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Auxiliary power systems deriving their energy from radionuclide decay have been under development for more than five years under Atomic Energy Commission sponsorship. Called SNAP, the program has been oriented toward satisfying the need for small, compact, lightweight and reliable power supplies for space systems. Radioisotopic power relates to the generation of heat by absorption of electromagnetic and particulate radiations emitted by a carefully sealed source of radioisotopes. The heat is then partially converted into electricity by a thermoelectric or thermionic conversion device, with the remainder rejected to the external environment. It is the intent of this paper to present a status report on SNAP radioisotopic devices and to explore some of the unique aspects of the technical problems encountered. Since these power systems have no moving parts, they lend themselves appropriately to the performance of the following types of space missions: 1. Satellites operating on equatorial orbits where the continuous power demand exceeds the storage cycle capability. 2. Satellites operating for considerable periods of time in the Van Allen radiation belts where solar cell output is significantly degraded. 3. Lunar explorations, such as NASA's Surveyor program, where storage requirements during the lunar night become excessive. 4. Venus surveys where solar energy would be excluded by the atmosphere. 5. Space probes away from the sun where the decrease in solar flux becomes significant.