<P> Nuclear power in space is the use of nuclear power in outer space , typically either small fission systems or radioactive decay for electricity or heat . Another use is for scientific observation , as in a Mössbauer spectrometer . One common type is a radioisotope thermoelectric generator , which has been used on many space probes and on manned lunar missions , and another is small fission reactors for Earth observation satellites such as the TOPAZ nuclear reactor . A radioisotope heater unit provides heat from radioactive decay of a material and can potentially produce heat for decades . </P>
how have scientists used nuclear power as a fuel for spacecraft
either small fission systems or radioactive decay for electricity or heat