A nuclear rocket is any spacecraft propulsion system that utilizes nuclear fission or fusion rather than chemical combustion. This broad category includes nuclear thermal rockets, which heat a fluid directly, and nuclear electric rockets, which generate power for ion thrusters. Engineers study these engines because their immense energy density allows for much higher exhaust velocities, drastically reducing the propellant mass required for interplanetary travel.
Heard on the job
Public safety concerns have kept the nuclear rocket grounded, but the physics make it the only practical choice for heavy outer-system payloads.