CNES Flight Experiments for Thermal Coatings Aging—THERME and SESAME

The in-flight evolution of thermo-optical properties of thermal coatings is of great concern when defining the thermal control system (TCS) of spacecraft. Experience (in-flight measurements and aging tests in laboratory) demonstrates that the parameter really affected by the in-orbit aging is the solar absorptivity a_S, 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 (a_S and e_IR). 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.<br/>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^{1, 2}. 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 constraints² and is now flying on ATHENA-FIDUS and KOREASAT from TAS and on a telecom mission from ADS.<br/>Moreover, CNES and ESA recently collaborate to develop the Euro Material Ageing (EMA) experiment. This will offer recurrent testing opportunities for European institutes to perform materials sciences research and validate new materials technologies in the real space environment on the International Space Station. EMA is currently under development and includes two main experiments:<br/>- SESAME for studying materials and coatings behavior in real space environment and recording in-orbit parameters such as atomic oxygen flux, contamination, radiation, for a better knowledge of the LEO space environment and its effects on materials.<br/>- IR-Coaster for characterizing the stability or evolution of biological and organic compounds when exposed to UV solar radiation.<br/>The intended duration of the EMA mission is between 12 and 18 months. For the first EMA flight opportunity, 141 passive materials samples and 8 active sensors are under the ISS ram direction, offering unrestricted view to space. Various European research groups samples have been selected following a competitive tender process managed by ESA. Exposed materials include novel types of innovative multilayer insulation (MLI), optical and thermal coatings, reinforced composites, 3D printed polymers and exotic metallic alloys... CNES and ESA are also providing samples from their own internal research programs and collaborations. Each material undergoes the measurements of their physical and chemical properties before and after flight. In particular, this allows the thermo-optical properties aging assessment of the new TCS coatings developed by CNES. The launch of the first EMA is planned by end of 2023.<br/><br/>In this paper, we present the overall concept and design of THERME and SESAME experiments. We discuss also in flight results of THERME and scientific objectives of SESAME for TCS coatings.<br/><br/>[1] S. Remaury, P. Nabarra, E. Bellouard and S. D’Escrivan. “In-Flight Aging of Thermal Coatings: THERME Experiment”, <u>Journal of spacecraft and rockets</u>, vol. 48, No.1, pp. 27-33, (2011).<br/>2 S. Remaury, S. d’Escrivan, and P. Nabarra, “New THERME Experiment Concept for Measuring Thermal Coating Aging in LEO an GEO”, <u>Journal of spacecraft and rockets,</u> vol. 53, No. 6, pp. 1105-1113, (2016).