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Earth imaging satellites in geostationary orbit (GEO) are proving to be of great public utility such as disaster prevention and damage mitigation by providing satellite images. JAXA has been conducting research and development of an optical space telescope that allows ground observations with a ground sampling distance less than 10 meters at nadir from GEO. These optical systems must have stable thermo-optical properties even in long-term operation. The authors have been evaluating the resistance of candidate materials to the space environment as part of their investigation into suitable materials for optical system mirrors. Cordierite ceramics have become promising candidate materials in addition to glass materials since their excellent physical properties, such as a high Young’s modulus, low bulk density, high thermal conductivity, and a low coefficient of thermal expansion (CTE). Cordierite ceramics have not been used for space optics, so how their optical properties change in a space environment needs to be evaluated. Cordierite ceramics have not been used for space optics, so how their optical properties change in a space environment needs to be evaluated. The thermooptical properties of mirrors may be degraded by prolonged exposure to the unique conditions in space, such as ultraviolet light, radiation, and atomic oxygen. The authors evaluated the changes in thermo-optical properties due to these environments for three candidate materials. Changes in optical parameters will be used to design optical system. It was confirmed that samples with the SiO2 protective coating showed no significant changes in their resistance to radiation and ultraviolet light, even after irradiation equivalent to twenty years of exposure to the space environment. The thermo-optical properties of silver coated samples without SiO2 protective coatings showed a significant degradation under atomic oxygen, which will be present in low-orbit applications. This is due to the surface formation of silver oxide.
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