Minimize CHEOPS (Characterizing ExOPlanets Satellite)

CHEOPS (CHaracterizing ExOPlanets Satellite)

Spacecraft     Launch    Mission Status     Sensor Complement    Ground Segment    References

CHEOPS is a minisatellite, the first mission in ESA's small Science Program, selected in October 2012; the project was adopted for implementation in February 2014. The SPC (Science Program Committee) of ESA decides on the program content, which is structured along the Cosmic Vision Plan since 2007.

The objective is to target nearby, bright stars already known to have planets orbiting around them. Through high-precision monitoring of the star’s brightness, scientists will search for the telltale signs of a ‘transit’ as a planet passes briefly across its face. This will allow an accurate measurement of the radius of the planet. For those planets with a known mass, the density will be revealed, providing an indication of the internal structure. 1)


Figure 1: Artist's impression of a planet transiting a star (image credit: ESA/ATG medialab)

The CHEOPS mission is devoted to the fine characterization of exoplanets orbiting around bright stars by using transit photometric measurements. The mission is envisaged as a partnership between the ESA Science Program and Switzerland, through the Swiss Space Office (SSO). The University of Bern leads a consortium of 11 ESA Member States contributing to the mission and represented in the CHEOPS Science Team. ESA is the mission architect responsible for overall mission definition and procurement of the spacecraft and launch. ESA is also responsible for the early operations phase that will be executed by the spacecraft contractor, Airbus Defence and Space – Spain. The science instrument is led by the University of Bern, with important contributions from Austria, Belgium, Germany and Italy. 2) 3) 4) 5) 6)

CHEOPS is considered as a test case for implementing a S-Mission (Small Mission) in the ESA Science Program, and it’s being implemented according to the following requirements:

- S-class missions must be science-driven and selected through an open call for missions (bottom-up process)

- The mission implementation cycle, from the call for proposals to launch, must be drastically shorter than for medium (M) and large (L) class missions

- Missions must be cost-capped, possibly with a proportionally larger Member States involvement than for M or L missions.

Science measurements

Observations of planetary transits for known exoplanet host stars brighter than the 12th magnitude in the V band. Photometric measurement accuracy < 20 ppm for stars with V < 9 (goal < 10 ppm) in 6 hours of integration time.

Mission profile

-Nominal orbit: sun-synchronous, 800 km, LTAN 6:00 hrs. Spacecraft compatible with several launchers as passenger (e.g. Soyuz under ASAP-S, VEGA using VESPA adapter, PSLV, etc.).
- Launch readiness: end 2017.
- Nominal in-orbit operations: 3.5 years.


- On-axis Ritchey-Chrétien telescope of 33 cm diameter, FOV 19 arcmin x 19 arcmin, back-illuminated frame transfer CCD detector.
- Mass: 60 kg; power 60 W.


- 3-axis stabilized platform, with pointing accuracy better than 8 arcsec rms over a 48 hour observation. Use of the star centroid from the science instrument for removing low frequency pointing errors.
- Spacecraft wet mass: < 280 kg. Telemetry rate: > 1 Gbit/day downlink.

Table 1: High-level CHEOPS mission requirements

Project organization & responsibilities: The CHEOPS Mission Consortium (CMC), led by the University of Bern (CH), is responsible for the following mission elements: 7)

• Development, procurement, qualification and calibration of the instrument

• Provision of AIT services in support to the satellite activities

• In-orbit monitoring, calibration and evaluation of the instrument performance

• Provision of the CHEOPS ground segment, including:

- The Ground station(s)

- The MOC (Mission Operations Center)

- The SOC (Science Operations Center)

ESA will be in charge of the overall mission architecture and is responsible for the following activities:

• Procurement of the CHEOPS platform accommodating the instrument assembly

• Procurement of the instrument detector CCD

• Integration, test and qualification of the complete satellite

• Procurement of the mission control system, satellite and instrument simulator and any required flight dynamics system

• Procurement of the launch opportunity and launch operations

• LEOP and in-orbit commissioning.

Mission architect, platform development



Member States (Consortium led by Switzerland with W. Benz as PI at the University of Bern)



Spacecraft operations

Member States (Operations will be implemented by Spain, as part of the Swiss-led Consortium)

Science operations

Member States (Operations will be implemented by the University of Geneva, as part of the Swiss-led Consortium)

Table 2: Responsibilities for the CHEOPS mission

Technology readiness requirements:

The Call for S1 mission requested the mission concept to be compatible with existing platforms, with minimum modification. All platform equipment must be off-the-shelf, flight qualified (TRL ≥ in ISO scale), and preferably flight proven (TRL ≥ 9). Modifications to existing products must be compatible with the development schedule (less than ~ 3 years for the platform) with satisfactory margins. Limited adaptation of the platform mechanical design and accommodation is an illustrative example of allowable modifications: although such modification may require the requalification of the platform structure, it can be achieved in less than 2 years with low schedule risks, and is therefore acceptable.

The science payload was requested to rely on available technologies (TRL > 5-6 in ISO scale), without ruling out new instrument developments, provided they can be safely achieved in less than three years (e.g. using some heritage or building on previous developments).

With the above approach, the major development risks are on the payload side. One could have equally requested a high TRL for the science instrument, as for the platform equipment. Such an approach was deliberately not followed for maximizing the “science accessibility” of the Call and the competition at mission level. Indeed, a higher payload TRL (TRL ≥ 7) would imply designing the mission around an existing instrument that would have been recently developed for a previous mission. As a matter of fact, most of the shortlisted missions, including CHEOPS, could not have been proposed under such condition.

Industrial implementation through ESA:

The approach retained for the industrial development of S-missions differs substantially from that for L and M missions, while obeying all the Agency rules. L or M missions are defined and implemented in three major phases:

1) The preparation phase: This phase starts by a Phase 0 performed by ESA, then includes a detailed study phase (Phases A and B1) where the mission definition and requirements are progressively matured and finally frozen, and where the mission critical technologies are developed in parallel aiming at TRL ≥ 6 (ISO scale) prior to starting the spacecraft development. The study phase is achieved with parallel industrial contracts, awarded in competition in two steps, for preserving the industrial competition for the spacecraft development. At the end of the preparation phase, the SPC in principle has available all critical elements for adopting the mission (i.e. deciding
on its implementation) in particular: the actual science that can be achieved, the development schedule, the development risks for all the space segment elements including the science payload, and the mission cost to ESA.

2) The implementation phase: Following the mission adoption by the SPC, the Agency initiates the implementation phase by first selecting the Prime contractor for the space segment development, and then the subsystem contractors and the equipment suppliers. Most of the industrial contractors are known at the end of phase B2, which is closed by the system PDR (Preliminary Design Review). The selection of all industrial contractors is made through ESA “procurement best practices”, i.e. in open competition with due consideration to specific procurement constraints, such as the geographical distribution of contracts. Following successful development and verification, the space segment is launched and the implementation phase ends with in-orbit commissioning.

3) The operational phase: The operational phase covers the nominal lifetime in-orbit plus possible extended time. The operations include: the control of the spacecraft, including communications; the execution of the required science observations (science operations); the data processing prior to delivery to the science community; and the data archiving.

For CHEOPS, the ESA Industrial Policy Committee approved the following approach, which is being implemented:

• A single Invitation to Tender was issued, covering both the study phase and the implementation phase for the platform. Obviously, this is possible only if 1) no technology developments are required and 2) the space segment requirements are mature enough at the end of Phase 0. Both conditions should be met by S-missions for enabling a fast implementation, and both were actually met by CHEOPS. A ceiling price was required from the bidders for the implementation phase.

• Parallel contracts were awarded for a “study and final contractor selection phase” limited to ~8 months, which was used for maturing the instrument definition and interfaces, and clearing critical contractual aspects. The parallel contracts were awarded to EADS CASA ESPACIO (ECE, Spain) and SSTL (UK). At the end of the study phase, the System Requirement Review was held with both competing contractors. The two shortlisted contractors were also required to convert their initial proposal to a firm fixed price proposal covering any requirement update resulting from the study phase, and providing their “best and final offer” prior to selection. ECE was finally selected by ESA for the platform development.

• While the platform contractor selection was made following ESA standard best practices, the platform equipment level was not subject to the same selection process. Indeed, since the concept is asking a fast development schedule and is therefore based on re-using existing “off-the-shelf” platforms with minimum modifications, it makes little sense to re-open the platform equipment selection through extensive ESA procurement best practices. The two competing platform contractors were requested to make best use of their respective heritage for meeting the high level requirements of S-missions, namely: a fast development schedule; mastered development risks; and cost ceiling. The faster selection of equipment suppliers, mostly through direct negotiations, also enabled to lower ESA internal costs.

The Science Program requirements regarding geographical return balance were taken into account by strongly constraining the over-returned countries (France and Germany in this case) in the platform Invitation to Tenders de facto excluding them from the platform competition. Therefore, any platform implemented under these conditions would contribute to the geographical return re-balance of the program – although modestly, since the CHEOPS contract value is only 6-7% of a typical Prime industrial contract for a M-class mission. In a way, the full CHEOPS contract is comparable in value to a subsystem of an M-mission spacecraft, and it is not excluded by ESA rules to make use of a simplified procurement scheme if, for some reason, the subsystem is required to be recurring from a previous mission.


Figure 2: Organogram of the CHEOPS project (image credit: CHEOPS Team)

Science objectives:

The discovery of planets around other stars (exoplanets) has opened one of the most interesting and exciting field in modern astronomy. After nearly two decades mainly dedicated to a census of exoplanets, emphasis is now shifting on the physical and chemical characterization of exoplanets and their systems. CHEOPS will be the first mission dedicated to search for transits of exoplanets by means of ultrahigh precision photometry on bright stars already known to host planets. The main science objective of the CHEOPS mission will be to study the structure of exoplanets smaller than Saturn orbiting bright stars and to measure the bulk density of super-Earths and Neptunes orbiting bright stars (Ref. 4).

