The mission requirements are defined to provide the extremely stable photometric environment necessary to achieve the low signal-to-noise ratio measurements of transiting small mass planets. Key requirements are mechanical and thermal stability and minimum amount of stray light. At the same time, a large fraction of the sky should be observable since CHEOPS is targeting bright stars.
The CHEOPS baseline concept is a telescope on a standard small satellite bus in LEO. To meet the above requirements, the telescope will be orbiting on a 6am/6pm SSO always pointing away from the Sun. Hence, the satellite will follow as close as possible the day-night terminator.
While this orbit satisfies the main requirements, other orbits have been studied in the early mission phases. For example, the option called “GTO extended” would allow a larger fraction of the sky to be seen albeit at the cost of increased technical effort and budget. The next table summarizes the studied orbit variants.
In either case, the launch will be a shared launch: CHEOPS will not utilize the full capacity of a VEGA class launch vehicle. The baseline mission duration is 3.5 years nominal mission.
Other Mission Orbit Options Previously Under Study
|Orbit characteristics||SSO 800 (baseline)||SSO 1200||GTO extended|
|Perigee altitude [km]||800||1200||10000|
|Apogee altitude [km]||800||1200||35943|
|Orbital period [min]||101||109||834|
|Mean local time of ascending node [h]||6 a.m.||6 a.m.||-|
|Percentage of the sky available for a minimum total duration of 15 days per year and target. Maximum interruption time is 20 minutes per orbit.||27%||47%||*|
|Percentage of the sky available for a minimum total duration of 60 days per year and target. Maximum interruption time is 50 minutes per orbit (for SSO).||58%||69%||100%|
The other orbit variants have been ruled out in the first mission design phase. The 1200 km orbital altitude was ruled out due to high radiation coming from the radiation belts, while the modified GTO orbit was not commensurate with the boundary conditions of a small mission. The baseline orbit is now a SSO with an altitude in the range of 650 to 800 km based on available launch opportunities.
Science targets will be compiled at the Science Operations Centre (SOC) located in Switzerland and communicated to the Mission Operations Centre, where spacecraft commanding sequences will be formed, verified and uplinked via ground station antennas. This is planned at Torrejon, Spain.
From a data point of view CHEOPS is a simple instrument. The data budget is estimated less than 1.2 Gb/day. We baseline an S-band system for TM/TC and for data downlink. Telemetry and raw science data will be archived at the SOC with a backup in Italy.