Animation visualising ESA's CHaracterising ExOPlanet Satellite, Cheops, in orbit above Earth.
Cheops will orbit Earth at an altitude of 700 km in a Sun-synchronous orbit, riding on the day-night terminator – such a configuration is also called dawn/dusk orbit. This orbit allows Cheops to point its instrument always towards the night side of Earth, in order to limit the impact on the measurements of sunlight and reflected stray light from Earth.
To maximize the fraction of the sky where Cheops can perform observations, the instrument can be pointed at any time anywhere in the sky up to 60 degrees from the anti-Sun direction.
The satellite will communicate several times per day with the main Cheops ground station, located at Torrejón de Ardoz, near Madrid, Spain, in order to downlink the science data and receive telecommands for further operations.
Cheops will make observations of exoplanet-hosting stars to measure small changes in their brightness due to the transit of a planet across the star's disc. The information will enable accurate and precise measurements of the sizes of the orbiting planets to be made. Cheops will target stars hosting planets in the super-Earth to Neptune size range. By combining sizes with existing measurements of the planet masses, Cheops will provide an estimate of bulk density – a first step towards characterising planets outside our Solar System.
Note: Some aspects have been simplified for the purpose of this animation.