grows as the propagation increases. “In reality, there is a huge tyranny of distance,” says Nathan Greiner, DRACO program director. He explains that nuclear-pumped spacecraft could better handle such a vast environment because of their maneuverability. “Nuclear thermal propulsion produces a higher specific impulse – also known as ‘gasoline consumption’ – than chemical propulsion,” says Greiner. “This allows nuclear thermal propulsion to perform large maneuvers faster than chemical propulsion.”

Felt met with the DRACO team and agreed to consider partnering with DARPA to take the technology to space force once it matures. The caveat – as with Project Kilopower – is that the space force must first find a “killer application” for it, Felt says. “Perhaps the ‘raison d’être’ would be more precise,” he adds.

Policy considerations

Whether this application is to be found in the cislunar domain or elsewhere, recent political developments will facilitate its trajectory. In 2019, the Trump administration issued Presidential National Security Memorandum-20, which governs the launch of space nuclear systems. He says nuclear power doesn’t have to be mission essential to get the green light. “You don’t need to get special presidential approval and prove there’s no other way to do it,” Felt says. “But I’m kind of joking with the team, ‘Just because someone’s going to let you do that doesn’t mean you should.’ “

The 2019 launch memorandum was followed in 2020 by a space policy directive defining a national strategy for space nuclear and propulsion and in January 2021 by a decree promoting small modular reactors for space.

DARPA plans to take DRACO for a first flight in 2025. SpaceNukes engineers have their own launch ambitions, but also contribute to the DARPA program. Poston is developing reference designs for DRACO and McClure is developing launch security protocols.

Nuclear reactors in space are not without danger. Accidents involving nuclear reactors could put Earth people at greater risk than with conventional spacecraft. And any technology involving uranium could raise international objections, especially if it uses highly enriched uranium (HEU), as Kilopower does. “The problem with HEU is that it’s a military grade material,” says McClure. “There’s always the worry that if we lost it at launch, some bad person would pick it up and do bad things.” Some policymakers and nonprofit groups fear that the production and use of more HEU, which the United States has worked to minimize for decades, poses proliferation risks.

Bomb-ready fuel isn’t completely banned, but its use would place the Kilopower design in the strictest launch approval category. The system can use a less controversial fuel called High Dose Low Enriched Uranium, or HALEU, which doesn’t present the same proliferation risks, but would add 700 kilograms to the mass of the reactor. “We prefer to use HEU because the system is lighter,” says Poston. But by using HALEU, approval to launch DRACO would be greatly simplified.

Zhanna Malekos Smith, Senior Associate at the Center for Strategic and International Studies, sees this moment as an opportunity for the country to “demonstrate its strategic determination and its intentions in space for safety, security and sustainability”. Of course, this sample frame goes both ways. “If the United States were to, say, cut corners to make these improved systems work, other states could potentially see this as something to emulate,” she says.

It remains to be seen whether the United States will set any example; the new White House has yet to define a position on space nuclear energy and propulsion. “These are seeds that were planted,” says Malekos Smith, “and it’s up to the rest of the Biden administration to decide whether they want to grow these seeds and grow them, or grow something else entirely, or eliminate them. . “

Felt isn’t sure the seeds will bloom in space force. But, he says, “I like having the option on the table.”