New details of the Trump administration’s plans for NASA, released Friday, revealed the White House’s desire to end the development of an experimental nuclear thermal rocket engine that could have shown a new way of exploring the Solar System.

Foremost among these cuts, the White House proposes to end NASA’s participation in the Demonstration Rocket for Agile Cislunar Operations (DRACO) project. NASA said this proposal “reflects the decision by our partner to cancel” the DRACO mission, which would have demonstrated a nuclear thermal rocket engine in space for the first time.

Less than two years ago, NASA and DARPA announced plans to move forward with the roughly $500 million DRACO project, targeting a launch into Earth orbit aboard a traditional chemical rocket in 2027. “With the help of this new technology, astronauts could journey to and from deep space faster than ever, a major capability to prepare for crewed missions to Mars,” former NASA administrator Bill Nelson said at the time.

The DRACO mission would have consisted of several elements, including a nuclear reactor to rapidly heat up super-cold liquid hydrogen fuel stored in an insulated tank onboard the spacecraft. Temperatures inside the engine would reach nearly 5,000° Fahrenheit, boiling the hydrogen and driving the resulting gas through a nozzle, generating thrust. From the outside, the spacecraft’s design looks a lot like the upper stage of a traditional rocket. However, theoretically, a nuclear thermal rocket engine like DRACO’s would offer twice the efficiency of the highest-performing conventional rocket engines. That translates to significantly less fuel that a mission to Mars would have to carry across the Solar System.