Antares has received a $161 million Strategic Breakthrough award to conduct a ground demonstration of its R1-S nuclear reactor and integrate the system with a spacecraft for flight certification, the company announced September 11. The award, issued through the Office of the Assistant Secretary of the Air Force for Space Acquisition and Integration, moves Antares toward an in-orbit reactor demonstration for U.S. Space Force missions.
Antares described the contract as the largest current U.S. defense award for space nuclear power. The company did not disclose the intended spacecraft, launch provider, orbital destination or target launch date.
The program will include a nuclear ground demonstration of R1-S, a space-oriented version of Antares’ heat-pipe microreactor architecture. Antares will then integrate the reactor with a spacecraft and complete the certification work required before launch.
The award also calls for Antares’ electricity-producing Mark-1 reactor to power an operational ground-to-space asset. Mark-1 is scheduled to begin testing in 2027 and operate for more than six months while coupled to a closed Brayton-cycle power conversion system using nitrogen as its working fluid.
A Brayton-cycle system converts reactor heat into electricity by circulating compressed gas through a turbine and generator. The closed configuration recirculates the working fluid rather than exhausting it, making the approach suitable for isolated installations and potentially adaptable to spacecraft, where continuous electrical generation must operate without atmospheric intake.
Heat pipes provide a largely passive method of moving thermal energy from the reactor core to the power conversion system. Their limited reliance on mechanical pumps can reduce the number of moving components, although an orbital reactor must still satisfy demanding requirements for thermal rejection, radiation shielding, autonomous control, launch survival and long-duration reliability.
From Criticality to Power Production
The Space Force award follows Antares’ first nuclear milestone. On June 4, the company’s Mark-0 microreactor completed a zero-power criticality demonstration at Idaho National Laboratory under Department of Energy authorization.
Criticality means that a reactor has established a self-sustaining nuclear fission chain reaction. The Mark-0 test verified core physics but was not designed to produce usable electricity. Mark-1 is intended to take the next step by generating power and operating continuously for an extended period.
Antares was the first private company to bring an advanced reactor to criticality through the Energy Department’s Reactor Pilot Program. The company has since been selected for additional military microreactor work, including an Army program involving a proposed reactor deployment at Fort Bragg, North Carolina.
The progression from Mark-0 to Mark-1 and R1-S separates three distinct development stages: validating reactor physics, demonstrating sustained electricity production and qualifying a reactor-based power system for spacecraft integration. Each stage introduces additional engineering, safety and operational requirements.
“Space has been core to the Antares thesis since the company was founded,” Antares co-founder and Chief Executive Jordan Bramble said. “This partnership with the Space Force turns that long-held vision into a codified space mission.”
Continuous Power for Maneuverable Spacecraft
The award supports the National Initiative for American Space Nuclear Power established under National Security and Technology Memorandum-3, as well as Executive Order 14369, “Ensuring American Space Superiority.” The federal policy calls for deploying reactors in orbit and developing a lunar surface reactor ready for launch by 2030.
Nuclear power could support spacecraft that require more continuous electricity than conventional solar arrays and batteries can provide, particularly during eclipses, operations far from the Sun or missions with high computing and communications loads. Potential military applications include persistent sensing, electronic warfare, high-throughput onboard processing and power-intensive payloads.
A reactor does not by itself provide propulsion. Its electricity must be paired with electric thrusters or another propulsion system before it can support sustained maneuvering. Nuclear electric propulsion could, however, reduce dependence on the available solar flux and provide steady power for long-duration orbit changes or cislunar operations.
The United States has launched only one nuclear fission reactor into space: the SNAP-10A experimental system in 1965. Radioisotope power systems used on planetary missions are different because they generate electricity from the natural decay of radioactive material rather than a controlled fission chain reaction.
Antares’ near-term schedule centers on the Mark-1 test campaign in 2027. A successful six-month power demonstration would provide operational data needed to support the subsequent R1-S spacecraft integration, flight certification and launch preparations.
