NERVA rocket, c 1965.

UNITED STATES - JUNE 06: Model. Nuclear rocket engines work by using fission to generate heat that is then used to eject hydrogen gas at high velocity. All rocket engines work according to the principle of Newton's Third Law of Motion: they move forward as mass is ejected at high velocity in the opposite direction. A nuclear-powered rocket engine has high 'specific impulse', a measure of an engine's efficiency, making it potentially suitable for deep space missions. In 1961, NASA issued a request for proposals for a nuclear rocket. The Nuclear Engine for Rocket Vehicle Applications (NERVA) programme was established in 1963, but although the testing of reactors showed considerable promise, the programme was cancelled in 1971 as NASA�s priorities changed. (Photo by SSPL/Getty Images)