SR-1 Freedom is a National Aeronautics and Space Administration (NASA) spacecraft that will fly a fission reactor to Mars and deliver a science payload called SkyFall, with launch targeted for late 2028. Its nuclear power module pairs a reactor designed by the Department of Energy’s Idaho National Laboratory with a closed Brayton cycle power conversion system from Sierra Lobo Inc., which turns reactor heat into electricity for the spacecraft’s electric propulsion.
NASA calls Space Reactor-1 (SR-1) Freedom the first fission-powered interplanetary spacecraft, and says it will be the first fission-powered spacecraft to travel outside Earth’s orbit. The mission was announced during NASA’s “Ignition” event. A NASA Glenn post published on 20 October 2026 says NASA and its partners have agreed to contract actions meant to speed up development of the power module.
SR-1 Freedom at a Glance
SR-1 Freedom is a NASA spacecraft of about 12,000 kilograms that uses a 20 kWe fission power module to run electric thrusters on a trip to Mars. NASA lists the Department of Energy (DOE) as its partner on the mission.
| Item | Detail |
|---|---|
| Mission | Demonstrate nuclear electric propulsion in deep space; Martian flyby and SkyFall payload deployment |
| Reactor builder | DOE’s Idaho National Laboratory (INL) |
| Power conversion builder | Sierra Lobo Inc., under a NASA Glenn contract |
| Launch target | Late 2028, arrival at Mars in 2029 |
| Module power | 20 kilowatts electric (kWe), closed Brayton cycle |
| Bus power | 48 kilowatts, Power and Propulsion Element |
| Thruster | Advanced Electric Propulsion System, a 12-kilowatt Hall thruster |
| Mass | About 26,455 pounds (12,000 kilograms) |
These figures come from the NASA Space Reactor-1 Freedom mission page, last modified 8 October 2026, and the NASA Glenn post of 20 October 2026.
The Nuclear Power Module
The nuclear power module on SR-1 Freedom is a fission reactor designed by DOE’s Idaho National Laboratory, joined to a closed Brayton cycle power conversion system supplied by Sierra Lobo Inc. The reactor generates heat through nuclear fission. The power conversion system turns that heat into electricity for the spacecraft’s systems and its electric propulsion.
The module sits at the end of the spacecraft’s boom. Sierra Lobo will provide its part under the Space Flight Systems Development and Operations Contract III (SFSDO III). NASA awarded that contract in 2024 to support spaceflight hardware design, development, testing and operations at Glenn Research Center in Cleveland.
| Part | Builder | Contract route |
|---|---|---|
| Fission reactor | DOE’s Idaho National Laboratory | Designed and developed by INL |
| Closed Brayton cycle power conversion system | Sierra Lobo Inc. | SFSDO III, awarded 2024, supports work at NASA Glenn |
| Integration | Not stated in the sources reviewed | The power conversion system is integrated with the reactor to complete the module |
The NASA Glenn post says NASA and its partners “have mutually agreed to key contract actions” to expedite development. Steve Sinacore, SR-1 Freedom program director, said: “These contract actions not only reduce technical risk and help us maintain momentum toward our 2028 launch, but they also strengthen collaboration across our partners.” The post gives no values or dates for those actions.
Vehicle and Propulsion
SR-1 Freedom’s spacecraft bus is the Power and Propulsion Element, and NASA lists leveraging existing technology and flight hardware to the maximum extent possible as a mission objective. NASA lists that bus as generating 48 kilowatts of electrical power. A NASA technical paper on the thermal architecture describes it as a repurposed Gateway Power and Propulsion Element spacecraft. The station it was first built for is covered in our Lunar Gateway explainer.
The spacecraft weighs about 26,455 pounds (12,000 kilograms). Its reactor fuel is high-assay low-enriched uranium (HALEU). Thrust comes from the Advanced Electric Propulsion System, a 12-kilowatt Hall thruster. Communications run over X-band through NASA’s Deep Space Network.
NASA lists six mission objectives:
- Operate a nuclear-electric propulsion system in space.
- Launch on an Earth escape trajectory.
- Use the 2028 Mars launch window.
- Leverage existing technology and flight hardware to the maximum extent possible.
- Deliver SkyFall to Mars intercept.
- Demonstrate reactor extended performance.
Safety Approach
NASA says the SR-1 Freedom reactor stays inactive until the spacecraft leaves Earth orbit.
NASA and DOE list four further safety measures for the program:
- Interagency safety reviews.
- Continuous oversight by a NASA chief safety officer.
- Regulatory compliance.
- A safety design process focused on hazards, failure tolerance and nuclear safety.
NASA’s mission page also says SR-1 will “set regulatory and launch precedent.”
SkyFall Payload
SkyFall is a set of three Mars helicopters that SR-1 Freedom will release in mid-air during the Martian flyby. The helicopters evolved from the Ingenuity Mars Helicopter design.
Each helicopter carries ground-penetrating radar and imagers. The helicopters can measure air temperature, wind speed and wind direction. NASA says their jobs are to collect scientific data, demonstrate exploration zones and identify potential water sources.
Each helicopter has two rotor blades, each 4 feet (1.4 meters) wide. The helicopters will operate where, according to NASA, gravity is one-third of Earth’s and surface air pressure is about 1% of Earth’s. NASA says the helicopters could last months and potentially years.
A landing-site selection process is planned. NASA is targeting flat, low-elevation areas with few geological obstacles.
Why SR-1 Comes Before the Lunar Reactor
NASA calls SR-1 Freedom a pathfinder for surface power on the Moon and Mars and for outer solar system exploration. It will inform and enable Lunar Reactor-1 (LR-1), a fission surface power system meant to keep NASA’s Moon Base running through periods of darkness, where solar power alone is not enough.
Flying a reactor first, without a lunar landing, does three things in NASA’s account:
- It reduces nuclear flight risk.
- It stimulates and qualifies the supply chain.
- It builds the workforce.
NASA describes SR-1 and LR-1 as the beginnings of a domestic nuclear-space industrial base. Its mission page says SR-1 will “activate the industrial base.” The lunar nuclear reactor explainer covers the LR-1 line that follows.
A NASA technical paper by Scott Thomas of Marshall Space Flight Center and Jared Berg of Glenn Research Center, presented at the 37th Thermal and Fluids Analysis Workshop (TFAWS) in Huntsville, Alabama, from 31 August to 4 September 2026, names three thermal challenges for the 20 kWe-class module:
- Very high waste heat rejection needs.
- Fitting a bus designed for a different mission.
- Unusual packaging constraints.
The paper gives no schedule and no mass.
What NASA Has Not Published
NASA has not published five details in the sources reviewed here:
- The launch vehicle.
- The total mission cost.
- Contract values, including the Sierra Lobo work and the recent contract actions.
- The reactor’s thermal power.
- The exact launch date.
Items to watch:
- Integration of the power conversion system with the INL reactor. NASA’s sources describe the integration only as the step that completes the module.
- Details of the contract actions. The NASA Glenn post describes them only in general terms.
- SkyFall landing-site selection. NASA says a selection process is planned.
- Launch vehicle announcement. None appears in the sources reviewed.
For more spacecraft and power-system explainers, browse the technology hub. Current schedule details appear on the NASA Space Reactor-1 Freedom mission page.