This paper presents the thermal control system of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). This instrument is part of the payload of the EOS-AM1 spacecraft, which is to be launched in June 1998.ASTER consists of three radiometers, a Master Power Supply (MPS), and a Common Signal Processor (CSP). The radiometers are the Visible and Near Infrared Radiometer (VNIR), the Short Wave Infrared Radiometer (SWIR), and the Thermal Infrared Radiometer (TIR). Each of these subsystems has an independent thermal control system. SWIR and TIR use the capillary pumped heat transport system (CPHTS), which is a new technology being developed for the EOS spacecraft, and the other ASTER components use conventional radiators and heaters.The thermal control systems were analyzed using thermal network methods to confirm that the temperatures are within the allowable range where electrical, mechanical, and optical performance specifications are met. The results indicate that the thermal requirements of each of the subsystems is met by the design approach with the accommodations provided on the EOS spacecraft. Therefore, each subsystem has been approved to proceed with engineering model development.