suitable only for shorter duration missions. Strontium-90 has also been proposed. Comparison of RHU with RTG While both RHUs and Radioisotope Thermoelectric Generators (RTGs) use the decay heat of a radioactive isotope, RHUs are generally much smaller as a result of omitting the thermocouples and heat sinks/radiators required to generate electricity from heat. Both RHUs and RTGs feature rugged, heat-resistant casings to safely contain the radioisotope in the event of a launch or re-entry vehicle failure. The total mass of a single one-watt RHU (including shielding) is about 40 grams. Similar schemes, such as thermionic generators, have also been used. GPHS The United States Department of Energy has developed the general-purpose heat source (GPHS) primarily for space use. These GPHSs can be used individually or in groups of up to eighteen for component heating, but are primarily used as the heat source for RTGs. Each GPHS contains four iridium-clad Pu-238 fuel pellets, standing 5 cm tall, 10 cm square and weighs 1.44 kg. See also Nuclear fuel Radioisotope generator Stirling radioisotope generator Radioisotope thermoelectric generator References External links NASA Radioisotope Power Systems website – RHU page Radioisotope heater unit fact sheet from NASA's Cassini mission website Nuclear power in space Nuclear technology Heaters