Radioisotope heater units (RHU) are small devices that provide heat through
radioactive decay. They are similar to tiny
radioisotope thermoelectric generators (RTG) and normally provide about one watt of heat each, derived from the decay of a few grams of
plutonium-238—although other
radioactive isotopes could be used. The heat produced by these RHUs is given off continuously for several decades and, theoretically, for up to a century or more.