The objectives of the thermal control design were to determine the system performance and system thermal response to satisfy structural components, both scientific and subsystem elements, temperature limits for all vehicle orientations. Thermal control heater power is provided to the observatory to maintain the observatory within its temperature limits for all conditions. The heater power required was analyzed to determine how much heater power, both raw and regulated, had to be supplied. Some selected electrical components were studied to determine their temperature. The approach taken was to establish the feasibility of passive thermal control.