Nuclear propulsion choices for space exploration

The potential for nuclear propulsion as a substitute for chemical propulsion for near term space exploration is examined. Chemical rockets face limitations on the generated thrust per unit mass of fuel. Increasing the specific impulse I_s by using nuclear propulsion significantly reduces the percentage of total mass that must be allocated for the propellant, thereby increasing the payload. For long distance missions, I_s becomes increasingly important, as the propellant mass approaches 100 percent of the total mass. The near term potential of nuclear propulsion is for a mission to Mars. The higher I_s reduces the mission duration from about a year for a chemical rocket, to a few weeks in the case of a nuclear rocket. This is crucial to avoid the effects of space radiation from solar flares on the astronauts, as well as the effects of gravity's absence on their muscular, bone, and other bodily functions. Details of the designs of solid core nuclear reactors for propulsion are discussed including the core design, shielding and reactivity control drums. The most promising options for near future space solar system space exploration are identified as the newer technologies of nuclear powered ion thrusters and gas core nuclear reactors.