CHEOPS (CHaracterising ExOPlanets Satellite) is a planned European space telescope for the study of the formation of extrasolar planets.
Slated for launch in 2017, the mission aims to bring an optical Ritchey–Chrétien telescope with an aperture of 30 cm, mounted on a standard small satellite platform, into a sun-synchronous orbit of about 800 km altitude. For the planned mission duration of 3.5 years, CHEOPS is to examine transiting exoplanets on known bright and nearby host stars. Its main goal will be to accurately measure the radii of the exoplanets for which ground-based spectroscopic surveys have already provided mass estimates. Knowing both the mass and the size of the exoplanets will allow scientists to determine the planets' approximate composition, such as whether they are gaseous or rocky.
Knowing where to look and at what time to observe makes CHEOPS the most efficient instrument to search for shallow transits and to determine accurate radii for planets in the super-Earth to Neptune mass range (1-6 REarth).
CHEOPS will measure photometric signals with a precision limited by stellar photon noise of 150 ppm/min for a 9th magnitude star. This corresponds to the transit of an Earth-sized planet orbiting a star of 0.9 Rsun in 60 days detected with a S/Ntransit >10 (100 ppm transit depth). For example an Earth size transit toward a G star creates a 80 ppm depth. The spacecraft will provide 54W of continuous power for instrument operations and allow for at least 1GBit/day downlink.
Organized as a partnership between the European Space Agency (ESA) and the Swiss Space Office, CHEOPS was selected in October 2012 from among 26 proposals as the first S-class ("small") space mission in ESA's Science Programme. The project is to be led by the Center for Space and Habitability at the University of Bern, Switzerland, with contributions from other Swiss and European universities.