Radioisotope Heater Unit - Spacecraft Use

Spacecraft Use

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. RHUs are especially useful on the Moon because of its lengthy and cold two-week night.

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.

The Cassini–Huygens spacecraft at Saturn contains eighty-two of these units (in addition to three main RTGs for power generation). The associated Huygens probe contains thirty-five.

The United States Department of Energy has developed the General Purpose Heat Source (GPHS) primarily for space use. These GPHSes can be used individually or in groups of up to eighteen for component heating and sources for RTGs. Each GPHS contains four iridium-clad Pu-238 fuel pellets, standing 5 cm tall, 10 cm square and weighs 1.44 kg.