Nuclear power has essentially three applications to spaceflight: to provide a source of heat to keep equipment warm (see radioisotope heater unit (RHU)), to provide a source of electricity to power equipment (see radioisotope thermoelectric generator (RTG)), or to provide a means of propulsion, either directly (see nuclear propulsion) or indirectly (see nuclear-electric propulsion). Research on nuclear power systems for prospective space applications began in the 1950s. Early American research on RTGs and space nuclear reactors was conducted under the auspices of the SNAP program. More recently, the SP-100 program focused on the design of larger reactors for use in space. With one exception, the United States has only flown nuclear material aboard spacecraft to power RTGs and RHUs: SNAPSHOT was the only American space mission ever to carry a working nuclear reactor. By contrast, numerous Soviet RORSAT missions have been reactor-powered.