For decades, nuclear propulsion has been a fixture of aerospace engineering proposals and government studies, always promising, never quite leaving the laboratory. That changes in 2028. NASA announced that it will launch a spacecraft called Space Reactor-1 Freedom, described by the agency as the first nuclear-powered interplanetary spacecraft, on a trajectory to Mars before the end of that year. The mission, called Skyfall, will carry a fleet of small helicopters to the Martian surface. But the helicopters may end up being the secondary headline. The reactor powering the journey could reshape how humanity moves through deep space. The propulsion technology onboard SR-1 Freedom is known as nuclear electric propulsion, or NEP. It operates through an onboard fission reactor that generates heat, which is then converted into electricity to drive highly efficient electric thrusters. This is distinct from the radioisotope thermoelectric generators that have powered instruments aboard probes like Voyager for decades; those systems use the heat from radioactive decay for power but play no role in propulsion. NEP is an active drive system, and unlike …