CHEOPS (CHaracterising ExOPlanet Satellite) is a small satellite weighing approximately 250 kilograms that fits inside a cube of 1.5 metres on a side when stowed.
Its instrument payload consists of a Ritchey-Chretien telescope with a 320-millimetre primary mirror and 68-millimetre secondary mirror. Its focal length is 1,600 millimetres, giving a focal ratio of F/5.
Visible light is captured between 0.4 and 1.1 micrometres by a CCD detector with 1024 x 1024 pixels and a pitch of 13 µm (e2V 47-20).
CHEOPS reuses some of the technologies—in particular the baffle and cover release system—from the CoRoT stellar seismology and exoplanet-hunting satellite that completed its mission on 17 June 2014.
The CHEOPS Instrument System (CIS) consists of four separate parts:
- OTA (Optical Telescope Assembly): this includes the telescope (black) and its optical structure with two star trackers (grey) to stabilize the image, the focal plane module (FPM, with the CCD) and radiators to cool the front-end electronics.
- BCA (Baffle and Cover Assembly): this includes the external baffle (grey, designed to suppress stray light from angles greater than 35° from the line of sight, mainly from Earth), the cover assembly (designed to protect the primary mirror up to launch) and a cover release mechanism to be used after launch.
- SEM (Sensor Electric Module): this includes the sensor control unit, used to control and read out the CCD, and the power conditioning unit used to condition/filter voltages supplied to the CCD.
- BEE (Back-End Electronics): this includes the data processing unit (DPU) and the power supply and distribution unit (PSDU) that powers the CIS.
In terms of photometric precision, CHEOPS is designed to offer very high levels of sensitivity. It will measure the brightness and orbital geometry of transits for stars with a magnitude between 6 (the brightest) and 12, or even 13 (the faintest), located almost anywhere in the sky. Unlike CoRoT, Kepler and TESS, all designed to inventory exoplanets, CHEOPS will precisely measure the geometric parameters of already known transits.
Operating at an altitude of 700 kilometres (620-800 kilometres to be precise) and inclined 98° to the equator, CHEOPS will orbit pole to pole at the day-night terminator, constantly alternating between dawn and dusk. In this 100-minute Sun-synchronous orbit it will circle the planet 14 times a day, observing without interruption night and day in glare-free conditions.
The satellite will be oriented with its back to the Sun to maintain a stable temperature as far as possible. The solar panels on its sunshield will also ensure a constant and maximum supply of power, as well as optimal cooling of its instruments. The telescope’s average operating temperature is –10°C: the optical elements are mounted on a carbon-fibre-reinforced polymer structure to reduce thermal variations, which can despite all efforts be too great in near-Earth orbit to achieve the desired degree of precision. The CCD detector is thus planned to operate at –40°C, with a margin of less than one-hundredth of a degree.
Three solar panels with a total surface area of 1.85 square metres deliver 138 watts of power to the satellite. Attitude control is provided by reaction wheels. The satellite’s altitude is boosted by two one-newton thrusters powered by 6.63 kilograms of hydrazine fuel stored in a 26-centimetre tank.
Sky projection with a preliminary list of possible targets for CHEOPS. White areas cannot be observed due to the Sun’s glare; orange areas can be observed for more than 2,000 hours a year. Credits: ESA, 2013