  The CHaracterising ExOPlanet Satellite (CHEOPS) was selected in 2012, as the
first small mission in the ESA Science Programme and successfully launched in
December 2019. CHEOPS is a partnership between ESA and Switzerland with
important contributions by ten additional ESA Member States. CHEOPS is the
first mission dedicated to search for transits of exoplanets using ultrahigh
precision photometry on bright stars already known to host planets. As a
follow-up mission, CHEOPS is mainly dedicated to improving, whenever possible,
existing radii measurements or provide first accurate measurements for a subset
of those planets for which the mass has already been estimated from
ground-based spectroscopic surveys and to following phase curves. CHEOPS will
provide prime targets for future spectroscopic atmospheric characterisation.
  Requirements on the photometric precision and stability have been derived for
stars with magnitudes ranging from 6 to 12 in the V band. In particular, CHEOPS
shall be able to detect Earth-size planets transiting G5 dwarf stars in the
magnitude range between 6 and 9 by achieving a photometric precision of 20 ppm
in 6 hours of integration. For K stars in the magnitude range between 9 and 12,
CHEOPS shall be able to detect transiting Neptune-size planets achieving a
photometric precision of 85 ppm in 3 hours of integration. This is achieved by
using a single, frame-transfer, back-illuminated CCD detector at the focal
plane assembly of a 33.5 cm diameter telescope. The 280 kg spacecraft has a
pointing accuracy of about 1 arcsec rms and orbits on a sun-synchronous
dusk-dawn orbit at 700 km altitude.
  The nominal mission lifetime is 3.5 years. During this period, 20% of the
observing time is available to the community through a yearly call and a
discretionary time programme managed by ESA.
