ESA Science & Technology - CHEOPS

Mission Summary
 

CHEOPS - CHaracterising ExOPlanet Satellite - will be the first mission dedicated to searching for exoplanetary transits by performing ultrahigh precision photometry on bright stars already known to host planets.

Artist's impression of CHEOPS. Credit: ESA - C. Carreau

CHEOPS will provide the unique capability of determining radii within ~10% accuracy for a subset of those planets, in the super-Earth to Neptune mass range, for which the mass has already been estimated using ground-based spectroscopic surveys. CHEOPS will also provide accurate radii for new planets discovered by the next generation of ground-based or space transits surveys (from super-Earth to Neptune-size). By unveiling transiting exoplanets with high potential for in-depth characterisation, CHEOPS will provide suitable targets for future instruments suited to the spectroscopic characterisation of exoplanetary atmospheres.

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.

On 19 October 2012, CHEOPS was selected for study as the first S-class mission in Cosmic Vision 2015-2025. The mission was formally adopted in early February 2014, with launch readiness planned for 2017.

The CHEOPS mission baseline relies completely on components with flight heritage. This is valid for the platform as well as for the payload components. For the latter, the team can exploit significant heritage from the CoRoT mission, minimising both cost and risk.

Science Objectives

The main science goals of the CHEOPS mission will be to measure the bulk density of exoplanets with sizes/masses in the super-Earth – Neptune range orbiting bright stars, and to select the optimal targets for future in-depth characterisation studies of exoplanets in these mass and size ranges. With an accurate knowledge of masses and radii, CHEOPS will set new constraints on the structure and therefore on the formation and evolution of planets in this mass range. In particular, CHEOPS will:

• Perform first-step characterisations of super-Earths, by measuring the radii and densities in a planetary mass range for which only a handful of data exist and to a precision never achieved before, and by identifying planets with significant atmospheres as a function of their mass, distance to the star, and stellar parameters. The presence (or absence) of large gaseous envelopes bears directly on fundamental issues such as runaway gas accretion in the core accretion scenario or the loss of primordial H/He atmospheres.
• Obtain new insights into the physics and formation processes of Neptune-like planets by measuring accurate radii and densities for these planets, deriving minimum values for their gas mass fractions, and inferring possible evolution paths.
• Provide suitable targets for future ground-based (e.g., E-ELT) and space-based (e.g., JWST) facilities with spectroscopic capabilities. With well-determined radii and masses, the CHEOPS planets will constitute the best sample of targets for such future studies, both within the solar neighbourhood and spread over the whole sky.
• Probe the atmospheres of known 'hot Jupiters' in order to study the physical mechanisms and efficiency of the energy transport from the dayside to the night side of the planet.

CHEOPS will also offer 20% of open time to the community, which will be allocated through competitive scientific review.

Spacecraft

CHEOPS will be a small spacecraft with a total launch mass of about ~ 250 kg. The baseline is to use a standard small satellite platform with some modifications. The spacecraft design has been consolidated in the phase A/B1 study, which will also lead to selection of the platform. The CHEOPS mission will be equipped with a single medium-size telescope of ~0.3 m aperture. All platform requirements are aimed at supporting the functionality of the telescope and its ultrahigh photometric precision. The main implications for the platform are related to pointing capabilities and the thermal environment for the payload.

The telescope will be mounted on a stiff optical bench, which defines the interface to the platform, and will be thermally decoupled. A sunshield mounted on the platform will protect the focal plane radiator and detector housing from solar illumination and also carry solar panels for the power subsystem. When stowed for launch, the satellite will measure about 1.5 m × 1.4 m × 1.5 m.
 
The spacecraft will be three-axis stabilised, with a pointing stability of eight arcsec rms during a 48-hour science observation. In a similar manner to the CoRoT mission, the payload will provide centroid data from the target star to the platform's attitude and orbit control system, to enable compensation of low-frequency pointing errors.

During each orbit, the spacecraft will be slowly rotated around the telescope line-of-sight to keep the focal plane radiator oriented towards cold space, enabling passive cooling of the detector.

Payload

To reach its science objectives, CHEOPS will be able to detect an Earth-size planet transiting a G5 dwarf star of 0.9 R_Sun with 6 ≤ m_V ≤ 9. Since the depth of such a transit is 100 ppm, this requires a photometric precision of 20 ppm in 6 hours of integration time, which corresponds to the transit duration of a planet with a revolution period of 50 days.

This precision will be achieved by using a single, frame-transfer, back-illuminated CCD detector with 1024×1024 pixels and a pixel pitch of 13 µm. The detector will be mounted in the focal plane of a ~33 cm diameter, f/8, on-axis Ritchey-Chrétien telescope; it will be passively cooled to < 233 K, with thermal stability < 10 mK.

A processing unit will perform image summation and data reduction. The units to be employed in the payload will all have a Technology Readiness Level (TRL) of greater than five (component and/or breadboard validation in the relevant environment). The detector is an existing component.

Operations

Launch readiness of CHEOPS is planned for 2017; the baseline scenario is a shared launch as auxiliary payload or co-passenger in Vega (VESPA adapter) or Soyuz (under ASAP-S), or in other small launch vehicles (e.g. PSLV). The baseline orbit is Sun-synchronous, with an altitude in the range between 650 and 800 km and a local time of the ascending node of 06:00. This choice permits the rear of the spacecraft to be permanently Sun-pointed, is optimal for uninterrupted observations, and keeps thermal variations of the spacecraft and Earth straylight on the satellite to a minimum as the orbital plane follows, as closely as possible, the day/night terminator.

Planning of the observations will be carried out at the Science Operations Centre and communicated to the Mission Operations Centre, where spacecraft commanding sequences will be formed, verified and uplinked via ground station antennas. The spacecraft telemetry will be routed from the Mission Operations Centre to the Science Data Centre for calibration, processing and archiving. It is planned that the Science Operations Centre will be the responsibility of the University of Geneva, and that the Mission Operations Centre will be the responsibility of ESA.

The data budget for CHEOPS is estimated at 1.2 Gb/day. An S-band system is presently baselined for data downlink, telemetry and telecommand. A mission duration of 3.5 years in orbit is baselined to enable the execution of the proposed core programme, with an allocation of 20% of the observing time open to the whole scientific community.

Partners

The CHEOPS mission is envisaged as a partnership between Switzerland and ESA's Science Programme, with important contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Sweden, and the United Kingdom.