Spacecraft require electrical energy. This energy must be available in the outer reaches of the solar system where sunlight is very faint. It must be available through lunar nights that last for 14 days, through long periods of dark and cold at the higher latitudes on Mars, and in high-radiation fields such as those around Jupiter. Radioisotope power systems (RPSs) are the only available power source that can operate unconstrained in these environments for the long periods of time needed to accomplish many missions, and plutonium-238 (238Pu) is the only practical isotope for fueling them. Plutonium-238 does not occur in nature. The committee does not believe that there is any additional 238Pu (or any operational 238Pu production facilities) available anywhere in the world.The total amount of 238Pu available for NASA is fixed, and essentially all of it is already dedicated to support several pending missions-the Mars Science Laboratory, Discovery 12, the Outer Planets Flagship 1 (OPF 1), and (perhaps) a small number of additional missions with a very small demand for 238Pu. If the status quo persists, the United States will not be able to provide RPSs for any subsequent missions.

This booklet discusses Systems for Nuclear Auxiliary Power (SNAP), called isotope power generators, that are based on using heat from the decay of radioisotopes to produce electricity. These are the SNAP systems with odd-numbered designators. The basics of radioisotope thermoelectric generators (RTGs) are discussed and their uses as power sources in space exploration and on earth are described. Various radioisotope heat sources are discussed and a table of RTGs built under the SNAP program listing their uses, electrical power, weight, the radioisotope used, the radioisotope's half-life, and the generator life is given.

After World War II, the US Atomic Energy Commission (AEC) began mass-producing radioisotopes, sending out nearly 64,000 shipments of radioactive materials to scientists and physicians by 1955. Even as the atomic bomb became the focus of Cold War anxiety, radioisotopes represented the government’s efforts to harness the power of the atom for peace—advancing medicine, domestic energy, and foreign relations. In Life Atomic, Angela N. H. Creager tells the story of how these radioisotopes, which were simultaneously scientific tools and political icons, transformed biomedicine and ecology. Government-produced radioisotopes provided physicians with new tools for diagnosis and therapy, specifically cancer therapy, and enabled biologists to trace molecular transformations. Yet the government’s attempt to present radioisotopes as marvelous dividends of the atomic age was undercut in the 1950s by the fallout debates, as scientists and citizens recognized the hazards of low-level radiation. Creager reveals that growing consciousness of the danger of radioactivity did not reduce the demand for radioisotopes at hospitals and laboratories, but it did change their popular representation from a therapeutic agent to an environmental poison. She then demonstrates how, by the late twentieth century, public fear of radioactivity overshadowed any appreciation of the positive consequences of the AEC’s provision of radioisotopes for research and medicine.