NASA Targets 2028 Mars Flight for First Interplanetary Reactor Propulsion Test

NASA wants to activate a working uranium-fission reactor aboard an uncrewed spacecraft heading toward Mars in late 2028. But Space Reactor-1 Freedom, or SR-1, does not yet have a booked launch, a named launch vehicle or a completed spacecraft. That leaves a substantial integration and approval campaign between the mission and its narrow interplanetary departure window.

The current target is December 2028, followed by a Mars encounter in 2029. If the flight proceeds and succeeds, it would provide the first interplanetary operating experience with a coupled U.S. fission-electric system: a reactor producing heat, machinery converting that heat into electricity and an electric thruster using the power to propel the spacecraft beyond Earth orbit.

SR-1 is planned as a roughly 12,000-kilogram spacecraft built around a high-assay low-enriched uranium reactor. NASA lists a 20-kilowatt-electric closed Brayton-cycle conversion system, which would circulate working fluid through rotating machinery to turn reactor heat into electrical power. Radiators would reject waste heat, while shielding would protect other spacecraft hardware.

That electricity would feed Hall-effect propulsion, including a 12-kilowatt thruster from the Advanced Electric Propulsion System. Hall thrusters accelerate propellant electrically rather than relying on combustion. Their low thrust is not suited to lifting a spacecraft from Earth, but their long operating duration can make them useful once a vehicle is already in space.

A conventional chemical rocket would therefore place SR-1 on an Earth-escape trajectory. NASA’s outline calls for the reactor to start within 48 hours after the spacecraft enters that trajectory. It is not intended to operate on the launch pad or during ascent, an important distinction for both mission architecture and the nuclear launch-approval process.

SR-1 is also different from the radioisotope power systems used by Voyager and the Perseverance Mars rover. Those systems obtain heat from the natural decay of radioactive material. SR-1 would instead maintain a controlled fission chain reaction, allowing it to generate sustained electrical power for propulsion as well as spacecraft systems.

Nor is this a nuclear thermal rocket, in which reactor heat would be applied directly to a propellant to generate thrust. SR-1 separates the reactor from the propulsion process: fission supplies heat, the Brayton system converts that heat into electricity and the Hall thruster converts electrical energy into spacecraft motion. Every conversion step adds hardware and thermal-management demands, but the architecture can support long-duration electrical operation far from the Sun.

The United States has operated a reactor in space before. SNAP-10A reached Earth orbit in 1965, as documented by the Department of Energy’s history of the System for Nuclear Auxiliary Power program. No reactor, however, has powered a spacecraft along an interplanetary trajectory beyond Earth orbit.

SR-1’s spacecraft bus would draw from the Power and Propulsion Element developed for NASA’s Gateway lunar station. NASA currently lists that Gateway-derived bus as generating 48 kilowatts, but adopting an existing design basis does not eliminate the central SR-1 challenge. The reactor, converter, radiators, shielding, bus and thruster must operate together as one unattended vehicle in deep space.

The mission would fly past Mars rather than land the reactor there or place it into permanent Mars orbit. At intercept, SR-1 is intended to deploy SkyFall, a separate payload containing three small helicopters derived from Ingenuity. The aircraft are planned to carry cameras, ground-penetrating radar and atmospheric sensors. NASA has scheduled the first SkyFall landing-site workshop for February 9-10, 2027, with a second workshop expected later that year.

The schedule still depends on completing spacecraft design, contracting, fuel and power-converter qualification, thermal testing, integration, the payload, nuclear launch approval and launch procurement. Those tasks are tightly coupled: changes in reactor shielding or radiator requirements, for example, can affect vehicle mass and launch arrangements even when the propulsion hardware itself is unchanged.

Timing is especially consequential because favorable Earth-Mars launch opportunities recur about every 26 months. A major delay would not necessarily end SR-1, but it could move the demonstration to a later planetary window.

Even a successful 20-kilowatt-class flight would be a technology demonstration rather than an immediate route to crewed Mars travel. NASA has estimated that future Mars transfer vehicles could need roughly 400 kilowatts to two megawatts of electrical power. SR-1’s nearer-term value would be proving that the complete fission-electric chain can start after Earth departure, reject its heat and keep producing useful propulsion power between planets. Reaching that point by December 2028 now depends as much on integration, approval and procurement as it does on the reactor itself.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.