The passive thermal control of the Cluster spacecraft is based on a low-emissivity concept, insulating the spacecraft from the exterior environment to the extent needed to survive the four hour eclipses in mission orbit, whilst still allowing the internally generated heat to be rejected. The thermal closeout is provided by three types of hardware: low-emissivity double foil shields on the upper and lower surfaces of the spacecraft; multi-layer insulation (MLI) on the top and bottom of the central cylinder, below the RCS ring and around the upper part of the satellite, enclosing the experiments; and thermal insulation of the inner sides of the solar-array panels and of the 400 N main engine. An Optical Surface Reflector (OSR) radiator is integrated into the top surface to allow for the high dissipation of the RF power amplifiers. An External Power Dumper (EPD) radiator located in the upper thermal shield within the central cylinder dissipates excess power generated by the solar arrays. Heaters are used to keep equipment within specified temperature ranges throughout all mission phases, including eclipses. Temperature control is achieved by a combination of thermostats with thermistor surveillance and of thermistor-guided software control.
The thermal design has been optimised for the almost constant solar-aspect-angle (SAA) range (90o< SAA < 96o) that will apply throughout the nominal mission phase. During the orbit transfer manoeuvres, however, the spacecraft may experience a much wider SAA range (65o < SAA < 115o). The heat-rejection concept that has been selected therefore permits the satellite to dissipate heat through either the upper or the lower thermal shield. With these precautions, Cluster can safely withstand the complete range of solar aspect angles that will be encountered.
A heated-environment concept has been chosen for the lower spacecraft compartment, comprising RCS equipment, batteries and battery regulators. The complexity and duration of the assembly and integration activities were greatly reduced by this approach compared to a solution with insulated components, but it requires somewhat more heater power during eclipses.
All external surfaces, including the solar cells, blankets, double foils and radiator will again be finished with an electrically conductive Indium Tin Oxide (ITO) coating to comply with the electrostatic requirements.