Radioisotope heater unit
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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. This heat is given off continuously for several decades. RHUs become necessary to heat critical components. Their function is to provide highly localised heating of sensitive equipment (such as electronics) in deep space. They are also used in RTGs to generate electricity by combining several tens of them together. Virtually all the deep space missions that have involved sending probes past Mars's orbit have used RHUs, because insolation decreases as the inverse square of distance from the Sun, requiring an additional heat source to keep the spacecraft systems at normal operating temperature, and to provide electricity in place of photovoltaic cells used closer to the Sun. 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 can be used singly or in groups of up to eighteen for component heating and sources for RTGs.
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