The need to control the mean internal temperature of the Able-4 Atlas lunar satellite within the range of 45 to 70 F led to the development of an active, lightweight closed loop temperature control system. The system senses internal temperatures and alters the effective radiation properties of the satellite so as to maintain the internal temperature with the specified range, even in the event of some adverse effects of the space environment on the radiation properties. The system consists of 50 rotatable four-bladed masks, each actuated by a spirally wound bimetal strip; the rotation of the masks (which control 20% of the satellite surface) exposes surface areas having differing absorbing and emitting characteristics. Both the problems encountered and the methods developed to meet these problems are discussed. (Author).