Most lunar and Martian surface probes use RHUs for heat, including many probes that use solar panels rather than RTGs to generate electricity. Examples include the
seismometer deployed on the Moon by
Apollo 11 in 1969, which contained 1.2 ounces (34 grams) of plutonium-238;
Mars Pathfinder; and the
Mars Exploration Rovers Spirit and
Opportunity..
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 238Pu. The ESA's
ExoMars Rosalind Franklin rover will use
americium-241 RHUs. The half-life of Am-241 is five times that of 238Pu, 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.
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==