CHEOPS will provide the unique capability of determining accurate radii for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys, providing on-the-fly characterization for exoplanets located almost everywhere in the sky. It will also provide accurate radii for new planets discovered by the next generation of ground-based transits surveys (Neptune-size and smaller). By unveiling transiting exoplanets with high potential for in-depth characterization, CHEOPS will provide “golden targets” for future missions, such as the recently selected M3-Plato mission, and for future instruments suited to the spectroscopic characterization of exo-planetary atmospheres.

CHEOPS is built to achieve a photometric precision similar to the Kepler mission of NASA while observing much brighter stars located almost anywhere on the sky. The CHEOPS target list will mainly consist of stars with small exoplanets previously detected by accurate Doppler surveys and known transiting Neptune-size planets detected by ground-based photometric surveys. CHEOPS is also currently envisioned following-up a significant number of targets from the planned TESS (Transiting Exoplanet Survey Satellite) mission of NASA.

In comparison with random searches such as those carried out by CoRoT and Kepler, CHEOPS will be incredibly more efficient. Knowing where to look and at what time to observe is priceless information particularly to detect (and characterize) long-period transiting systems (up to 50 days of revolution period).

Target observability and sky coverage: The key to precise photometric transit measurements is to keep the noise in the measurements to an absolute minimum. Besides the noise associated with the detector itself, stray light is the main source of noise. This stray light is minimized through the design of the telescope itself, the orbit chosen, and by limiting the directions in which the telescope points in order to avoid solar light reflected by the Earth and/or the Moon reaching the detector. With the current design of the CHEOPS telescope, in order to meet the photometric requirements mentioned above, the following conditions have to be met:

- the angle between the line-of-sight and the sun must be larger than 120º

- the angle between the line-of-sight and any illuminated part of the Earth must be larger than 35º. In addition, the target must have a projected altitude from the surface of the Earth equal or higher than 100 km

- the angle between the line-of-sight and the Moon must be larger than 5º.

The prime mission goals of CHEOPS are:

1) Performing first-step characterizations of super-Earths: Measuring the radius, density, and inferring the presence or absence of a significant atmospheric envelope for super-Earths in a wide range of environmental conditions. This will be achieved by searching for shallow transits on bright stars (6 < V < 9 mag) already known to host planets in this mass range with revolution periods up to 50 days.

2) Obtaining new insights into the physics and formation processes of Neptunes: Measuring accurate radii for Neptunes, determining precise densities, deriving minimum values of their gas mass fractions, and inferring possible evolution paths. This will be achieved by characterizing the transit light curves of tens of Neptunes with revolution periods up to 13 days, previously detected in transit across stars brighter than the 12th magnitude in the V band.

3) Building a collection of “golden targets” for exo-planetology: The small mass exoplanets transiting bright stars observed by CHEOPS will be the best suited targets for in-depth atmospheric characterization by spectroscopic facilities contemporaneous or subsequent to the mission (e.g., the James Webb Space Telescope).


Figure 3: Illustration of the CHEOPS mission (image credit: Airbus DS, Ref. 12)

Some background on Exoplanet missions

The first discoveries of exoplanets in the 1990s, by ground-based observatories, completely changed our perspective of the Solar System and opened up new areas of research that continues today. This infographic highlights the main space-based contributors to the field, including not only exoplanet-dedicated missions, but also exoplanet-sensitive missions, past, present and future. 8)

One of the first exoplanet-sensitive space telescopes was the CNES-led CoRoT (Convection, Rotation and planetary Transits) mission, which launched in 2006. While focused on studies of stars, it uncovered exoplanets using the transit method.

NASA’s 2009 Kepler mission is an exoplanet discovery machine, accounting for around three-quarters of all exoplanet discoveries so far. It looks at a specific patch of sky for a lengthy period of time, so is sensitive to a great number of faint stars.


Figure 4: Overview of exoplanet mission timeline as well as contributing missions (image credit: ESA)

Meanwhile, telescopes that were launched even before the first discoveries were confirmed, such as the NASA/ESA Hubble Space Telescope, have been contributing to the evolving field.

ESA’s Gaia mission, through its unprecedented all-sky survey of the position, brightness and motion of over one billion stars, is providing a large database to search for exoplanets. These will be uncovered through the changes in the star’s motion as the planet or planets orbit around it, or by the dip in the star’s brightness as the planet transits across its face.

The most recent addition to the exoplanet-hunting fleet is NASA’s TESS (Transiting Exoplanet Survey Satellite), launched in April 2018. It is the first all-sky transit survey satellite.

But discovering an exoplanet is just the beginning: dedicated telescopes are needed to follow up on this ever-growing catalog to understand their properties, to get closer to knowing if another Earth-like planet exists, and better understanding what conditions are needed for planet formation and the emergence of life.

ESA is launching three dedicated exoplanet satellites in the next decade, each tackling a unique topic.

1) CHEOPS (Characterizing Exoplanet Satellite) will observe bright stars known to host exoplanets, in particular Earth-to-Neptune-sized planets, anywhere in the sky. By knowing exactly where and when to look for transits, and being able to return to repeatedly observe the same targets, Cheops will become the most efficient instrument to study individual exoplanets. It will record the precise sizes of these relatively small planets and combined with mass measurements already calculated from other observatories, will enable the planet’s density to be determined, and thus make a first-step characterization of the nature of these worlds. — Cheops will also identify candidates for additional study by future missions. For example, it will provide well-characterized targets for the international James Webb Space Telescope, which will perform further detailed studies of their atmospheres.

2) Plato (PLAnetary Transits and Oscillations of stars) mission, is a next-generation planet hunter with an emphasis on the properties of rocky planets up to the habitable zone around Sun-like stars – the location from a star where liquid water can exist on the planet’s surface. Importantly, it will also analyze the planet’s host star, including its age, and thus give insight into the evolutionary state of the entire extrasolar system.

3) Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large-survey) mission, will perform a chemical census of a large and diverse sample of exoplanets by analyzing their atmospheres in great detail.

With the complementary work of both ground- and space-based observatories, we will get closer to understanding one of humanity’s biggest questions: are we alone in the Universe?


Figure 5: Exoplanet imaginarium (image credit: ESA) 9)


The Spacecraft platform has undergone a successful SRR (System Requirements Review) in March 2014 leading to the selection of the platform contractor by end of April 2014, with a scheduled PDR (Preliminary Design Review) for September 2014, and a fast implementation phase with a readiness for launch in 2018. 10)

The short platform development schedule (~2.5 years) precludes any new technology development, requiring preferably TRL 6 (qualified items) for all elements of the platform. Therefore, as specified in the ITT (Invitation To Tender), the proposed CHEOPS platform had to be an adaptation of an existing product, flight qualified and with heritage for all aspects including equipment qualification, design, development and verification approach (Ref. 4).

From the mission analysis and to satisfy the science requirements, the driving design requirements for the platform design are:

• Sun-synchronous orbit with a local time in the ascending mode (LTAN) to be at 6:00 or at 18:00 hours (decision open in order to increase possibility to find a co-passenger for launch) with an altitude in the range between 650 and 800 km, enabling to have the sun permanently on the backside of the spacecraft and minimizing Earth stray-light.

• A 3-axis stabilized spacecraft, keeping the telescope line of sight fixed in inertial reference frame with an APE (Absolute Performance Error) better than 8 arcsec for pointing accuracy (for 68% of the observation time), slowly rotating around the telescope line of sight for maintaining the focal plane radiator oriented towards cold space (i.e., at zenith of the subsatellite point), enabling a passive cooling of the CIS (Contact Image Sensor) detector to 233 K.

• Instrument to avoid pointing at the sun once cover is opened and provision of a fixed sunshield to protect the instrument radiators from sun illumination and providing it with a stable thermal environment.

• Required compatibility with a variety of different launcher vehicles and thus an envelope launch environment requirements have been defined, presently covering baseline launchers: VEGA and Soyuz, and optional launchers: Rockot, PSLV, Dnepr and Falcon-9. (It is planned to select the Launcher Vehicle at the latest for the mission level Critical Design Review).

• Mass of the spacecraft is approximately 290 kg to be compatible with a shared launch as co-passenger.

• Compatibility with several Ground Station coverage scenarios, e.g. using Redu, Harwell, Malindi and/or Kiruna, and allowing for 1.2 Gbit/day downlink in S-band.

• Deorbiting of the spacecraft within 25 years at the end of the mission, sized for a worst case 800 km orbit, to comply with ESA orbit debris mitigation requirements.

The project development is organized on two main phases:

• Phase 1 (A/B1) – Competitive definition phase to consolidate all CHEOPS mission requirements, provide evidence that the proposed spacecraft design meets those technical, quality and programmatic requirements and the budget constraints. This phase is ending with the SRR.

• Phase 2 (B2/C/D/E1) – Implementation phase encompassing activities to freeze the spacecraft design until Launch, LEOP and In-Orbit Commissioning.

And, as a key element in the CHEOPS implementation approach, ESA released a single ITT (Invitation To Tender) for the satellite procurement covering both phases calling for a full implementation proposal, with a corresponding ceiling price, to be converted in a firm-fixed price at the end of Phase 1.

• In July 2014, ESA announced that the CHEOPS project had entered the implementation phase (meaning that the spacecraft has gone from selection to implementation in less than 18 months). Capped to an ESA cost of €50 million, CHEOPS is being developed in collaboration with the Swiss Space Office (SSO), a division of the Swiss State Secretariat for Education, Research and Innovation (SERI), and the University of Bern, Switzerland. The Swiss organizations lead the consortium of 11 ESA Member States contributing to the mission and represented in the CHEOPS Science Team. The spacecraft will be built by Airbus Defence and Space, Spain. 11)

- The science instrument is led by the University of Bern, with important contributions from Italy, Germany, Austria, and Belgium. Other contributions to the science instrument in the form of hardware, or in the science operations and exploitation, are provided by the United Kingdom, France, Hungary, Portugal and Sweden. The MOC (Mission Operations Center) is under the responsibility of Spain, while the SOC (Science Operations Center) is located at the University of Geneva, Switzerland.

