thumb|Diagram of a radioisotope heater unit
A radioisotope heater unit (RHU) is a small device that provides 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.
In spacecraft, RHUs are used to keep other components at their operational temperatures, which may be very different to the temperature of other parts of the spacecraft. In the vacuum of space any part of the spacecraft which doesn't receive direct sunlight will cool down so much that electronics or delicate scientific instruments break down. They are simpler and more reliable than other ways of keeping components warm, such as electric heaters..
ISRO included two radioisotope heater units developed by India's Department of Atomic Energy (DAE) in the propulsion module of Chandrayaan-3 on a trial basis which worked flawlessly.
Isotope
Radioisotope heater units for NASA missions have used plutonium-238 as the isotope for heat sources, since the radioactive half-life of 87.7 years means that the decay of the isotope will not limit the mission lifetime. The isotope produces 0.57 watts of thermal power per gram of <sup>238</sup>Pu.
The ESA's ExoMars Rosalind Franklin rover will use americium-241 RHUs. The half-life of Am-241 is five times that of <sup>238</sup>Pu, with a concomitant reduction in power-density.
Soviet missions have used other isotopes, such as the polonium-210 heat source used in the Lunokhod lunar rovers. With a half-life of 138.376 days, polonium-210 produces more thermal power per unit mass, but is suitable only for shorter duration missions. Strontium-90 has also been proposed.