radioisotope heater unit (RHU)
Each RHU provides about one watt of heat, derived from the decay of a few grams of a radioactive substance, such as plutonium 238-dioxide, contained in a platinum-rhodium alloy cladding. Heat is transferred to spacecraft structures, systems, and instruments directly without moving parts or intervening electronic components.
RHUs are very compact, 3.2 centimeters (1.3 inches) long and 2.6 centimeters (1 inch) in diameter. The fuel pellet is about the size and shape of a pencil eraser weighing approximately 2.7 grams (0.1 ounces). All together each RHU weighs about 40 grams (1.4 ounces).
RHUs were used, for example, by the Apollo 11-17 (1969-72) missions, the Soviet Luna 17 (1970) and 21 (1973), and Galileo. Cassini and Huygens are equipped with 117 40-gram RHUs for temperature regulation.
Source: U.S. Department of Energy
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