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Aradioisotope heater unit (RHU) is a small device that provides heat throughradioactive decay.[1] They are similar to tinyradioisotope thermoelectric generators (RTG) and normally provide about one watt of heat each, derived from the decay of a few grams ofplutonium-238—although otherradioactive 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.[2]
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.[2]
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 theseismometer deployed on the Moon byApollo 11 in 1969, which contained 1.2 ounces (34 grams) of plutonium-238;Mars Pathfinder; and theMars Exploration RoversSpirit andOpportunity.[3] RHUs are especially useful on the Moon because of its lengthy and cold two-week night.
Virtually everydeep space mission beyondMars uses both RHUs and RTGs. Solarinsolation decreases with the square of the distance from theSun, so additional heat is needed to keep spacecraft components at nominaloperating temperature. Some of this heat is produced electrically because it is easier to control, but electrical heaters are far less efficient than a RHU because RTGs convert only a few percent of their heat to electricity and reject the rest to space.
TheCassini–Huygens spacecraft sent toSaturn contained eighty-two of these units (in addition to three main RTGs for power generation). The associatedHuygens probe contained thirty-five.
ISRO included two radioisotope heater units developed by India'sDepartment of Atomic Energy (DAE) in the propulsion module ofChandrayaan-3 on a trial basis which worked flawlessly.[4]
Radioisotope heater units for NASA missions have used plutonium-238[3] 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 of238Pu.[5]
The ESA'sExoMarsRosalind Franklin rover will useamericium-241 RHUs. The half-life of Am-241 is five times that of238Pu, with a concomitant reduction in power-density.
Soviet missions have used other isotopes, such as thepolonium-210 heat source used in theLunokhod lunar rovers.[6][7] 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.[7]
While both RHUs andRadioisotope Thermoelectric Generators (RTGs) use the decay heat of a radioactive isotope, RHUs are generally much smaller as a result of omitting thethermocouples 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 asthermionic generators, have also been used.
TheUnited States Department of Energy has developed thegeneral-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 fouriridium-clad Pu-238 fuel pellets, standing 5 cm tall, 10 cm square and weighs 1.44 kg.
