The in-flight evolution of thermo-optical properties of thermal coatings is of great concern when defining the thermal control system (TCS) of spacecraft. In-flight measurements and aging tests in laboratory demonstrates that the parameter really affected by the in-orbit aging is the solar absorptivity, which often increases when coatings are under space environment conditions. To define spacecraft TCS, thermal engineers have to take into account, from beginning of life (BOL) to end of life (EOL), the thermo-optical properties evolutions of the external coatings they intend to use. Radiative surfaces sizes cope with the hottest conditions of the mission, which generally depend on the EOL properties of external coatings. But the larger the surfaces, the higher the heating power needed for the coldest conditions of the mission, which generally depend on BOL properties. Improving the knowledge of the in-orbit evolutions of the solar absorptivity of thermal coatings is consequently of paramount importance to optimize the TCS designs, and then to better master the on-board heating power consumption. In this context, CNES has developed since 1990 a very simple and low cost experiment, called "THERME", to evaluate the degradation of space coatings in real orbital conditions. This experiment was previously carried on low Earth orbit (LEO) spacecraft such as SPOT 2 and 5, HELIOS 2A and 2B and Demeter, all removed from operational service. In the last few years, the THERME experiment design was extended for geostationary (GEO) mission typical constraints and is now flying on ATHENA-FIDUS and KOREASAT from TAS and on a telecom mission from ADS. During the ICES 2024, we present the overall concept and design of THERME. We discuss also in methodology for exploitation of flight results of THERME, of main flight results and we identify the axes of improvement for future missions.
Calarco et al. (Sun,) studied this question.