The new cost estimate for the US National Aeronautics & Space Administration’s (NASA’s) nuclear-powered Mars mission – Space Reactor-1 (SR-1) Freedom – has been put at $2,1bn, according to documents viewed by Politico. The mission is scheduled to launch in late December 2028, with a one-year flight transit arriving at Mars in late 2029.
It will be powered by a nuclear electric propulsion (NEP) system generating 20 kW using high-assay, low-enriched uranium (HALEU) fuel. NASA is repurposing the Power and Propulsion Element (PPE) from the cancelled Lunar Gateway orbiting space station to accelerate the project’s development timeline.
According to the budget documents reviewed by Politico, NASA plans to spread the $2.1bn across the next four fiscal years: $640m (FY26); $890m (FY27); $415m (FY28); and $180m (FY29). NASA will pull that money from the agency’s exploration account and from funding it received through the party-line reconciliation bill passed last year.
However, the $2.1bn does not cover the cost of SkyFall, the accompanying science payload consisting of three (or up to six) Ingenuity-class helicopters tasked with scouting landing zones and subsurface water ice on Mars. NASA hasn’t released a cost estimate for SkyFall, but Mars researchers have warned it could cannibalise the agency’s science budget.
The House Appropriations Committee has only recommended $10m specifically for the project in FY27. The remaining $880m required for that year depends on Congress approving the reallocation of funds from the suspended Lunar Gateway programme. If this is not achieved, the integration timeline will slip, forcing a mandatory two-year delay until the next Earth-Mars orbital window opens in 2030.
Nuclear propulsion systems operate by using a nuclear reactor to generate heat, which then creates thrust to propel a spacecraft. Unlike chemical rockets that rely on burning propellants, nuclear systems use atomic fission to achieve vastly superior fuel efficiency. Nuclear propulsion reduces transit times to Mars by up to 50% compared with chemical rockets, cutting the journey from nine months down to roughly four to five months. This reduces the amount of cosmic radiation astronauts absorb and lessens the long-term effects of zero gravity on the human body.
NASA and the Department of Energy (DOE) primarily focus on two distinct types of nuclear propulsion – NEP and Nuclear Thermal Propulsion (NTP).
SR-1 Freedom mission uses NEP. This system prioritises long-duration efficiency over raw power. The nuclear reactor generates intense heat, which a thermal converter (such as a Stirling engine or Brayton cycle turbine) converts into electrical power. This electricity power is fed into an electric thruster (an ion engine or Hall thruster), where it strips electrons from a propellant gas such as xenon or krypton to create positive ions. High-voltage electromagnetic fields accelerate these ions out of the back of the spacecraft at up to 30 kilometres per second. While the push is gentle, the engine can run continuously for months or years, eventually reaching speeds much higher than chemical rockets.
NTP systems work by directly converting thermal energy into kinetic energy. A compact nuclear reactor splits uranium atoms (typically HALEU) to generate extreme heat. Liquid hydrogen propellant is pumped directly through the ultra-hot reactor core. The liquid hydrogen rapidly expands into a superheated gas. The expanding gas shoots out of a conventional rocket nozzle at extreme speeds, pushing the spacecraft forward. While NASA continues to research NTP for future rapid human transit, NEP was chosen for SR-1 Freedom because it excels at efficiently hauling heavy cargo over long-duration timelines, making it ideal for carrying the SkyFall helicopter payload to Mars.
“We’re trying to leverage as much as we can with as little new development as possible. I know that’s always a challenge. It always sounds good on paper, but that’s really the intent,” Lori Glaze, the Acting Associate Administrator for NASA’s exploration systems development, told a June meeting with the National Academies’ Aeronautics and Space Engineering Board. “We are realigning all the resources within the topline to make sure we can cover the items that were identified in Ignition, and right now, it all fits,” she said.
She added that funding will come from NASA’s FY27 budget request as well as the 2025 Working Families Tax Cut Act, which included $2.6bn for the cancelled Gateway project. Board members questioned the timeline. One noted that a two-year development schedule is more typical of a cubesat than a nuclear interplanetary spacecraft. “It is ambitious. It’s a challenge,” Glaze admitted but said NASA is doing everything it can to rise to the occasion.
How NASA will shuffle funding already allocated in the 2026 budget to pay for the Mars mission is not clear, or how it can be worked into the 2027 appropriations process. However, NASA officials remain optimistic that funding for the programme will be found. As to SkyFall, NASA said in a separate statement that the helicopters will be funded in part by the science directorate’s Mars Future Missions budget line, without providing a cost estimate.
“While the final programme-wide budget numbers are being carefully refined as mission architectures undergo formal design maturation, by getting both a launch and a ride to Mars with SR-1, this mission fits well with the administrator’s objective of reducing costs and concentrating efforts on missions with near term science and exploration opportunities,” NASA said.
