The Top-level science requirements for CHEOPS 

The science objectives of CHEOPS impose a number of top-level science requirements which impact on all aspects of the mission, including payload and spacecraft design as well as operations and the choice of satellite orbit. A detailed description, including motivation and justification, of the science requirements can be found in the CHEOPS Science Requirement Document which can be downloaded here. The top level requirements are summarised below: 
1. Photometric Precision:
CHEOPS will target the host stars of super-Earth sized exoplanets detected from the ground by means of high-precision radial velocity surveys. It will also target transiting Neptune-like planets detected by new generation of ground-based transit surveys (eg. the Next-Generation Transit Survey - NGTS -  based at, and currently running from, ESO’s Paranal  Observatory site in Chile) . Additional targets will come from the TESS satellite, which is foreseen to identify large numbers of short-period exoplanets orbiting late-type stars. Two different photometric requirements have been derived to account for the particularities of the different detection methods and the size of the corresponding exoplanets. 
In the first, CHEOPS shall be able to detect Earth-size planets transiting G5 dwarf stars (stellar radius of 0.9 Rsolar) with V-band magnitudes in the range 6 ≤ V ≤ 9 mag. Given the relative sizes of the exoplanet and stellar radii, the depth of such transits is 100 parts-per-million (ppm), which requires a photometric precision of 20 ppm (goal: 10 ppm) to be detected with statistical significance ( signal-to-noise ratio of 5). This sensitivity is to be achieved in a total integration time of 6 hours, which corresponds to the duration of the transit of such a planet with a revolution period of 50 days. The signal-to-noise ratio called for is sufficient to establish the detection of the transit and to determine the presence or absence of a significant atmosphere for planets with masses from that of the Earth to Neptune. 
The second requirement focuses on the characterisation of the transits of Neptune-sized planets, which calls for transit detections with much higher signal-to-noise. In particular, CHEOPS shall be able to detect Neptune-size planets transiting K-type dwarf stars (stellar radius of 0.7 Rsolar) with V-band magnitudes as faint as V=12 mag (goal: V=13 mag) with a signal-to-noise ratio of 30. Such transits have a depth of 2 500 ppm, and so a sensitivity of around 85 ppm is required. This shall be achieved in a total integration time of 3 hours, which corresponds to the duration of the transit of such a planet with a revolution period of 13 days. 
Achieving such signal-to-noise ratios will enable characterisation of the transit light curve, and the possibility to accurately derive parameters such as the impact paramter, transit duration and stellar limb-darkening coefficients. 
In both cases the necessary signal-to-noise ratio may be achieved in a single transit or, in the case of (a) planets with revolution periods of ≤ few days where the transit duration may be a few hours only, or (b) obvserations during which data cannot be collected observation due to Earth occultation or passage through the South Atlantic Anomaly, in more than one. 
CHEOPS shall maintain the photometric performances noted above during all observations (out of interruptions) for a duration of up to 48 hours (including interruptions). The time scale is driven by the need to cover uncertainties in the (a) transit time ephemeris for radial velocity targets; (b) the error on the orbital eccentricity which can dominate the uncertainty for planets with longer periods and (c) to enable observations of phase curves of hot/short-period giant plants. 
2. Target Observability and Sky Coverage:

The key to making precise measurements of the radii of small exoplanets is to keep the noise in the measurements to an absolute minimum. Stray light from the Earth is a major source of noise, and will dominate the small transit signal unless steps are taking to minimise its contribution through (a) careful design of the instrument, including the telescope and its baffling; (b) the choice of the operational orbit of the satellite, and (c) by limiting the directions in which the telescope points during nominal observations.  

As a follow-up mission, CHEOPS will observe stars that are known to host exoplanets. In principle these can be found over the whole sky, and it is essential to maximise the fraction of the sky which CHEOPS can observe down to the photometric precision limits outlined above.  In addition, there are some regions of the sky for which preferential access is required to take into accound detection biases associated with the surveys that will provide CHEOPS targets. Taking these into account, for low-mass/Super-Earth size planets discovered through radial velocity surveys, CHEOPS shall cover 50% of the whole sky for 50 (goal: 60) cumulative (goal: consecutive) days of observations per year, per target, with an observation duration of longer than 50% of the spacecraft orbit duration (>50 min for 100-min spacecraft orbital period). The figure of 50 days is derived from the maximum revolution period of the exoplanets we wish to observe, which corresponds that of a planet at the inner edge of the habitable zone of K stars.
The NGTS survey, which is foreseen to provide the majority of the Neptune-size exoplanets, will cover around 10% of the Southern sky. The sky coverage requirement can therefore be relaxed, however as the objective is to  characterise the transits, a much higher fraction of each transit needs to be observable. In this case, 25% of the whole sky, with 2/3 in the southern hemisphere, shall be accessible for 13 days (cumulative; goal: 15 days) per year and per target, with an observation duration of great than 80% of the spacecraft orbit duration (>80 min for 100-min spacecraft orbit).
3. Temporal sampling and precision of the light curves:
Characterisation of the light curve requires temporal resolution of the transit ingress and/or egress, which is particularly important for lifting the degeneracy between transit duration and impact parameter. For a Neptune-size exoplanet transiting a 0.7 Rsolar radius star, the ingress duration is minutes. CHEOPS shall be able to provide one photometric measurement per minute (goal: one every 30 s). 
A second timing requirement is motivated by planned searches for previously-unidentified exoplanets in known systems using the method of Transit Time Variation (TTV). To enable this, CHEOPS shall have be able to provide photometric measurements with an uncertainty in the time stamp (UTC) of 1 second.

4. Mission Lifetime
The nominal required duration of CHEOPS operations is 3.5 years (excluding commissioning, goal 5 years), based on a design reference mission for the Core Programme outlined as follows. Transit detection on bright stars identified by radial velocity surveys will need as a minimum a total of 600 days to observe 150 targets, assuming two observations of duration of 48 hrs. Targets coming from ground-based surveys (NGTS) will cover a range of V-band magnitudes, and will each require up to 10 observations to achieve the required signal-to-noise. A total of 150 days is required to characterise just over 100 known transits, assuming an observing time of 12 hours per transit and an efficiency of 80%. Phase curve measurements of a handful (5) of hot Jupiters will require a total of 70 days, based on the need to make multiple observations of each target in order to achieve sufficient photometric precision.  
Factoring in (a) an overhead per observation for slewing, pointing acquistion and set-up; (b) that 20% of the total available observing time shall be available to Guest Observers; (c) an allocation for activities required both to monitor and characterise instrument performance and to execute planned and unplanned platform/spacecraft activities (d) a scheduling efficiency of 80%, sets the required mission duration to 3.5 years.
The rate of exoplanet discovery in the past years show that new exciting targets are likely to be discovered in coming years - an extended mission lifetime of 5 years would enhance the science return of the mission, enabling eg. collection of additional data for targets discovered during later phases of the mission, follow-up of candiate planets detected by TESS, extended monitoring for transit-timing variations, collection of additional data for targets discovered during later phases of the mission.