Interplanetary spacecraft which are operating in the outer solar system are normally powered by Radioisotopic Thermoelectric Generators. The European (ESA) scientific spacecraft Rosetta however, which shall explore the comet „Wirtanen” begining in the year 2011, shall be powered by low-intensity, low-temperature solar cells even for Sun distances of up to 5.25 AU. To keep the solar array area within feasible sizes, strict power saving is mandatory. Heater power is saved by mitigating the large solar flux differences between hot and cold cases by the thermal design.The resulting thermal design features are louver radiators, superior performance insulation, two stage radiant cooler and electrical heaters. Heater power is saved by reducing the difference between the external heat load in the hot and cold cases by the configuration and the attitudes of the spacecraft which shadow louver radiators and payload from direct sun radiation. Additionally the temperature ranges of the units are wider as usual to allow the minimisation of radiator areas and heater power. Those louvers which are not shadowed from direct solar radiation are covered by external optical-solar-reflector coated panels.