U.S. Plans to Launch Nuclear Fission-Powered Spacecraft to Fly by Mars by End of 2028

The National Aeronautics and Space Administration (NASA) is advancing the "Space Reactor-1 'Freedom'" (SR-1 Freedom) mission, with the goal of launching a nuclear fission-powered spacecraft by the end of 2028 to validate deep-space nuclear electric propulsion technology. According to existing mission documentation, the mission plans to fly by Mars in 2029 and deploy the "SkyFall" payload.

The SR-1 "Freedom" is neither a fusion spacecraft nor a crewed Mars vehicle. Its core experiment is to operate a small uranium fission reactor, an energy conversion system, and Hall-effect thrusters in coordination beyond Earth orbit, accumulating engineering data for future higher-power deep-space missions.

The spacecraft is expected to have a mass of approximately 12 metric tons, with the reactor fueled by high-assay low-enriched uranium (HALEU) and designed to produce approximately 20 kilowatts of electric power. Heat generated by the reactor will be converted into electricity through a closed Brayton cycle system, which will then drive the electric propulsion system. Unlike ground-based nuclear power plants, the spacecraft cannot rely on large volumes of water to dissipate heat, so it must reject waste heat through radiator panels in the vacuum environment.

NASA's Glenn Research Center is participating in advancing this project. The project team plans to reuse certain power and propulsion components originally developed for the Gateway lunar space station, while building on the technical foundation accumulated from previous experiments such as Kilopower. Project personnel have stated that nuclear power systems do not depend on sunlight and can provide continuous power to spacecraft in deep-space environments far from the Sun, an advantage that solar arrays are difficult to replicate.

Under the mission concept, the SR-1 "Freedom" will start up its reactor after being injected into an escape trajectory from Earth by a chemical rocket. According to documentation, the reactor will not operate on the launch pad or during the rocket's ascent phase, and is expected to be started approximately 48 hours after leaving Earth. This arrangement helps reduce nuclear safety risks during the launch phase, but the fuel, control mechanisms, and reactor structure must still pass safety assessments under launch vibration, mission abort, and extreme accident scenarios.

The mission's SkyFall payload is planned to include three small rotorcraft developed from the technology of the Ingenuity Mars helicopter. These helicopters will be used for aerial reconnaissance of the Martian surface, carrying cameras, ground-penetrating radar, and meteorological measurement instruments, with mission objectives including terrain mapping, investigation of potential exploration areas, and detection of subsurface water ice signatures.

NASA has previously used radioisotope power systems to power probes such as Voyager, New Horizons, Curiosity, and Perseverance, but these devices rely on the natural decay of plutonium-238 to generate heat and do not involve controlled chain fission reactions. The United States also launched the SNAP-10A fission reactor into Earth orbit in 1965. The significance of the SR-1 "Freedom" lies in its plan to use an operating fission reactor to power propulsion during interplanetary flight beyond Earth orbit.

At present, the end of 2028 remains a target date for the mission and does not imply that the spacecraft, launch vehicle, and nuclear launch approvals have all been completed. The SR-1 "Freedom" still needs to complete system integration, reactor and energy conversion system verification, thermal control and shielding design confirmation, launch safety reviews, and payload development. If key milestones are delayed, the mission may need to wait for the next suitable Mars launch window.