of a nuclear thermal rocket engine. In July 2023, Lockheed Martin was awarded the contract to build the spacecraft and BWX Technologies (BWXT) would have developed the nuclear reactor. A launch was expected in 2027, but this was put on indefinite hold due to nuclear reactor test requirements, later compounded by proposed cuts in the FY2026 budget by the second Donald Trump administration before being cancelled.

In June 2025, the European Space Agency