RHU

An RHU (radioisotope heater unit) is a small cylindrical heat source, typically about 2.5 centimeters in diameter and 1.3 centimeters thick, that converts the thermal energy from radioactive decay directly into usable warmth. It contains a radioisotope, historically plutonium-238, sealed in multiple containment layers. Unlike a radioisotope thermoelectric generator (RTG), which produces electricity from the temperature difference created by decay heat, an RHU simply releases that heat into its surroundings for direct consumption by spacecraft instruments and propellant tanks.

RHUs operate continuously without moving parts, switches, or external power input. A single unit generates roughly 1 watt of thermal power steadily and reliably for years. They are mounted directly against components that need warming, such as fuel lines, battery packs, or sensitive electronics, with minimal insulation between them. This makes them ideal for deep space missions where solar panels become useless and radiators cannot dissipate enough heat to prevent instrument failure in the extreme cold near Jupiter, Saturn, or beyond.

The plutonium-238 fuel (which had a half-life of about 88 years at the time of major Space Shuttle and planetary probe missions) is pressed into a ceramic pellet and encased in iridium. That capsule is then wrapped in molybdenum and steel, creating a robust package that survives launch vibration, reentry, and potential impact. Each unit weighs roughly 40 grams. A spacecraft may carry dozens of RHUs distributed around the bus, with dozens or hundreds total across a mission.

The main constraint on RHU use has always been availability and politics, not engineering. Plutonium-238 production was a controlled state monopoly; most supplies came from the U.S. Department of Energy or the Russian space agency. When domestic plutonium production ended in the United States in 1988, supply became scarce, and missions had to conserve units or redesign to use fewer of them. By the early 2010s, RHU availability was a genuine bottleneck for outer-planet missions.

RHUs remain irreplaceable for certain applications. No battery survives the thermal stress and duration required. Radiators cannot reject enough heat in the absence of sunlight. Heaters powered by solar panels or primary batteries add mass, complexity, and failure modes. A radioisotope source that simply emits heat, asks nothing, and fails only if its containment cracks, remains the most reliable option for spacecraft that must operate for a decade or more in the outer solar system.