- The Prime contractor for CHEOPS is Airbus Defence and Space, Spain (note: former EADS CASA Espacio S.L. Is now part of Airbus Defence and Space).


Figure 6: ECE-CASA spacecraft design (image credit: ESA, CHEOPS Team)

CHEOPS platform: 12)

The CHEOPS spacecraft is based on the Airbus Defence and Space AstroBus family of low cost satellite platforms (following on from e.g. Spot- 6 & -7, KazEOSat-1), and the ninth for an ESA program following on from Sentinel-5 Precursor and the MetOp Second Generation satellites.

The satellite is composed by a prismatic body platform, where the Optical Instrument is mounted on its top surface, plus three body-mounted solar array panels (Figure 6). There is a fixed sunshield in the central solar panel, to provide the required shadow to Instrument radiators.

The three body-mounted solar array panels have at 67 degree deviation between them, in order to have best illumination in any mission attitude. The bottom surface is dedicated to the launcher interface, for which a 937 standard clamp-band system is proposed. The propulsion module thrusters and S-band –Z antenna are in bottom panel with the minimum protrusion downwards from separation plane. The top surface of the platform prismatic structure is dedicated to the accommodation of the Instrument elements, the Optical Telescope Assembly and Baffle and Cover Assembly.

Two Instrument electronic units are accommodated inside the satellite platform, Sensor Electronics Module and the Back-End Electronics.

The structure design is made of sandwiches with metallic skins for top-bottom-side panels to benefit of the strong SEOSAT/Ingenio heritage. The panels accommodate the platform and instrument equipment. The prismatic hexagonal body allows the side panels to be deployed using special removable hinges for units’ assembly and electrical and functional integration campaigns.

The thermal control design driver is to achieve a stable payload thermal environment in orbit while minimizing the perturbations on instrument radiator. For such purpose, a sunshield prevents the sun to illuminate instruments radiator under any observation condition. Besides, passive thermal control means are MLI (Multi-Layered Insulation) to insulate from cold space and external fluxes, use of massive parts to improve thermal stability in time, high conductance link between the sensitive parts and their dedicated radiator, radiative surfaces to reject heat and cool down the sensitive parts and conductive thermal insulation. Additionally, heating lines are controlled by the instrument electronics in operating mode, while the survival mode heating lines are operated by the platform. 13)

The platform thermal architecture is based on the AS250 avionics architecture adapted to the CHEOPS mission specific needs and observation constraints, whose main constituents are:

- MLIs to provide insulation from the external environment

- Radiator foils to allow rejection of the heat dissipated by electronic units

- Black coatings to increase the radiative coupling inside the platform

- Interface fillers to increase the thermal contact with the mounting panel

- Thermal washers to conductively decouple from the mounting panel

- Active thermal control (foil heaters and temperature sensors) to maintain all the equipment above their minimum design temperature during cold phases.

The electrical architecture for CHEOPS (Figure 7) is fully recurrent of the AS250 electrical architecture with the corresponding tuning for adaptation to CHEOPS needs, namely:

- A compact S-band communication transceiver (TRCV)

- New models of AOCS sensors and actuators sized to CHEOPS needs: reaction wheels and magnetorquers

- A 30 kg propulsion module.


Figure 7: CHEOPS electrical architecture (image credit: Airbus DS)

The CHEOPS avionics architecture is built around a core DHS (Data Handling Subsystem) made of the OBC (On-Board Computer), the RIU (Remote Interface Unit), and a set of two redundant 1553 buses, one dedicated to the platform including the power management unit and star tracker, the other dedicated to the instrument.

Analog/discrete interfaces are handled through the RIU that is in charge of acquiring the housekeeping information of the spacecraft. It is also in charge of managing through dedicated boards the propulsion module items (valves, pressure transducer, cat-bed heaters), the AOCS components (magnetometers, magnetorquers and reaction wheels) and to control through high power commands the configuration of the satellite units.

The S-band transceiver and the reaction wheels interface with the RIU through an UART link and further with the OBC through the 1553 RIU interface. These UART interfaces are used for commanding and housekeeping/monitoring telemetry acquisition.

The synchronization between the spacecraft equipment is provided by means of synchronization lines distribution generated by the OBC and distributed via the RIU towards the client units (instrument, star tracker).


Figure 8: Satellite units’ layout in the hexagonal body surfaces (image credit: Airbus DS)

The PCDU (Power Conditioning and Distribution Unit) handles the switching and protection of the power lines for all the satellite units. Most units are protected by a LCL (Latch-up Current Limiter) against any over-current anomaly. Vital units, transceiver and the OBC, that allow recovery and controllability of the satellite in case of critical anomaly, are powered through FCLs (Fault Current Limiters) that maintain powering of the protected unit even after an anomaly.

The non-regulated primary power bus is directly connected to the battery. Its voltage is thus fully dependent of the battery state of charge and of the instantaneous power consumption. It ranges from 22 V to a maximum voltage of 34 V. Secondary power distribution is ensured either within the unit for elaborated units [STR (Star Tracker)], or distributed by the RIU for the most elementary ones (AOCS sensors/actuators).

The Solar Array (SA) accommodation to the primary power bus is performed by DET (Direct Energy Transfer) and is implemented by switches in parallel with SA sections (shunt regulation). Excess of SA power is dissipated within the PCDU.

AOCS (Attitude and Orbit Control Subsystem): The CHEOPS AOCS is mostly inherited from the AS250 avionics product line with some adaptations. The AOCS design for CHEOPS has shown that a GPS receiver is not needed, as opposed to the AS250 baseline. Although this decision leads to a lack of an on-board accurate state vector measurement, a two-steps thorough analysis proved the feasibility of the solution despite this modification. First, it was confirmed that science pointing requirements are not degraded by this issue (regarding parallax effects), and second the analysis showed that degradation of performances of on-board AOCS functions using on-board propagated state vector are acceptable (roll guidance, magnetic model and orbit control functions). In order to avoid long term drifting of the onboard propagator, a periodic state vector update from the ground is used, based on Doppler measurements. Another change with respect to the AS250 architecture is the removal of a CSS (Coarse Sun Sensor). Since CHEOPS features a dusk-dawn orbit, the B-dot law used in Safe Mode ensures the convergence of solar arrays towards Sun direction by design, without further reorientation of the axis perpendicular to the orbit as it is the case in a barbecue configuration. This fact renders the CSS unnecessary for this mission. Additionally, the design incorporates payload-based data of the direction of the target star centroid, which is used by the AOCS to estimate and compensate for thermoelastic deformations between the star tracker and the instrument. This approach allows improving the pointing performance of the satellite, reaching a value of a few arcseconds. 14)


Figure 9: CHEOPS satellite (left)) and AOCS architecture (right), image credit: Airbus DS

The AOCS must ensure an accurate and stable pointing during the observation, with an APE (Attitude Performance Error) of < 4 arcseconds (8 arcseconds in case of optical head failure) at a 68% confidence level. The inertial direction of observation of the star can be selected by the ground segment with the following constraints:

- The Sun must not be inside a cone around the line of sight of the telescope (Sun exclusion angle) of at most 120º

- The target shall not be occulted by the Earth, taking a slightly enhanced Earth radius by 100 km to avoid atmospheric glow

- Additionally, an Earth stray light exclusion angle of 35º and a Moon exclusion angle of 5º are also applicable.

As shown in Figure 9, the X axis of the spacecraft is nominally aligned with the instrument LoS (Line of Sight). This means that there is a degree of freedom around roll axis during observations, since the rotation of the image around the star center is irrelevant for the scientific purposes, because an integration of the incoming light is performed, which is independent of the roll angle. This degree of freedom is used to keep the instrument radiators away from Nadir direction for thermal stability purposes. This is a constraint in the body Z axis which unambiguously defines the reference quaternion to be computed by the guidance law and followed by the control algorithms.

Another important mission constraint is related to the need to avoid the sunlight intrusion in the payload CCD. Indeed, in order to ensure this occurrence immediately launch, a cover is included at the aperture of the instrument which is released after initial stabilization phase. The three-axes attitude control during nominal phases of the mission guarantee by design the compliance of this constraint. For the Safe Mode, an extensive analysis has been conducted to characterize the probability of such event, which proves to be sufficiently low and only possible under very special initial conditions.

Regarding orbit control, the mission needs ΔV maneuvers to correct launcher injection errors, to conduct collision avoidance maneuvers and to carry out a deorbiting campaign at the end of the satellite lifetime. Nevertheless, due to the mission features, orbit control to compensate for disturbance forces is not needed during nominal life, which simplifies the operations on-ground and reduces the propellant budget.

Figure 9 depicts the final architecture implemented for CHEOPS. One compact and accurate multi-head STR (Star Tracker ) is used as attitude sensor for nominal operations. Two optical heads are mounted in a direction close to the instrument line of sight and used in hot redundancy. Two electronic units are embarked also in cold redundancy, each one of them cross-strapped to the two optical heads of the star sensor. The optical heads are accommodated on the payload panel to optimize performance because of lower thermoelastic deformations, and their orientation is chosen to ensure their availability for all mission phases.

A cluster of four reaction wheels is used in hot redundancy for three axes attitude control in normal mode, and three internally redundant magnetorquers are used for wheels offloading. These two groups of actuators are also used in Safe Mode, where the magnetorquers also provide attitude control. Two magnetometers are used for magnetic field measurement in Safe Mode.

The propulsion subsystem simple architecture reduces overall mass, propellant and cost (including design and integration). Uniquely used for orbit corrections, it is based on four thrusters aligned in the same direction and one tank of mono propellant. As the propulsion subsystem is never used for Safe Mode neither for normal operations outside orbit corrections, it allows decreasing the required propellant mass. Furthermore, this architecture has been extensively used in other projects with excellent results.

