Radioisotope heater unit

[1] They are similar to tiny radioisotope thermoelectric generators (RTG) and normally provide about one watt of heat each, derived from the decay of a few grams of plutonium-238—although other radioactive isotopes could be used.

In the vacuum of space any part of the spacecraft which doesn't receive direct sunlight will cool down so much that electronics or delicate scientific instruments break down.

Solar insolation decreases with the square of the distance from the Sun, so additional heat is needed to keep spacecraft components at nominal operating temperature.

ISRO included two radioisotope heater units developed by India's Department of Atomic Energy (DAE) in the propulsion module of Chandrayaan-3 on a trial basis which worked flawlessly.

Both RHUs and RTGs feature rugged, heat-resistant casings to safely contain the radioisotope in the event of a launch or re-entry vehicle failure.