Future space exploration missions require advanced thermal control systems (TCS) to dissipate heat from spacecraft, rovers, or habitats to external environments. We propose to develop a lightweight, reliable TCS to effectively maintain cabin and equipment temperatures under widely varying heat loads and ambient temperatures. The proposed system uses freeze-tolerant radiators, which eliminate the need for a secondary circulation loop or heat pipe systems. Each radiator has a self-regulating variable thermal conductance to its ambient environment. The variable conductance will enable the TCS to maintain the cabin and equipment at a tightly controlled temperature. The TCS uses a nontoxic working fluid that is compatible with existing lightweight aluminum heat exchangers. The TCS is lightweight, compact, and requires very little pumping power. In Phase I, we will prove the feasibility of our approach through performance demonstration of a key component in the TCS system and detailed system design and analysis. In Phase II we will build a TCS demonstrator and obtain test data to show its unique performance advantages.