RF communications: The communications subsystem has been designed to meet the uplink and downlink requirements considering that the baseline nominal ground station is located at Torrejon and the backup station is Vilspa-1. The use of Kiruna or Svalbard stations is a possible option to increase LEOP and/or decommissioning coverage. The CHEOPS proposed communications subsystem is based on the following architecture. 15)


Figure 10: Topology of the Cheops communications subsystem (image credit: CHEOPS Team)

This subsystem is composed by two transceivers (nominal and redundant), two couplers and four patch antennas (two for transmission and two for reception). The transceivers include a transmitter, in cold redundancy with the one in the other chain, a receiver, in hot redundancy with the one in the other chain, and adequate filtering to ensure self-compatibility and avoid interferences.

The architecture considers antennas with the same circular polarization (RHCP) for the uplink. In order to avoid signal cancelling, and discriminate the two different paths, there is a privileged path with no attenuation, and an attenuated path with 14 dB attenuation. Therefore, in case of signal combination, no destructive results will appear.

For the downlink, different circular polarization (RHCP and LHCP) will be used for each antenna. Due to CHEOPS mission attitude, there will be a change from one antenna (one polarization) to the other antenna (other polarization) during one pass. Therefore, it requires the ground station capability to simultaneously receive both polarizations and be able to select the best signal.


2033.66 MHz

TC modulation

16 kHz subcarrier PM

Bit rate (RF)

4 kbit/s


Pseudo-randomization decoding performed at OBC


Threshold: -120 dBm carrier ±140 kHz @32 kHz/s


Threshold: -125 dBm carrier ±140 kHz @32 kHz/s

Receiver noise figure

≤ 3 dB

Power consumption

4.5 W (per receiver)

Table 3: Uplink specification


2208.5 MHz

TM modulation

SRRC (Square-Root Raised Cosine Signals), OQPSK (Offset Quadrature-Phase Shift Keying)

Bit rate (RF)

1143 kbit/s

Output power

33 dBm (2 W)

Bandwidth @99% power

1,5 MHz

SRRC roll-off



Reed Solomon (255,223) (performed at OBC), Pseudo-randomization (performed at OBC)

Power consumption

14 W per transmitter

Table 4: Downlink specification


Figure 11: Antenna location in spacecraft (image credit: CHEOPS Team)


Figure 12: COM DEV STC-MS03 transceiver (image credit: COM DEV)


ESA (European Space Agency)

Primary goal

Characterize transiting exoplanets orbiting bright host stars


Known exoplanet host stars with a V-magnitude ≤ 12 anywhere in the sky

Spacecraft characteristics

Size: 1,6 m height, 290 kg mass, 200 W nominal operation, 30 kg propulsion tank

On board image memory

Solid State Mass Memory in OBC up to 3.8 Gbit


End Q2 2018 (shared flight opportunity)


Lifetime: 3.5 years science operations (5 years goal) – First S-class mission in Cosmic Vision 2015-2025 and the first one Airbus DS Spain has won in an open competition


On axis telescope with a diameter of 32 cm; capability of determining planets radii within ~10% accuracy


Three-axis satellite attitude control and instrument-in-the-loop.
Sensors: Magnetometers, star trackers
Actuators: Magneto-torquers, reaction wheels, propulsion module
Pointing stability < 4º
Attitude: defined by the telescope Line-of-sight and the direction of nadir

Airbus DS responsibilities

AIRBUS DS Spain was awarded with the contract to manufacture and integrate the satellite platform, the integration of the scientific instrument, the realization of the satellite functional and environmental campaigns, the launch campaign and the LEOP and IOC phases

Table 5: CHEOPS key features (Ref. 10)


Figure 13: Summary of the milestones achieved and planned until completion (image credit:ESA) 16)

Launch: The CHEOPS minisatellite was launched as a secondary payload on 18 December 2019 (08:54:20 UTC)on an Arianespace Soyuz rocket, designated VS23, from the the Guiana Space Center in Kourou. — The primary mission on this Soyuz flight is the first COSMO-SkyMed Second Generation (CSG-1), radar surveillance satellite for ASI, the Italian space agency. The Soyuz 2-1b (Soyuz ST-B) rocket used a Fregat upper stage. It provided an estimated total lift performance of 3,250 kg, including the satellites and ASAP-S multi-passenger dispenser system. Flight VS23 was Arianespace’s third launch in 2019 using a medium-lift Soyuz, and the ninth overall this year across its full family of launchers – which also includes the heavy-lift Ariane 5 and lightweight Vega. 17) 18) 19)

Figure 14: ESA's Cheops satellite lifts off from Europe’s Spaceport in Kourou, French Guiana. The Soyuz-Fregat launcher will also deliver the Italian space agency’s Cosmo-SkyMed Second Generation satellite (CSG-1), and three CubeSats – including ESA’s OPS-SAT – into space today. Cheops is ESA’s first mission dedicated to the study of extrasolar planets, or exoplanets. It will observe bright stars that are already known to host planets, measuring minuscule brightness changes due to the planet’s transit across the star’s disc (video credit: ESA/CNES/Arianespace) 20)

Orbit: The CHEOPS baseline orbit satisfying the science requirements is a 6:00 hr (baseline) or 18:00 hr (backup) sun-synchronous orbit (dawn/dusk orbit). The two satellites will separate in turn into their own orbits soon after ascent, with CHEOPS operating in LEO (Low Earth Orbit) at an altitude of 700 km and an inclination of ~98º, enabling to have the Sun permanently on the backside of the satellite and minimizing Earth straylight.

Secondary payloads

• CHEOPS (CHaracterizing ExOPlanets Satellite) minisatellite of ESA with a mass of ~300 kg.

• EyeSat, a 3U CubeSat (5 kg) student satellite of the University Technological Institute (IUT) in Cachan, France and CNES.

• ANGELS (ARGOS Néo on a Generic Economical and Light Satellite), a microsatellite (30 kg) of CNES and NEXEYA, an innovative industrial group active in the aerospace market.

• OPS-SAT, a 3U CubeSat (6 kg) of ESA/ESOC that will allow on-orbit testing and demonstration of experimental and innovative software that may be used in future ESA missions and programs.

The CHEOPS satellite will observe individual bright stars that are known to host exoplanets, in particular those in the Earth-to-Neptune size range. By targeting known planets, CHEOPS will know exactly when and where to point to catch the exoplanet as it transits across the disk of its host star. Its ability to observe multiple transits of each planet will enable scientists to achieve the high-precision transit signatures that are needed to measure the sizes of small planets.

Figure 15: Animation of CHEOPS observing in space (video credit: ESA/ATG medialab)

Legend to Figure 15: In this view the satellite's telescope points to several different stars, one after the other. The beam represents the light from the star that CHEOPS is observing at a given point in time.

Mission status

• January 10, 2022: ESA’s exoplanet mission Cheops has revealed that an exoplanet orbiting its host star within a day has a deformed shape more like that of a rugby ball than a sphere. This is the first time that the deformation of an exoplanet has been detected, offering new insights into the internal structure of these star-hugging planets. 21)

- The planet, known as WASP-103b is located in the constellation of Hercules. It has been deformed by the strong tidal forces between the planet and its host star WASP-103, which is about 200 degrees hotter and 1.7 times larger than the Sun.


Figure 16: ESA’s exoplanet mission Cheops has revealed that an exoplanet orbiting its host star within a day has a deformed shape more like that of a rugby ball than a sphere. This is the first time that the deformation of an exoplanet has been detected, offering new insights into the internal structure of these star-hugging planets (image credit: ESA)

Tidal tug

- We experience tides in the oceans of Earth mainly due to the Moon tugging slightly on our planet as it orbits us. The Sun also has a small but significant effect on tides, however it is too far from Earth to cause major deformations of our planet. The same cannot be said for WASP-103b, a planet almost twice the size of Jupiter with 1.5 times its mass, orbiting its host star in less than a day. Astronomers have suspected that such a close proximity would cause monumental tides, but up until now they haven’t been able to measure them.

- Using new data from ESA’s Cheops space telescope, combined with data that had already been obtained by the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope, astronomers have now been able to detect how tidal forces deform exoplanet WASP-103b from a usual sphere into a rugby ball shape. 22)

- Cheops measures exoplanet transits – the dip in light caused when a planet passes in front of its star from our point of view. Ordinarily, studying the shape of the light curve will reveal details about the planet such as its size. The high precision of Cheops together with its pointing flexibility, which enables the satellite to return to a target and to observe multiple transits, has allowed astronomers to detect the minute signal of the tidal deformation of WASP-103b. This distinct signature can be used to unveil even more about the planet.

- “It’s incredible that Cheops was actually able to reveal this tiny deformation,” says Jacques Laskar of Paris Observatory, Université Paris Sciences et Lettres, and co-author of the research. “This is the first time such analysis has been made, and we can hope that observing over a longer time interval will strengthen this observation and lead to better knowledge of the planet’s internal structure."

Inflated planet

- The team was able to use the transit light curve of WASP-103b to derive a parameter – the Love number – that measures how mass is distributed within a planet. Understanding how mass is distributed can reveal details on the internal structure of the planet.


Figure 17: Artist impression of planet WASP-103b and its host star. ESA’s exoplanet mission Cheops has revealed that an exoplanet orbiting its host star within a day has a deformed shape more like that of a rugby ball than a sphere (image credit: ESA)

- “The resistance of a material to being deformed depends on its composition,” explains Susana Barros of Instituto de Astrofísica e Ciências do Espaço and University of Porto, Portugal, and lead author of the research. “For example, here on Earth we have tides due to the Moon and the Sun but we can only see tides in the oceans. The rocky part doesn’t move that much. By measuring how much the planet is deformed we can tell how much of it is rocky, gaseous or water.”

