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- PLATO Assessment Study Report (Yellow Book)
The measurement principle of PLATO is to carry out high precision, long (months to years), uninterrupted photometric monitoring in the visible band of very large samples of bright (mV ≤ 11–13) stars. The resulting light curves will be used for the detection of planetary transits, from which the planetary radii will be determined, and for the asteroseismology analysis to derive accurate stellar parameters and ages. Since the PLATO targets are bright, the masses of the detected planets can be determined from radial velocity observations at ground-based observatories.
The key scientific requirement to detect and characterise a large number of terrestrial planets around bright stars determined the design of the payload module. The module provides a wide field-of-view (FoV) to maximise the number of the sparsely distributed bright stars in the sky with one pointing, and allows the satellite to cover a large part of the sky. In addition, it provides the required photometric accuracy to detect Earth-sized planets and a high photometric dynamic range, allowing astronomers to observe bright stars (mV < 11) as well as fainter stars down to V-magnitude of 16. This performance is achieved by a multi-telescope instrument concept, which is novel for a space telescope.
The payload consists of 24 'normal' cameras with CCD-based focal planes, operating in white light. They will be read out with a cadence of 25 s and will monitor stars with mV > 8. Two additional 'fast' cameras with high read-out cadence (2.5 s) will be used for stars with mV ~4–8. The 'normal' cameras are arranged in four groups of six. Each group has the same field-of-view but is offset by a 9.2° angle from the payload module +Z axis, allowing astronomers to survey a total field of about 2250 deg2 per pointing, but with different sensitivities over the field.
The ensemble of instruments is mounted on an optical bench. The cameras are based on a fully dioptric design with 6 lenses. Each camera has an 1100 deg2 field-of-view and a pupil diameter of 120 mm and is equipped with a focal plane array of 4 CCDs each with 4510x4510 pixels of 18 μm size, working in full frame mode for the 'normal' camera and in frame transfer mode for the 'fast' cameras.
The current baseline observing plan for the 4-year nominal science operations consists of long-duration observations of two sky fields lasting two years each. An alternative scenario is for operations split into a long-duration pointing lasting three years and a one-year step-and-stare phase with several pointings. In view of the exceptionally fast development of exoplanet science, the final observing strategy will be investigated throughout the mission development and decided two years before launch. Depending on the selected strategy, the mission will be able to cover between 10 per cent and 50 per cent of the sky during the nominal observing time.
The PLATO satellite will be built and verified for an in-orbit lifetime of 6.5 years, accomodating consumables for 8 years, which offers the possibility of mission operation extensions.
PLATO can be launched on a Soyuz-Fregat rocket for injection into a Lissajous orbit around the L2 Lagrangian point. To protect the instrument from solar light, it has to rotate by 90° around the line-of-sight (LoS) every 3 months.
ESA provides the spacecraft, the CCDs, the mission operations, and parts of the science operations. The PLATO Mission Consortium, funded by national Funding Agencies, provides the payload and contributions to the science operations.