Radioisotope heater unit
Radioisotope heater unit
A radioisotope heater unit (RHU) is a small device that provides heat from radioactive decay. Each unit typically gives about one watt of heat and can keep producing heat for decades, potentially many decades longer. Plutonium-238 is commonly used because it generates steady heat as it slowly decays; its half-life is about 87.7 years and it delivers roughly 0.57 watts of heat per gram. Other isotopes have been used or considered, such as americium-241 (ExoMars), polonium-210 (older Lunokhod rovers), and strontium-90.
Why RHUs are used in spacecraft:
In the vacuum of space, parts that don’t get direct sunlight can get extremely cold. RHUs provide a simple, reliable source of heat to keep important equipment at safe operating temperatures. They are rugged and designed to safely contain the radioactive material, making them a dependable alternative or supplement to electric heaters.
Typical characteristics:
A single 1-watt RHU, with shielding, weighs about 40 grams. RHUs are smaller and lighter than radioisotope thermoelectric generators (RTGs) because they only produce heat, not electricity. They are designed to be durable and safe for launch and operation.
Space mission examples:
RHUs have powered heat for many lunar and Martian missions, including instruments on Apollo 11’s Moon seismometer, Mars Pathfinder, and the Mars Exploration Rovers Spirit and Opportunity. The Cassini–Huygens spacecraft also used many RHUs to supplement the mission’s heat needs.
Other related heat sources:
General-purpose heat sources (GPHS) are used mainly to heat RTGs. A GPHS module contains four Pu-238 fuel pellets encased in iridium, stands about 5 cm tall and 10 cm square, and weighs around 1.44 kg.
In summary, RHUs are compact, reliable heat sources that keep spacecraft components warm in harsh space environments, enabling long-lasting missions even where sunlight is scarce.
This page was last edited on 1 February 2026, at 21:27 (CET).