NASA intended to propel spacecraft to Mars using nuclear power

The NERVA program was an American nuclear thermal space propulsion project conducted between 1960 and 1972. The technique consisted of heating hydrogen with a nuclear reactor to eject it at high speed, and thus propel a manned spacecraft toward Mars. An experimental model with a thrust reaching 350 kN was tested on the ground in 1968 and 1969 before the program was halted in 1972.