- The Love number for WASP-103b is similar to Jupiter, which tentatively suggests that the internal structure is similar, despite WASP-103b having twice the radius.

- In principle we would expect a planet with 1.5 times the mass of the Jupiter to be roughly the same size, so WASP-103b must be very inflated due to heating from its star and maybe other mechanisms,” says Susana.

- “If we can confirm the details of its internal structure with future observations maybe we could better understand what makes it so inflated. Knowing the size of the core of this exoplanet will also be important to better understand how it formed.”

- Since the uncertainty in the Love number is still quite high, it will take future observations with Cheops and the James Webb Space Telescope (Webb) to decipher the details. The extremely high precision of Webb will improve the measurements of tidal deformation of exoplanets, enabling a better comparison between these so-called “hot Jupiters” and giant planets in the Solar System.

Mysterious motion

- Another mystery also surrounds WASP-103b. The tidal interactions between a star and a very close-in Jupiter-sized planet would usually cause the planet’s orbital period to shorten, bringing it gradually closer to the star before it is eventually engulfed by the parent star. However, measurements of WASP-103b seem to indicate that the orbital period might be increasing and that the planet is drifting slowly away from the star. This would indicate that something other than tidal forces is the dominant factor affecting this planet.

- Susana and her colleagues looked at other potential scenarios, such as a companion star to the host affecting the dynamics of the system or the orbit of the planet being slightly elliptical. They weren’t able to confirm these scenarios, but couldn’t rule them out either. It is also possible that the orbital period is actually decreasing, rather than increasing, but only additional observations of the transits of WASP-103b with Cheops and other telescopes will help shed light on this mystery.

- “The size of the effect of tidal deformation on an exoplanet transit light curve is very small, but thanks to the very high precision of Cheops we are able to see this for the first time,” says ESA’s Project Scientist for Cheops, Kate Isaak. “This study is an excellent example of the very diverse questions that exoplanet scientists are able to tackle with Cheops, illustrating the importance of this flexible follow-up mission.”

• June 28, 2021: While exploring two exoplanets in a bright nearby star system, ESA’s exoplanet-hunting Cheops satellite has unexpectedly spotted the system’s third known planet crossing the face of the star. This transit reveals exciting details about a rare planet “with no known equivalent”, say the researchers. 23)

- The discovery is one of the first results from ESA’s Cheops (CHaracterising ExOPlanet Satellite), and the first time an exoplanet with a period of over 100 days has been spotted transiting a star that is bright enough to be visible to the naked eye.

- Named Nu2 Lupi, this bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf). In 2019, the High Accuracy Radial velocity Planet Searcher (HARPS) at the ESO 3.6-meter telescope in Chile discovered three exoplanets (named ‘b’, ‘c’ and ‘d’, with the star deemed to be object ‘A’) in the system, with masses between those of Earth and Neptune and orbits lasting 11.6, 27.6 and 107.6 days. The innermost two of these planets – b and c – were subsequently found to transit Nu2 Lupi by NASA's Transiting Exoplanet Survey Satellite (TESS), making it one of only three naked-eye stars known to host multiple transiting planets.

- “Transiting systems such as Nu2 Lupi are of paramount importance in our understanding of how planets form and evolve, as we can compare several planets around the same bright star in detail,” says Laetitia Delrez of the University of Liège, Belgium, and lead author of the new finding.

- “We set out to build on previous studies of Nu2 Lupi and observe planets b and c crossing the face of Nu2 Lupi with Cheops, but during a transit of planet c we spotted something amazing: an unexpected transit by planet ‘d’, which lies further out in the system.”


Figure 18: This infographic reveals the details of the Nu2 Lupi planetary system, which was recently explored by ESA’s exoplanet watcher Cheops. - This bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf), as shown to the left of the frame, and is known to host three planets (named ‘b’, ‘c’ and ‘d’, with the star deemed to be object ‘A’). The relative sizes, orbital periods, and possible compositions of these three planets are depicted to the centre and lower right of the frame, while planet d’s comparative position within our Solar System is shown to the upper right (as defined by the amount of incident light it receives from its star, Nu2 Lupi), - Cheops explored this planetary system to better characterize its two inner planets, b and c, as these were known to pass in front of their host star (a ‘transit’). However, while doing so, Cheops unexpectedly spotted planet d also transiting Nu2 Lupi – the first time an exoplanet with a period of over 100 days has been spotted transiting a star bright enough to be visible to the naked eye. -Transits create the valuable opportunity to study a planet’s atmosphere, orbit, size and interior, and allow scientists to compare multiple planets around the same star to understand how they have formed and evolved. The transiting behavior of all three planets of the Nu2 Lupi system enabled Cheops to refine the planetary characteristics and compositions depicted here [image credit: ESA; data: L. Delrez et al (2021)]

- Using the high-precision capabilities of Cheops, planet d was found to be about 2.5 times the radius of Earth, confirmed to take just over 107 days to loop once around its star and, using archival observations from ground-based telescopes, found to have a mass of 8.8 times that of Earth.

- “The amount of stellar radiation reaching planet d is also mild in comparison to many other discovered exoplanets; in our Solar System, Nu2 Lupi d would orbit between Mercury and Venus,” adds co-author David Ehrenreich of the University of Geneva, Switzerland. “Combined with its bright parent star, long orbital period, and suitability for follow-up characterization, this makes planet d hugely exciting – it is an exceptional object with no known equivalent, and sure to be a golden target for future study.”

- Most long-period transiting exoplanets discovered to date have been found around stars that are too faint to allow detailed follow-up observations, meaning that little is known about their planets’ properties. Nu2 Lupi, however, is bright enough to be an attractive target for other powerful telescopes based in space — such as the NASA/ESA Hubble Space Telescope or the forthcoming NASA/ESA/CSA James Webb Space Telescope — or large observatories on the ground.

- “Given its overall properties and orbit, this makes planet d a uniquely favorable target for studying an exoplanet with a mild-temperature atmosphere around a star similar to the Sun,” says Laetitia.

- By combining new Cheops data with archival data from other observatories, the researchers were able to accurately determine the mean densities of all of Nu2 Lupi’s known planets, and put strong constraints on their possible compositions.


Figure 19: This artist’s impression shows the Nu2 Lupi planetary system, which was recently explored by ESA’s exoplanet watcher Cheops (CHaracterising ExOPlanet Satellite). This bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf), and is known to host three planets – named ‘b’, ‘c’ and ‘d’, with the star deemed to be object ‘A’ – with masses between those of Earth and Neptune and orbital periods lasting 11.6, 27.6 and just over 107 days. The inner two of these planets were found to cross the face of their star as seen from Earth, an event known as a transit. Transits create the valuable opportunity to study a planet’s atmosphere, orbit, size and interior, and allow scientists to compare multiple planets around the same star to understand how they have formed and evolved over time. - While exploring these two transiting planets, Cheops spied something unexpected and exciting: the third and outermost planet, Nu2 Lupi d, also completing a transit. The discovery is one of the first results from Cheops, and the first time an exoplanet with a period of over 100 days has been spotted transiting a star bright enough to be visible to the naked eye (image credit: ESA)

- They found planet b to be mainly rocky, while planets c and d appear to contain large amounts of water enshrouded in envelopes of hydrogen and helium gases. In fact, planets c and d contain far more water than Earth: a quarter of each planet’s mass is made up of water, compared to less than 0.1% for Earth. This water, however, is not liquid, instead taking the form of high-pressure ice or high-temperature steam.

- “While none of these planets would be habitable, their diversity makes the system even more exciting, and a great future prospect for testing how these bodies form and change over time,” says ESA Cheops project scientist Kate Isaak. “There is also the potential to search for rings or moons in the Nu2 Lupi system, as the exquisite precision and stability of Cheops could allow detection of bodies down to roughly the size of Mars.“

- Cheops is designed to collect ultra-high precision data of individual stars known to host planets, rather than sweeping more generally for possible exoplanets around many stars – and this focus and precision is proving exceptionally useful in understanding the star systems around us.

- “These exciting results demonstrate once again the huge potential of Cheops,” adds Kate. “Cheops will allow us not just to better understand known exoplanets, as shown in this and other early results from the mission, but also to discover new ones and reveal their secrets.” 24)

• January 25, 2021: ESA’s exoplanet mission Cheops has revealed a unique planetary system consisting of six exoplanets, five of which are locked in a rare rhythmic dance as they orbit their central star. The sizes and masses of the planets, however, don’t follow such an orderly pattern. This finding challenges current theories of planet formation. 25)

- The discovery of increasing numbers of planetary systems, none like our own Solar System, continues to improve our understanding of how planets form and evolve. A striking example is the planetary system called TOI-178, some 200 light-years away in the constellation of Sculptor.

- Astronomers already expected this star to host two or more exoplanets after observing it with NASA’s Transiting Exoplanet Survey Satellite (TESS). New, highly precise observations with Cheops, ESA’s Characterizing Exoplanet Satellite that was launched in 2019, now show that TOI-178 harbors at least six planets and that this foreign solar system has a very unique layout. The team, led by Adrien Leleu of University of Geneva and the University of Bern in Switzerland, published their results today in Astronomy & Astrophysics. 26)

- One of the special characteristics of the TOI-178 system that the scientists were able to uncover with Cheops is that the planets – except the one closest to the star – follow a rhythmic dance as they move in their orbits. This phenomenon is called orbital resonance, and it means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits.

- A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede. For every orbit of Europa, Ganymede completes two orbits, and Io completes four (this is a 4:2:1 pattern).


