CHEOPS (CHaracterising ExOPlanet Satellite) thermal design and thermal analysis
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CHEOPS - CHaracterising ExOPlanet Satellite - is the first ESA’s Science Programme of class S (Small mission). It is dedicated to searching for exoplanetary transits by performing ultra-high precision photometry on bright stars already known to host planets. The mission's main science goals are to measure the bulk density of super-Earths and Neptunes orbiting bright stars and provide suitable targets for future in-depth characterisation studies of exoplanets in these mass and size ranges. This mission is being developed in collaboration with the Swiss Space Office and the University of Bern, Switzerland, which is in charge of the instrument. CHEOPS instrument is based around a CCD detector in the focal plane of a 33.5 cm diameter on-axis telescope. The mission requirements are defined to provide to the detector an extremely stable photometric environment necessary to achieve the low signal-to-noise ratio measurements of transiting small mass planets (Figure 1). Key requirements are mechanical and thermal stability and minimum amount of stray light. At the same time, a large fraction of the sky should be observable since CHEOPS is targeting bright stars. The CHEOPS baseline concept is a telescope on a standard small satellite bus in LEO. To meet the above requirements, CHEOPS will be launched on a circular Sun- synchronous orbit, at an altitude of between 650 and 800 km, always pointing away from the Sun. Hence, the satellite will follow as close as possible the day-night terminator. The detector operates between 0.4 and 1.1 micron and requires both a low temperature (-40°C) and a thermal stability better than 10 mK. To this end, the focal plane assembly (FPA) thermal control includes a large passive radiator always shaded by a dedicated Sun-shield, a high conductance thermal path between the detector and the radiator, high thermal inertia capacitors to help temperature stabilization, thermal insulation and a PID-law controlled heating line to achieve the required stability. The front end electronic (FEE), in the direct vicinity of the FPA, can operate at a warmer temperature but requires as well a stringent thermal stability. Therefore the FEE thermal control presents similar features than the FPA though it is thermally decoupled from it. Less demanding in stability, but more exposed to varying external environment, the Telescope thermal control includes 2 sets of MLI and several local heating lines to counter gradients. The verification of the instrument thermal control relies first on thermal analysis before the thermal test campaign in 2015. An extensive survey has been performed to analyse the Instrument thermal performance in all the possible attitudes, with azimuth and elevation steps of 5° to 10° inside the required range (+/-60° away from anti-Sun direction). The stability and gradients maps of the telescope and mirrors, the FPA and the FEE thermal performance have been worked out, allowing to identify the attitude worst condition (both worst hot case and worst cold case) and to define the preliminary thermal control architecture. Then followed a more detailed thermal analysis which results showed that the demanding stability requirements for the FPA CCD and FEE are achievable, with a proper tuning of the PID controllers gains.