A radioisotope heater unit is a compact thermal management device containing a small pellet of radioactive material, typically plutonium-238. As the isotope decays, it emits steady heat. Engineers place them near sensitive electronics or fluid lines on deep space probes to prevent freezing in extreme cold, without drawing power from the spacecraft's electrical system.
Heard on the job
The rover relies on several RHUs to keep the battery core above survival temperature during the Martian night.