This paper describes a novel thermal control system for future Mars landers and rovers designed to keep battery temperatures within the −10 °C to +25 °C temperature range. To keep the battery temperatures above the lower limit, the system uses: 1) a phase change material (PCM) thermal storage module to store and release heat and 2) a loop heat pipe (LHP) to transfer heat from a set of Radioisotope Heater Units (RHUs) to the battery. To keep the battery temperature below the upper limit, a thermal control valve in the LHP opens to redirect the working fluid to an external radiator where excess heat is dumped to the atmosphere.The PCM thermal storage module was designed and fabricated using dodecane paraffin wax (melting point, − 9.6 °C) as the phase change material. A miniature ammonia loop heat pipe with two condensers and an integrated thermal control valve was designed and fabricated for use with the PCM thermal storage unit. The results from an ongoing experimental simulation of the Mars '03/'05 rover thermal performance in the Martian environment are described in the paper. The tests were performed for various internal configurations of the PCM and LHP arrangements in worst-case hot and cold environment. Based on the results of these tests, the Mars '03/'05 rover thermal design development will be finalized. Many lessons already have been learned during the development and implementation of these thermal technologies for Mars rover battery thermal control. Recommendations for the design and operation of loop heat pipe and phase change material thermal energy storage systems for future space missions are made herein.