Figure 20: This graphic shows a representation of the TOI-178 planetary system, which was revealed by ESA’s exoplanet watcher Cheops. The system consists of six exoplanets, five of which are locked in a rare rhythmic dance as they orbit their central star. The two inner planets have terrestrial densities (like Earth) and the outer four planets are gaseous (with densities like Neptune and Jupiter). The five outer planets follow a rhythmic dance as they move in their orbits. This phenomenon is called orbital resonance, and it means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits. While the planets in the TOI-178 system orbit their star in a very orderly manner, their densities do not follow any particular pattern. One of the exoplanets, a dense, terrestrial planet like Earth is right next to a similar-sized but very fluffy planet ­­– like a mini-Jupiter, and next to that is one very similar to Neptune. Astronomers did not expect to find this lay-out in a planetary system, and this discovery challenges current theories of planet formation. In this graphic, the relative sizes of the planets are to scale, but not the distances and the size of the star (image credit: ESA/Cheops Mission Consortium/A. Leleu et al.)

- In the TOI-178 system, the resonant motion is much more complex as it involves five planets, following a 18:9:6:4:3 pattern. While the second planet from the star (the first in the pattern) completes 18 orbits, the third planet from the star (second in the pattern) completes nine orbits, and so on.

- Initially, the scientists only found four of the planets in resonance, but by following the pattern the scientists calculated that there must be another planet in the system (the fourth following the pattern, the fifth planet from the star).

- “We predicted its trajectory very precisely by assuming that it was in resonance with the other planets,” Adrien explains. An additional observation with Cheops confirmed that the missing planet indeed existed in the predicted orbit.


Figure 21: This is an artist’s impression of the TOI-178 planetary system, which was revealed by ESA’s exoplanet watcher Cheops. The system consists of six exoplanets, five of which are locked in a rare rhythmic dance as they orbit their central star. The two inner planets have terrestrial densities (like Earth) and the outer four planets are gaseous (with densities like Neptune and Jupiter). The five outer planets follow a rhythmic dance as they move in their orbits. This phenomenon is called orbital resonance, and it means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits. In this artist impression, the relative sizes of the planets are to scale, but not the distances and the size of the star (image credit: ESA)

- After they had uncovered the rare orbital arrangements, the scientists were curious to see whether the planet densities (size and mass) also follow an orderly pattern. To investigate this, Adrien and his team combined data from Cheops with observations taken with ground-based telescopes at the European Southern Observatory’s (ESO) Paranal Observatory in Chile. 27)

- But while the planets in the TOI-178 system orbit their star in a very orderly manner, their densities do not follow any particular pattern. One of the exoplanets, a dense, terrestrial planet like Earth is right next to a similar-sized but very fluffy planet ­­– like a mini-Jupiter, and next to that is one very similar to Neptune.

- “This is not what we expected, and is the first time that we observe such a setup in a planetary system,” says Adrien. “In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory.”

- Catastrophic events such as giant impacts could normally explain large variations in planet densities, but the TOI-178 system would not be so neatly in harmony if that had been the case.

- “The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” explains co-author Yann Alibert from the University of Bern.

- Revealing the complex architecture of the TOI-178 system, which challenges current theories of planet formation, was made possible thanks to almost 12 days of observations with Cheops (11 days of continuous observations, plus two shorter observations).

- “Solving this exciting puzzle required quite some effort to plan, in particular to schedule the 11-day continuous observation needed in order to catch the signatures of the different planets,” says ESA Cheops project scientist Kate Isaak. “This study highlights very nicely the follow-up potential of Cheops – not only to better characterize known planets, but to hunt down and confirm new ones.”

- Adrien and his team want to continue to use Cheops to study the TOI system in even more detail.

- “We might find more planets that could be in the habitable zone – where liquid water might be present on the surface of a planet – which begins outside of the orbits of the planets that we discovered to date,” says Adrien. “We also want to find out what happened to the innermost planet that is not in resonance with the others. We suspect that it broke out of resonance due to tidal forces.”

- Astronomers will use Cheops to observe hundreds of known exoplanets orbiting bright stars.

- “Cheops will not only deepen our understanding of the formation of exoplanets, but also that of our own planet and the Solar System,” adds Kate.

• September 28, 2020: ESA’s new exoplanet mission, Cheops, has found a nearby planetary system to contain one of the hottest and most extreme extra-solar planets known to date: WASP-189 b. The finding, the very first from the mission, demonstrates Cheops’ unique ability to shed light on the Universe around us by revealing the secrets of these alien worlds. 28)

- Launched in December 2019, Cheops (Characterizing Exoplanet Satellite) is designed to observe nearby stars known to host planets. By ultra-precisely measuring changes in the levels of light coming from these systems as the planets orbit their stars, Cheops can initially characterize these planets — and, in turn, increase our understanding of how they form and evolve.

- The new finding concerns a so-called ‘ultra-hot Jupiter’ named WASP-189 b. Hot Jupiters, as the name suggests, are giant gas planets a bit like Jupiter in our own Solar System; however, they orbit far, far closer to their host star, and so are heated to extreme temperatures.


Figure 22: Key parameters of the WASP-189 planetary system as determined by ESA’s exoplanet mission Cheops (image credit: ESA)

- WASP-189 b sits around 20 times closer to its star than Earth does to the Sun, and completes a full orbit in just 2.7 days. Its host star is larger and more than 2000 degrees hotter than the Sun, and so appears to glow blue. “Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest,” says Monika Lendl of the University of Geneva, Switzerland, lead author of the new study. “WASP-189b is also the brightest hot Jupiter that we can observe as it passes in front of or behind its star, making the whole system really intriguing.” 29)

- First, Monika and colleagues used Cheops to observe WASP-189 b as it passed behind its host star – an occultation. “As the planet is so bright, there is actually a noticeable dip in the light we see coming from the system as it briefly slips out of view,” explains Monika. “We used this to measure the planet’s brightness and constrain its temperature to a scorching 3200ºC.”

- This makes WASP-189 b one of the hottest and most extreme planets, and entirely unlike any of the planets of the Solar System. At such temperatures, even metals such as iron melt and turn to gas, making the planet a clearly uninhabitable one.

- Next, Cheops watched as WASP-189 b passed in front of its star – a transit. Transits can reveal much about the size, shape, and orbital characteristics of a planet. This was true for WASP-189 b, which was found to be larger than thought at almost 1.6 times the radius of Jupiter.


Figure 23: ESA’s exoplanet mission Cheops has observed the WASP-189 system and determined key parameters about the star and its planet, WASP-189b. Cheops observed WASP-189b at is passed behind its host star – an occultation – and recorded the dip in light from the entire system as it briefly slipped out of view. It also observed the planet passing in front of the star – a transit. During a transit the planet temporarily blocks a tiny fraction of light from the star. Occultations and transits allow scientists to determine parameters such as the planet’s brightness, size, shape and orbital characteristics, as well as information about the star (image credit: ESA)

- “We also saw that the star itself is interesting – it’s not perfectly round, but larger and cooler at its equator than at the poles, making the poles of the star appear brighter,” says Monika. “It’s spinning around so fast that it’s being pulled outwards at its equator! Adding to this asymmetry is the fact that WASP-189 b’s orbit is inclined; it doesn’t travel around the equator, but passes close to the star’s poles.”

- Seeing such a tilted orbit adds to the existing mystery of how hot Jupiters form. For a planet to have such an inclined orbit, it must have formed further out and then been pushed inwards. This is thought to happen as multiple planets within a system jostle for position, or as an external influence – another star, for instance – disturbs the system, pushing gas giants towards their star and onto very short orbits that are highly tilted. “As we measured such a tilt with Cheops, this suggests that WASP-189 b has undergone such interactions in the past,” adds Monika.

- Monika and colleagues used Cheops’ highly precise observations and optical capabilities to reveal the secrets of WASP-189 b. Cheops opened its ‘eye’ in January of this year and began routine science operations in April, and has been working to expand our understanding of exoplanets and the nearby cosmos in the months since.

- “This first result from Cheops is hugely exciting: it is early definitive evidence that the mission is living up to its promise in terms of precision and performance,” says Kate Isaak, Cheops project scientist at ESA.

- Thousands of exoplanets, the vast majority with no analogues in our Solar System, have been discovered in the past quarter of a century, with many more to come from both current and future ground-based surveys and space missions.


Figure 24: Artist impression of exoplanet WASP-189b orbiting its host star. The system was observed by ESA’s exoplanet mission Cheops to determine key characteristics. For example, the host star is larger and more than 2000 degrees hotter than our own Sun, and so appears to glow blue. The planet has an inclined orbit – it doesn’t travel around the equator, but passes close to the star’s poles. It is one of the hottest and most extreme extra-solar planets known to date, and falls into the class of ultra-hot Jupiters (image credit: ESA)

- “Cheops has a unique ‘follow-up’ role to play in studying such exoplanets,” adds Kate. “It will search for transits of planets that have been discovered from the ground, and, where possible, will more precisely measure the sizes of planets already known to transit their host stars. By tracking exoplanets on their orbits with Cheops, we can make a first-step characterization of their atmospheres and determine the presence and properties of any clouds present.”

- In the next few years, Cheops will follow up on hundreds of known planets orbiting bright stars, building on and extending what has been done here for WASP-189b. The mission is the first in a series of three ESA science missions focusing on exoplanet detection and characterization: it has significant discovery potential also – from identifying prime targets for future missions that will probe exoplanetary atmospheres to searching for new planets and exomoons.

- “Cheops will not only deepen our understanding of exoplanets,” says Kate, "but also that of our own planet, Solar System, and the wider cosmic environment.”

• July 27, 2020: Is there a planet like our Earth out there? Scientists have discovered over 4000 exoplanets, or planets outside of our Solar System, that ESA's CHEOPS satellite will study in depth. Project scientist Kate Isaak discusses the types of exoplanets discovered and what we can learn from them. 30)

Figure 25: Meet the Experts: Other worlds (video credit: ESA)

• April 16, 2020: Cheops, ESA’s new exoplanet mission, has successfully completed its almost three months of in-orbit commissioning, exceeding expectations for its performance. The satellite, which will commence routine science operations by the end of April, has already obtained promising observations of known exoplanet-hosting stars, with many exciting discoveries to come. 31)

- “The in-orbit commissioning phase was an exciting period, and we are pleased we were able to meet all requirements,” says Nicola Rando, Cheops project manager at ESA. “The satellite platform and instrument performed remarkably, and both the Mission and Science Operation Centers supported operations impeccably.”

