Antares Nuclear Microreactor Achieves Criticality, Securing $161 Million Space Force Contract

Aerospace contractors maintain that solar arrays and chemical batteries are the only viable power sources for orbital assets, forcing military satellites to strictly ration their maneuvers. The U.S. Space Force is now funding the hardware to break that constraint. On September 11, 2026, the military branch awarded Antares Nuclear a $161 million Strategic Breakthrough contract to develop a space-flight certified microreactor, building directly on the company's historic terrestrial reactor demonstration three months earlier.[2]

The federal award tasks the California-based startup with conducting a ground demonstration of its R1-S nuclear system and subsequently integrating the reactor with a spacecraft for flight certification. The military objective is to deploy a medium-power reactor by 2031 to support heavy computational loads, directed energy weapons, and electromagnetic warfare systems in orbit—hardware that requires continuous, high-output electricity that solar panels cannot reliably provide.[2][4]

"Space has been core to the Antares thesis since the company was founded," said Jordan Bramble, CEO and co-founder of Antares. "This partnership with the Space Force turns that long-held vision into a codified space mission."[2]

The orbital contract serves as a direct financial endorsement of the company's terrestrial engineering. On June 4, 2026, Antares achieved zero-power fueled criticality with its Mark-0 demonstrator at the Idaho National Laboratory. The unit became the 53rd reactor built at the site since 1951, and the event marked the first time in more than 40 years that a privately developed, non-light-water reactor reached criticality within the United States.[1][2][3]

Criticality occurs when a nuclear fission chain reaction becomes self-sustaining. During the June 4 test, the Mark-0 reactor sustained this reaction at a zero-power level, meaning each fission event released sufficient neutrons to maintain the series of reactions without producing significant thermal energy. The demonstration served to validate the core physics parameters of the company's sodium heat-pipe-cooled design.[3]

Unlike conventional nuclear plants that rely on high-pressure water pumps, the Antares system uses passive liquid-metal heat pipes to transport heat away from the core. The reactor is fueled by tri-structural isotropic (TRISO) particles containing high-assay low-enriched uranium, a configuration designed to operate autonomously for years without refueling.[3]

The terrestrial version of the hardware is engineered for rapid deployment to remote or off-grid locations. Following the successful zero-power test, Antares is preparing to run a full-power, electricity-producing demonstration of its Mark-1 reactor for six months in 2027. That system will integrate the nuclear core with a nitrogen closed Brayton cycle power conversion unit.[2][3][4]

The U.S. Army is closely monitoring the terrestrial development through its Janus Program, which aims to deploy microreactors to military bases to ensure energy resilience during grid outages. Following the June 4 test, Antares stated that the established licensing pathway keeps the company on schedule to deploy operational electricity-producing microreactors to U.S. military installations by the end of September 2028.[3]

The rapid timeline reflects a broader federal strategy to accelerate nuclear deployment. The Mark-0 test was conducted under the Department of Energy's Reactor Pilot Program, an initiative launched by a 2025 executive order that allows companies to test early-stage designs at federal laboratories under agency authorization, bypassing the standard Nuclear Regulatory Commission commercial licensing process for initial validation.[1][3]

While the $161 million Space Force contract secures the orbital pathway, the terrestrial timeline depends entirely on the upcoming 2027 full-power demonstration. The next verifiable checkpoint arrives when the Mark-1 reactor attempts to convert its sustained fission reaction into actual electricity on the grid.[2]