Nuclear energy remains an attractive potential means of propulsion for future spacecraft. When compared with conventional rocket engines, a nuclear propulsion system would in theory be less massive, and could provide sustained thrust with greater energy. Many believe nuclear-powered spacecraft can and should be built, but first many technical problems and other hurdles must be overcome.

Both the U.S. and Soviet space programs were researching nuclear propulsion as far back as the early 1960s, and since then, dozens of ideas for nuclear propulsion systems—and the spacecraft they would power—have been proposed. Each system, however, is based around one of the two methods of generating nuclear energy: fission and fusion.

Fission Propulsion

Fission is the act of splitting a heavy atomic nucleus into two lighter ones, which results in a tremendous release of energy. Common fuels for fission reactions are plutonium and enriched uranium...