- Launched in December 2019, Cheops opened its eye to the Universe at the end of January and shortly after took its first, intentionally blurred images of stars. The deliberate defocusing is at the core of the mission’s observing strategy, which improves the measurement precision by spreading the light coming from distant stars over many pixels of its detector.

- Precision is key in today’s exoplanet research. More than 4000 planets – and counting – are known to be orbiting stars other than the Sun. A key follow-on is to start to characterize these planets, providing constraints on their structure, formation and evolution.

- Taking the steps to characterize exoplanets through the precise measurement of their sizes – in particular those of smaller planets – is exactly the mission of Cheops. Before being declared ready for the task, however, the small, 1.5 meter sized satellite had to pass a large number of tests.

Outstanding performance

- With the first series of in-flight tests, performed between January and February, the mission experts started analyzing the response of the satellite, and in particular of the telescope and detector, in the actual space environment. Proceeding into March, Cheops focused on well-studied stars.


Figure 26: An image of the star HD 88111, which is not known to host any exoplanets, taken by Cheops during the missions' in-orbit commissioning (image credit: ESA)

- “To measure how well Cheops performs we first needed to observe stars whose properties are well known, stars that are well-behaved – hand-picked to be very stable, with no signs of activity” says Kate Isaak, Cheops project scientist at ESA.

- This approach enabled the teams at ESA, the mission consortium, and Airbus Spain – the prime contractor – to verify that the satellite is as precise and stable as needed to meet its ambitious goals.

- “The pointing is extremely stable: this means that while the telescope observes a star for hours while the spacecraft moves along its orbit, the image of the star remains always within the same group of pixels in the detector,” explains Carlos Corral van Damme, ESA’s System Principal Engineer for Cheops.

- “Such a great stability is a combination of the excellent performance of the equipment and of the bespoke pointing algorithms, and will be especially important to fulfill the scientific objectives of the mission. The thermal stability of the telescope and the detector has also proven to be even better than required,” adds Carlos.

- The commissioning period demonstrated that Cheops achieves the required photometric precision and, importantly, it also showed that the satellite can be commanded by the ground segment team as needed to perform its science observations.

- “We were thrilled when we realized that all the systems worked as expected or even better than expected,” says Cheops Instrument Scientist Andrea Fortier, who led the commissioning team of the consortium for the University of Bern, Switzerland.

Figure 27: Cheops: the science begins (video credit: ESA/ATG medialab)

Time for exoplanets

- During the final two weeks of in-orbit commissioning, Cheops observed two exoplanet-hosting stars as the planets ‘transited’ in front of their host star and blocked a fraction of starlight. Observing transits of known exoplanets is what the mission was built for – to measure planet sizes with unprecedented precision and accuracy and to determine their densities by combining these with independent measurements of their masses.

- One of the targets was HD 93396, a subgiant yellow star located 320 light-years away, slightly cooler and three times larger than our Sun. The focus of the observations was KELT-11b, a puffy gaseous planet about 30% larger in size than Jupiter, in an orbit that is much closer to the star than Mercury is to the Sun.

- The measurements made by Cheops are five times more accurate than those from Earth, explains Willy Benz, Principal Investigator of the Cheops mission consortium, and professor of astrophysics at the University of Bern. “That gives us a foretaste for what we can achieve with Cheops over the months and years to come,” he says.

- A formal review of the satellite performance and ground segment operations was held on 25 March, and Cheops passed it with flying colors. With this, ESA handed over responsibility for operating the mission to the consortium led by Willy Benz.

- Fortunately, the commissioning activities were not affected much by the ensuing emergency caused by the coronavirus pandemic, which resulted in social distancing measures and restrictions to movement across Europe to prevent the spread of the virus.

- “The ground segment has been working very smoothly from early on, which enabled us to fully automate most of the operations for commanding the satellite and downlinking the data already in the first few weeks after launch,” explains Carlos. “By the time the crisis emerged in March, with the new rules and regulations that came with it, the automated systems meant that the impact on the mission was minimal.”

- Cheops is currently transitioning towards routine science operations, which are expected to begin before the end of April. Scientists have started observing some of the ‘early science targets’ – a selection of stars and planetary systems chosen to showcase examples of what the mission can achieve: these include a ‘hot super-Earth’ planet known as 55 Cancri e, which is covered in a lava ocean, as well as the ‘warm Neptune’ GJ 436b, which is losing its atmosphere due to the glare from its host star. Another star on the list of upcoming Cheops observations is a white dwarf, the first target from ESA’s Guest Observers Program, which provides scientists from beyond the mission consortium with the opportunity to use the mission and capitalize on its observational capabilities.


Figure 28: During its in-orbit commissioning, ESA's Cheops mission observed the transit of KELT-11b in front of its host star. HD 93396 is a subgiant yellow star located 320 light-years away, slightly cooler and three times larger than our Sun. It hosts a puffy gaseous planet, KELT-11b, about 30% larger in size than Jupiter, in an orbit that is much closer to the star than Mercury is to the Sun. - The light curve of this star shows a clear dip caused by the eight hour-long transit of KELT-11b, which enabled scientists to determine very precisely the diameter of the planet: 181,600 km – with an uncertainty just under 4300 km. The measurements made by Cheops are five times more accurate than those from Earth, providing a taster for the science to come from the Cheops mission. In this graphic, the Sun is shown as a comparison, along with the diameter of Earth and Jupiter (calculated from the mean volumetric radius), image credit: ESA/Airbus/CHEOPS Mission Consortium


Figure 29: Artist's impression of star HD 93396 and its hot Jupiter planet, KELT-11b (image credit: ESA)

• March 26, 2020: Airbus has received confirmation from ESA of a successful end to the In Orbit Commissioning (IOC) of CHEOPS after the IOC review yesterday. This critical phase was performed by Airbus in Spain with the support of the Instrument Team (University of Bern), Mission Operation Center (INTA), Science Operation Center (University of Geneva) and ESA. 32) 33)

- The IOC phase started on 7th January and over the past two and a half months Airbus has conducted the operations to verify the performance of the satellite (platform and instrument), the ground segment and the science package. During this time the main goal was to consolidate the documentation, processes and procedures for use during the operational phase.

- ESA recognized the great job done by the Airbus teams and stated there were no issues preventing routine operations from starting and confirmed hand-over of the mission operations from Airbus to INTA and the mission consortium.

- Fernando Varela, Head of Space Systems in Spain, said: "The in-orbit delivery of the CHEOPS satellite is the culmination of the Airbus participation in the program. It is the first European exoplanetary mission and the first ESA mission built by Airbus in Spain. The professionalism of the technical and engineering teams at Airbus was key to this success."

- CHEOPS will be controlled by INTA and the mission consortium (University of Geneva and University of Bern). Nevertheless, Airbus is also ready to assist during the operational phase for the whole mission life.

- CHEOPS marks the first time that Airbus in Spain has been the prime contractor for the whole mission, from satellite development, through launch, to LEOP and IOC. The entire mission development was completed in record time without delays and met the very tight budget. To do this, Airbus managed a team of 24 companies from 11 European countries, seven of them Spanish, confirming Airbus as the driving force behind the space industry in Spain.

- As a reminder, CHEOPS is the first in ESA's FAST TRACK missions program whose main characteristics are low cost and a challenging budget. CHEOPS will characterize exoplanets orbiting nearby stars, observing known planets in the size range between Earth and Neptune and precisely measuring their radii to determine their density and understand what they are made of.

• February 7, 2020: ESA’s exoplanet-observer Cheops acquired the first image of its initial target star, following the successful telescope cover opening on 29 January 2020 (Figure NO TAG#). The intentionally blurry image is a product of the specially designed telescope optics, which are deliberately defocused to maximize the precision of Cheops’ measurements and enable its unprecedented study of exoplanets, or planets in other solar systems. 34)

- The acquisition of this image marks a key milestone in the extensive testing phase of Cheops, the Characterizing ExOPlanet Satellite, before it embarks on its mission to study planets around nearby stars.

- “This is a defining moment for the mission,” says Nicola Rando, ESA project manager for Cheops.

- “To the engineers and scientists across Europe who have worked and continue to work on Cheops, this image represents the culmination of many years of dedication and effort – designing, planning, coordinating and building this new and unique satellite.”

- The image features a stellar field centered on HD 70843, a yellow-white star located around 150 light years away. The team responsible for in-orbit commissioning of the satellite selected this star as the telescope’s first target because of its brightness and its location in the sky, which made it ideal for testing purposes.

- “The first images that were about to appear on the screen were crucial for us to be able to determine if the telescope's optics had survived the rocket launch in good shape,” explains Willy Benz, Principal Investigator of the Cheops mission consortium from the University of Bern, Switzerland.

- When the first images of a field of stars appeared on the screen, it was immediately clear to everyone that we did indeed have a working telescope.

- The Cheops telescope deliberately delivers defocused images of a target star onto the detector – a charge-coupled device, or CCD – in order to distribute the light from each star over many pixels. This makes the measurements of starlight more precise, as they are much less sensitive to small differences in the response of individual pixels in the CCD and to variations in the telescope pointing.

- Having a precise measurement of the stars’ brightness and its variation is of critical importance to the scientists striving to learn as much as possible about the planets known to orbit those stars. A planet transiting in front of a star in Cheops’ view causes the star to dim – a barely detectable dip that can reveal key information about the planet’s properties, most importantly its size.

- “Now that Cheops has observed its first target, we are one step closer to the start of the mission science,” says Kate Isaak, ESA Cheops project scientist.

- “This beautifully blurred image carries the promise of a new, deeper understanding of worlds beyond our Solar System.”

