US to Allocate $161 Million for Orbital Nuclear Reactor Development
U.S. company Antares has won a U.S. Air Force competition to develop a space nuclear reactor demonstrator and will receive $161 million for the project.
Antares announced this in a press release.
Under the program, the company will conduct a ground demonstration of the R1-S nuclear reactor, then integrate it with a spacecraft for flight certification before a possible launch.
The technical specifications of the prospective R1-S have not been publicly disclosed. It is known, however, that the ground-based R1 reactor family uses HALEU fuel in the form of TRISO particles, a graphite core, sodium heat pipes, and a closed-loop nitrogen Brayton cycle. Antares states that the family provides electrical power ranging from 100 kW to 1 MW and has a service life of more than six years. However, these figures do not directly apply to the space-based R1-S.
The company will also use the Mark-1 reactor to power an unnamed “operational ground-space asset.” This likely refers to ground infrastructure supporting space operations, although Antares has not specified its type or location.
On June 4, 2026, the Antares Mark-0 microreactor reached criticality for the first time at Idaho National Laboratory with authorization from the U.S. Department of Energy. This made Antares the first private company to bring an advanced reactor to criticality under the U.S. Department of Energy’s pilot reactor program.
The Mark-0 was a zero-power reactor and did not generate electricity. It was intended to verify core physics, reactivity margin, and the operation of control and emergency shutdown systems. In 2027, the next Mark-1 reactor is expected to operate for more than six months together with an energy conversion system based on a closed-loop nitrogen Brayton cycle.
U.S. Space Nuclear Programs
The United States is developing several space nuclear power programs for scientific, research, and military missions. The NSTM-3 initiative, approved in April 2026, calls for developing small- and medium-power reactors for operation in orbit and on the lunar surface, as well as preparing to deploy systems producing at least 100 kW in the 2030s.
NASA and the Pentagon will conduct separate competitions but, where possible, will use common reactor design elements, nuclear fuel, energy conversion systems and ground infrastructure.
The first flight demonstration is to be the Space Reactor-1 Freedom spacecraft, which NASA plans to launch to Mars in late 2028. The HALEU-fueled reactor will produce 20 kW of electrical power using a closed-loop Brayton cycle and will power a 12-kilowatt Hall-effect thruster.
The next stage will be Lunar Reactor-1, which NASA and the U.S. Department of Energy plan to prepare for launch in 2030. It is intended to power lunar infrastructure for at least five years, regardless of illumination or temperature. The exact installation site on the lunar surface has not yet been announced.
The Pentagon is pursuing a separate program and, subject to funding, is to deploy a medium-power reactor by 2031 for an unnamed military space mission. The military is to determine potential missions and payloads for small-, medium- and large-power reactors, but the results of this analysis have not been disclosed.
It was previously reported that in late August, the U.S. Army selected companies to build more than 20 nuclear microreactors. Antares was one of the competition winners.