Thermal control mirrors provide passive thermal control for spacecraft. They help reduce the spacecraft operating temperature by reflecting incident solar radiation and simultaneously radiating internally generated heat into the surrounding environment. The basic construction of these thermal control mirrors, also known as Optical Solar Reflectors (OSRs) or Second Surface Mirrors (SSMs), consists of a thin glass substrate with a rear surface silver coating. The paper will describe the development of a new glass composition, CMO, which has a lower solar absorptance and a higher emissivity than conventional ceria-doped microsheet substrates along with developments in the optical coatings used in the SSM. Preliminary results on a new low absorptance conductive coating are also described. The results of a comprehensive space qualification are also reported showing that the new SSM components are stable to the high radiation fluences to be found in space.