A passive thermal control system for a satellite IR sensor was designed for operation at cryogenic temperature below 150°K. The system's purpose is to remove heat from the sensor and maintain it at the required operating temperature. The basic approach is to minimize the heat transfer to and the heat dissipation within, the sensor. This is achieved by conducting heat from the sensor to a phase-change canister (PCC) which provides thermal storage when needed. Heat is distributed from the PCC via diode monogroove heat pipes to the one of two 45 sq ft radiators which has a favorable view of coldspace. Heat is transferred over the face of the radiator using simple fixed conductances axial groove heat pipes from which it is radiated to space (Fig. 1). The heat pipes are oriented from the PCC to the radiator in one plane, thereby making the system ground testable. A heat pipe radiator was selected for this system because of its inherent reliability. Conventional systems such as solid evaporative coolers of fluid pumping systems are inherently heavier and less reliable than heat pipe systems. Refrigeration systems are heavier and have unacceptably large power consumption rates for the required heat rejection load.