Radioisotope heater units 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 (in nominal amounts) for up to a century or more.[1]
In spacecraft RHUs are necessary to heat critical components and subsystems. RHUs also reduce spacecraft complexity, by making heater subsystems unnecessary. By having as few heating subsystems as possible, the overall complexity of the spacecraft can be reduced.[1]
RHUs provide local heat to sensitive equipment in deep space. While both RHUs and RTGs use the decay heat of a radioactive isotope (usually Pu-238), RHUs are generally much smaller and omit the thermocouples and heat sink/radiators required to generate electricity from heat. Both feature rugged, heat-resistant casings to safely contain the radioisotope in the event of a launch vehicle failure or an accidental collision with the earth from deep space.
Virtually every deep space mission beyond Mars uses both RHUs and RTGs. Solar insolation decreases with the square of the distance from the Sun so additional heat is needed to keep spacecraft components at nominal operating 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.
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 early ALSEP seismometer deployed on Apollo 11; Mars Pathfinder; and the Mars Exploration Rovers Spirit and Opportunity. RHUs are especially useful on the moon because of its very long and cold 2-week night.
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The Cassini–Huygens spacecraft at Saturn contains 82 of these units (in addition to three main RTGs for power generation); the associated Huygens probe contains 35. The total mass of a single RHU (including shielding) is about 40 grams. Similar schemes such as thermo-ionic generators have also been used.
The United States Department of Energy has developed the General Purpose Heat Source (GPHS). Each GPHS contains four iridium-clad Pu-238 fuel pellets, stands 5 cm tall, 10 cm square and weighs 1.44 kg.
These GPHSes can be used singly or in groups of up to eighteen for component heating and sources for RTGs.