Radioisotope heater unit (RHU)

Design and operation

At the core of an RHU is a small quantity of a heat-producing radioisotope enclosed in a robust container. Heat flows from the isotope to the surrounding structure and nearby components. Some RHUs are paired with a small thermocouple or similar device to convert a tiny portion of the temperature difference into useful electricity, but most are optimized strictly for thermal output.

• Core: isotope material in pellet or ceramic form.
• Encapsulation: multiple containment layers for mechanical and chemical protection.
• Heat interface: mounting surfaces or thermal straps that deliver warmth to instruments.

Common isotopes and containment

Plutonium-238 oxide is commonly used in space applications because it produces heat steadily over decades and has chemical stability. RHU designs emphasize containment and impact resistance: the radioactive material is typically sealed within chemically inert forms and clad in materials chosen to survive reentry, impact, and corrosion, reducing the risk of environmental release.

Applications and importance

RHUs are widely used on spacecraft, planetary landers, probes, and some remote terrestrial installations to keep batteries, instruments, and propulsion components within operational temperature ranges. They are especially valuable for missions to cold regions, shadowed craters, or distant planets where sunlight is weak. RHUs complement larger radioisotope power systems that generate electricity; they are smaller, simpler, and used where only heat is needed.

Safety, regulation, and distinctions

Because RHUs contain radioactive material, their manufacture, testing, and flight certification follow strict regulatory and safety procedures. RHUs differ from radioisotope thermoelectric generators (RTGs) primarily in purpose and scale: RTGs are built to supply continuous electricity for spacecraft systems, while RHUs provide localized thermal control.