The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

adioisotope heater unit (RHU)
A device used to keep instruments warm aboard a spacecraft
on a lunar or interplanetary mission. 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. RHUs were used, for example, by the
Apollo 11 to 17 missions (1969–1972), the Soviet Luna
17 (1970) and 21 (1973), and Galileo. Cassini and the
radioisotope thermoelectric generator (RTG) 337
Huygens Probe are equipped with 117 40-gram RHUs for
temperature regulation.
radioisotope thermoelectric generator (RTG)
A power source often used for deep space missions
because of its long life, steady output, and independence
of solar illumination. RTGs employ banks of thermoelectric
elements (typically silicon-germanium unicouples) to
convert the heat generated by the decay of a radioisotope,
radioisotope thermoelectric
generator Engineers examine
the interface surface on the
Cassini spacecraft before
installing the third radioisotope
thermoelectric generator (RTG).
The other two RTGs are the long
black objects projecting to the
left and right near the base of
Cassini in this photo. NASA