Cassini's Huygens Probe ALSO used RHU's! (35)

[link to sci.esa.int]

Thermal Subsystem (THSS)
While the PSE is thermally controlled by the Orbiter, the Probe's THSS must maintain all experiments and subsystem units within their allowed temperature ranges during all mission phases. In space, the THSS partially insulates the Probe from the Orbiter and ensures only small variations in the Probe's internal temperatures, despite the incident solar flux varying from 3800 W m-2 (near Venus) to 17 W m-2 (approaching Titan after 22 days of the coast phase following Orbiter separation).
Probe thermal control is achieved by:
• MLI covering all external surfaces, except for the small 'thermal window' (see below) of the Front Shield

• 35 Radioisotope Heater Units (RHUs) on the Experiment and Top Platforms continuously providing about one Watt each even when the Probe is dormant

• a white-painted 0.17 m2 thin aluminium sheet on the Front Shield's forward face acting as a controlled heat leak (about eight Watts during cruise) to reduce the sensitivity of thermal performances to MLI efficiency
The MLI is burned and torn away during entry, leaving temperature control to the AQ60 high-temperature tiles on the Front Shield's front face, and to Prosial on the Front Shield's aft surface and on the Back Cover.
During the descent phase, thermal control is provided by foam insulation and gas-tight seals. Lightweight open-cell Basotect foam covers the internal walls of the DM's shells and Top Platform. This prevents convection cooling by Titan's cold atmosphere (70 K at 45 km altitude) and therefore thermally decouples the units mounted on the Experiment Platform from the cold aluminium shells. Gas-tight seals around all elements protruding through the DM's shell minimise gas influx. In fact, the DM is gas tight except for a single 6 cm2 hole in the Top Platform that equalises pressure during launch and descent to Titan's surface.