- Over coming weeks, teams at the Mission Operations Center in Torrejón, near Madrid, Spain, together with colleagues from the Universities of Bern and Geneva, Switzerland, will conduct an extensive series of tests on Cheops’ instrument and detector to characterize their in-orbit performance. The same tests will be used to confirm that the scientific data taken by Cheops is processed appropriately.

- Launched on 18 December 2019, Cheops is ESA’s first mission dedicated to the study of exoplanets. Rather than search for new planets, the mission will follow up on hundreds of known planets in orbit around bright stars, with sizes smaller than Saturn’s, that have been discovered by other methods.

- Cheops will observe individual stars as a planet transits in front of them and blocks a fraction of the starlight, using the dip in the light level to measure the planet’s size with exquisite precision. By combining these very accurate and precise sizes with existing measurements of planet masses, it will be possible to determine the bulk densities of large numbers of planets in the size range between Earth and Neptune, which provide vital clues to the planets’ composition and structure. This first-step characterization is a critical step towards understanding how these small extrasolar worlds form and evolve.

- Routine science operations, during which Cheops will observe many hundreds of exoplanet transits, are foreseen to start at the beginning of April.


Figure 30: Cheops image of its first target star. Image of HD 70843, the star chosen as the first target for Cheops, ESA’s Characterizing ExOPlanet Satellite. The star, located around 150 light years away in the constellation of Cancer, is visible at the center of the image, surrounded by fainter stars in the background. The peculiar shape of the stars in the image is a result of the deliberate defocusing of the Cheops optics, which spreads the light from each star over many pixels. This makes the measurements of the starlight more precise, as they are much less sensitive to small differences in the response of individual pixels in the CCD and to variations in the telescope pointing. The triangular appearance of the stars is a known effect of the three struts that support the telescope’s primary mirror. The image covers about 1000 x 1000 pixels, with each pixel edge representing a tiny angle of about 0.0003 degrees on the sky, equivalent to less than one thousandth of the full Moon’s diameter. The inset in the lower right corner shows a region covering about 100 x 100 pixels, centred on the target star (image credit: ESA/Airbus/CHEOPS Mission Consortium)

• January 29, 2020: Six weeks after the launch of Cheops the telescope cover was opened as part of the mission’s in-orbit commissioning. 35)

- Since the launch of Cheops on 18 December 2019, the project has progressed smoothly and successfully through its planned operations and test activities.

- The LEOP (Launch and Early Orbit Phase) was flawlessly completed on 22 December, and the instrument was switched on for the first time on 8 January 2020. By late January, a number of health tests and calibration activities had demonstrated that the instrument was behaving as expected and ready to have its focal plane exposed to light.

- Eventually, one of the mission’s major moments of truth arrived at 07:38 CET this morning, when the telescope cover was opened. This irreversible operation allowed the instrument to see the sky for the first time and was a high point for the project team achieved through years of work.

- “Opening the cover of the telescope baffle is a critical operation for Cheops, enabling the telescope to observe its target stars, and we are extremely pleased that it was performed flawlessly,” says Nicola Rando, ESA Cheops project manager.

Figure 31: Cheops observing in space. In this animation the satellite's telescope points to several different stars, one after the other. The beam represents the light from the star that Cheops is observing at a given point in time. 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 (video credit: ESA/ATG medialab)

- The main goal of Cheops is to observe nearby, bright stars that are already known to host planets. The mission will observe these planets, as they transit in front of their host star and block a fraction of its light, to measure their sizes with unprecedented precision and accuracy.

- The signal of an exoplanet transit can be extremely weak, especially for the smallest planets. Measuring it with the high precision needed to investigate the planet properties represents an observational challenge that can only be met from space, and requires the instrument and satellite to be highly stable.

- One of the key mission elements is a 95 cm long baffle that shields the Cheops telescope from straylight and minimizes light contamination from unwanted sources, such as light reflected by our own planet Earth. The Cheops baffle cover, which can be compared to the lens cap of a camera, covered the front end of the telescope baffle and protected the science instrument from dust and sources of bright light (for example, sunlight) during testing, launch and the first phases of in-orbit commissioning (see Figure 53).

- Opening the baffle cover in orbit was a mission-critical procedure that was closely monitored by members of the in-orbit commissioning team at the Mission Operations Center in Torrejón, near Madrid, Spain.

- The opening mechanism relied on a titanium fastening bolt surrounded by a cylinder made of a shape memory alloy. Heating the cylinder caused it to elongate and break the bolt, which subsequently allowed a spring-loaded hinge to swing the cover open and expose the telescope to the sky. The telescope cover is now permanently locked in the open position by a hook engaged in a ratchet.

- “The opening mechanism is known to be extremely reliable, as it was extensively tested on ground and already flown on previous space missions, but it was still quite a nerve-wracking moment to witness, and we are all very excited now that the telescope has opened its eye to the Universe,” says Francesco Ratti, ESA Cheops instrument engineer.

- Executing this critical procedure is a major step along the path towards Cheops routine observations. Over the coming weeks, the in-orbit commissioning team will test the instrument’s detector and its performance, making sure it behaves as designed and as required to detect exoplanet transits and tackle the mission’s ambitious science questions.

• January 14, 2020: ESA's Cheops orbit trail is shown here as a long streak against a backdrop of stars as it orbits the Earth after its successful launch on 18 December 2019. 36)

- The coordinates of the center of this 2048 x 2048 pixel image are: right ascension 11 h 56 m 58.00 s and declination +27º 30’ 45.0’’ (J2000). The visible trail seen running from bottom to top in the image is due to sunlight reflected by the Cheops spacecraft, which is in a sun-synchronous orbit with an altitude of 700 km and a local time of the ascending node of 6:00 am.

- The image spans only 12 arcminutes across, so Cheops spent a very short time in the field of view – around 400 ms. The estimated r’-band magnitude of CHEOPS in this image is 7.8 ± 0.3 (calculation by M. Sestovic, University of Bern).


Figure 32: The 6-minute long exposure was taken at 13:18 UTC on 11 January 2020 with the 1 m SAINT-EX robotic telescope, located at the National Astronomical Observatory of Mexico at San Pedro Martir, Mexico (image courtesy of the SAINT-EX team, University of Bern)

• December 18, 2019: Signals from the spacecraft, received at the mission control center based at INTA in Torrejón de Ardoz near Madrid, Spain, via the Troll ground tracking station at 12:43 CET confirmed that the launch was successful (Ref. NO TAG#.

- “Cheops will take exoplanet science to a whole new level,” says Günther Hasinger, ESA Director of Science. “After the discovery of thousands of planets, the quest can now turn to characterisation, investigating the physical and chemical properties of many exoplanets and really getting to know what they are made of and how they formed. Cheops will also pave the way for our future exoplanet missions, from the international James Webb Telescope to ESA’s very own Plato and Ariel satellites, keeping European science at the forefront of exoplanet research.”

- Cheops will not focus on the search for new planets. Instead, it will follow-up on hundreds of known planets that have been discovered through other methods. The mission will observe these planets exactly as they transit in front of their parent star and block a fraction of its light, to measure their sizes with unprecedented precision and accuracy.


Figure 33: CHEOPS spacecraft attitude and pointing directions in the operational orbit (image credit: ESA, (Ref. 16))

The combination of the accurate and precise sizes determined by CHEOPS with masses determined from other measurements will be used to establish the bulk density of the planets, placing constraints on their composition; these, together with information on the host stars and the planet orbits, will provide key insight into the formation and evolutionary history of planets in the super-Earth to Neptune size range.


Figure 34: CHEOPS nominal attitude definition (image credit: Airbus DS, ESA)

IOC (In Orbit Commissioning)

The CHEOPS commissioning phase starts immediately after the launch and early operations phase (LEOP) that will last for one week. The commissioning phase is expected to last two months and will verify all performance aspects of the mission. The phase will end with the IOCR (In-Orbit Commissioning Review). The LEOP and IOC operations are centralized at the National Institute of Aerospace Technology (INTA) in Torrejon de Ardoz close to Madrid. INTA hosts the mission operations center (MOC) for the CHEOPS mission. The responsibility for LEOP and IOC has been delegated to Airbus DS Spain by ESA (Ref. 6).

The commissioning is divided into four different phases from IOC-A to IOC-D where different requirements are validated. The different phases differ in objectives, involvement of the different stakeholders and in complexity. Starting from instrument functional checkouts in the early phase A and CCD calibration measurements when the aperture cover is still closed to the cover opening and first light end of phase A. Figure NO TAG# gives an idea of how the first light of CHEOPS can look like. It shows a simulated image of the payload where the PSF has been introduced as measured on ground. It shows a view to 55 Cancri, which is located at the very center of the image. The target shown here is a relatively bright star of magnitude 6 imaged using an exposure time of 0.5s.


Figure 35: Simulated full frame image of 55 Cancri (center of the image), image credit: CHEOPS Team

The subsequent phase B will focus on optical verifications of the instrument including distortion, plate scale and PSF measurements; verify pointing stability with and without payload in the loop under different conditions including occultation and different field crowding. In addition, the straylight contribution to the noise budget is measured for different pointing configurations. All the read-out modes of the payload are tested and the SAA model, which is applied for CHEOPS, is validated. In the end of phase B all monitoring and characterization activities that are intended to be performed in a moderate frequency over the mission lifetime are exercised as a reference. Those include PSF, dark current, stray-light, flat field, gain and FWC monitoring as well as a so-called pinning curve measurement. The pinning curve is a measure for radiation damage on the CCD detector. The phase B is therefore very densely packed and gives a comprehensive view on the mission performance. Phase C verifies end-to-end science performance using, among other measurements, reference transits including the full data processing chain.

Phase D demonstrates the nominal mission capabilities including the one-week mission planning and MTL uplink. The phase can also contain low priority activities that are needed to be re-run from the previous phases. In general, phase D verifies the capability of the full space and ground segment to go into the nominal mission phase. At the end of the phase, the IOC is concluded with the IOC review, which marks the point of the handover of the satellite to the mission consortium.