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CHEOPS (for Characterizing Exoplanet Satellite) is a European Space Agency (ESA) space telescope for determining the radii of extra-solar planets. It was launched in December 2019 and has started its 3.5-year primary mission (a 3-year extension was approved in 2023). It is a 32-cm Ritchey-Chrétien telescope tailored to take accurate photometric measurements of the transits of transiting planets of known mass, to enable estimation of their density, e.g., to classify them as rocky planets or gas planets. In contrast to the missions of Kepler Telescope and Transiting Exoplanet Survey Satellite to locate planets, CHEOPS aims to gather more precise information about previously-discovered planets.