This list of nuclear power systems in space includes nuclear power systems that were flown to space, or at least launched in an attempt to reach space. Such used nuclear power systems include: * radioisotope heater units (RHU) (usually produce heat by spontaneous decay of 238Pu) * radioisotope thermoelectric generators (RTG) (usually produce heat by spontaneous decay of 238Pu and convert it to electricity using a thermoelectric generator) * miniaturized fission reactors (usually produce heat by controlled fission of highly enriched 235U and convert it to electricity using a thermionic converter)