NASA’s reconsidering the nuclear propulsion ideas shelved back in the 1960’s. The basic concept is you use a fission reactor to superheat a gas or fluid reaction mass. The specific impulse, the amount of acceleration per unit of reaction mass, of a nuclear engine is greater than chemical reactions.
Going to Mars using a nuclear rocket might take two months one way, instead of six. That makes a human mission to Mars feasible.
Read The Curve of Binding Energy, by John McPhee to learn about the other nuclear rocket proposal. Instead of a reactor, Project Orion would use nukes for acceleration.
