A radioisotope rocket or radioisotope thermal rocket is a type of thermal rocket engine that uses the heat generated by the decay of radioactive elements to heat a working fluid, which is then exhausted through a rocket nozzle to produce thrust. They are similar in nature to nuclear thermal rockets such as NERVA, but are considerably simpler and often have no moving parts. Alternatively, radioisotopes may be used in a radioisotope electric rocket, in which energy from nuclear decay is used to generate the electricity used to power an electric propulsion system.