Nuclear power has played a significant role in the exploration of the solar system, in many cases enabling missions that could not have been achieved otherwise. First flown by the United States in 1961 (Table 1-1), radioisotope power systems (RPSs) possess unique capabilities relative to other types of space power systems. RPSs generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically 238Pu) into electricity via one of many power conversion processes. Potential advantages of RPSs are their long life, robustness, compact size, and high reliability. They are able to operate continuously, largely independent of orientation to and distance from the Sun, and can be designed to be relatively insensitive to radiation and other environmental effects. These properties have made RPSs ideally suited for many robotic missions in the extreme environments of outer space and on planetary and satellite surfaces.