Future exploration spacecraft will need to operate in extreme thermal environments, with highly variable conditions for heat rejection by thermal radiation. Thermal storage is a critical technology for these missions, since it enables the spacecraft's thermal management system to be sized for average conditions instead of the least favorable. We propose a new technology for thermal storage based on an innovative, non-venting phase-change heat exchanger. Creare's technology overcomes some of the key limitations of phase-change heat exchanger technology. Our design maximizes the amount of phase-change material, eliminates large thermal resistances due to the poor thermal conductivity of the phase-change material, and accommodates differences in solid vs. liquid volume without a heavy structure. In Phase I we will prove the feasibility of this approach through thermal, fluid, and structural analysis; assessment of critical trade-offs; proof-of-concept demonstrations; and design of a prototype phase-change heat exchanger sized to meet requirements for future space exploration missions. In Phase II we will design, build, and demonstrate operation of the prototype phase-change heat exchanger.