Space Nuclear Power Moves From Paper to Hardware: A DARPA Reactor Test and NASA's SR-1 Freedom

The United States has started and canceled space reactor programs for decades. In the same week of October 2026, two announcements pointed toward hardware. Space Nuclear Power Corporation (SpaceNukes) said DARPA awarded it an agreement to ground-test an actively controlled, autonomous version of the Kilopower reactor. Separately, NASA reported progress on the nuclear power module for Space Reactor-1 (SR-1) Freedom, the fission-powered spacecraft it plans to launch to Mars in late 2028.

The DARPA Award: Teaching a Space Reactor to Control Itself

On Oct. 21, 2026, SpaceNukes announced that DARPA had awarded it an Other Transaction Agreement under the Fast Capacitance (Fat Cat) program, managed by Dr. Tabitha Dodson in DARPA's Defense Sciences Office. The company says the program aims to advance autonomous reactor operation for national security missions.

Kilopower is a small fission reactor concept developed by Los Alamos National Laboratory and NASA to supply reliable electricity in space or on planetary surfaces, where solar power is limited.

The new work builds on KRUSTY (Kilopower Reactor Using Stirling Technology), a 2018 test the company calls "the only full-power ground test of a new American space reactor in five decades." Key members of the SpaceNukes team did that work while employed at Los Alamos and NASA.

The difference is how the reactor is controlled. KRUSTY was passively controlled. Under the DARPA agreement, SpaceNukes and Los Alamos will design, build and run a follow-on nuclear ground test of a Kilopower reactor with active control rod operation, which the company says supports missions that need power to be managed dynamically and to respond quickly as conditions change.

The program moves through design, procurement and integration gates before the test. Integration will take place at SpaceNukes' workshop in Medina, Ohio, led by Chief Engineer Marc Gibson. The release does not give the contract's value or a test date. "There is no substitute for demonstrated technology when American national security objectives are at stake," said CEO Andy Phelps.

NASA's SR-1 Freedom: The Power Module Takes Shape

On Oct. 20, 2026, NASA said it and its partners had agreed to key contract actions to speed up SR-1 Freedom, which NASA describes as the first fission-powered spacecraft to travel beyond Earth orbit.

Nuclear electric propulsion (NEP) uses heat from a fission reactor to generate electricity, which then powers electric thrusters. It produces low thrust but is very efficient over long periods.

According to NASA:

• Sierra Lobo Inc. will provide the power module's closed Brayton cycle power conversion system, which turns the reactor's heat into electricity. The work falls under a NASA contract awarded in 2024.
• The Department of Energy's Idaho National Laboratory is designing and developing the fission reactor, which will be integrated with the conversion system to complete the nuclear power module.
• The reactor stays inactive until the spacecraft leaves Earth orbit.
• SR-1 Freedom is set to launch in late 2028 carrying SkyFall, a team of three Mars helicopters based on the Ingenuity design.

Program director Steve Sinacore said the contract actions "reduce technical risk" as NASA works toward its launch target. EconoTimes adds that the spacecraft will use the Power and Propulsion Element developed for NASA's Gateway program.

MIT Technology Review adds detail: the reactor will have a capacity of at least 20 kilowatts, will be switched on about two days after launch, and the trip to Mars should take about a year, with arrival in 2029. Sinacore told the magazine that "procurement of major components is underway."

Why This Time Might Be Different

This section is analysis, drawing on MIT Technology Review's reporting.

MIT Technology Review notes the United States has started and terminated several space nuclear programs over about 75 years. The most recent was DRACO, a NASA and Department of Defense nuclear propulsion project that ended in 2025. The magazine says it was canceled over high experimentation costs, an inability to compete with conventional rockets on price, and difficulty doing ground tests safely and effectively.

Two things stand out now. First, both efforts are about electric power, not nuclear-thermal thrust. Second, both have concrete hardware milestones: a nuclear ground test for SpaceNukes, and named suppliers and a national laboratory for SR-1. MIT Technology Review calls SR-1 a roughly $2 billion bet and reports that the urgency is tied partly to Chinese and Russian discussions of placing a reactor on the Moon in the mid-2030s. NASA says SR-1 should also help pave the way for fission systems for its planned Moon base, a goal we covered in our piece on the NASA and Energy Department space nuclear agreement.

The timeline is tight. Lindsey Holmes, a vice president at Analytical Mechanics Associates, called it "an aggressive timeline" to MIT Technology Review. Flight qualification, launch safety reviews and radiation shielding all still need to come together, and the DARPA test has no public date or price.

Why It Matters

• Space reactors are moving from studies to contracts. There is a ground test with a government sponsor and a flight program with named suppliers.
• Control is the new technical question. KRUSTY showed a passive design could work. An actively controlled reactor is closer to what long missions and bases would need.
• Timelines are the risk. A late-2028 launch and an undated ground test leave little room for slips, and earlier programs were canceled over cost.
• The Moon and Mars goals depend on it. Surface bases and deep-space transit both need reliable power that sunlight cannot provide.