Kurzbeschreibung
(Englisch)
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We propose for the RAMSES mission a compact, lightweight, and uniquely capable high-resolution Vis-NIR multispectral imaging system, called Colour High-resolution Apophis Narrow-angle CamEra System (CHANCES)*. CHANCES combines extensive heritage from our previous developments and novel and innovative aspects tailored to the objectives of the RAMSES mission, the capabilities of its baseline payload, and the characteristics of its target asteroid, 99942 Apophis. The mission will provide a unique opportunity to study the 375-m sized asteroid before, during, and after its close Earth flyby on April 13, 2029, and to identify possible changes at the surface resulting from gravitational interactions with the Earth. CHANCES provides unprecedented imaging capabilities which will be key to detect and analyse changes caused by tidal effects, which might be very subtle.
The proposed instrument heavily relies on heritage from the CaSSIS (ExoMars TGO) and CoCa (Comet Interceptor) imagers. It will share a common digital processing unit and flight software with CaSSIS/CoCa so that all interfaces between the spacecraft, the instrument and operation teams will be the same, ultimately benefiting all missions. We have been operating the CaSSIS instrument on Mars’ orbit since 2016 (more than 50’000 precisely targeted images) and have unique expertise with all aspects of the operation of imagers, from target selection by the science team to the delivery and archiving of calibrated products to the planetary science archive. We were also previously involved in the Rosetta mission and have experienced the challenges associated with the acquisition of spacecraft data in the vicinity of irregular small bodies. The new instrument design also has heritage from other Bernese contributions to optical instruments, in particular the CHEOPS and PLATO space telescopes (particularly the lightweight structure elements and straylight mitigation).
Our proposed high-resolution multi-spectral imager ideally complements the set of cameras already part of the baseline payload. Despite the limited allocated mass and volume, we achieve a high angular resolution by implementing a compact and lightweight Maksutov telescope with a 400 mm focal length and 80 mm aperture. Combined with the 5 μm pixel pitch of the detector, CHANCES will achieve the desired 10 cm/pixel resolution at a distance of 8 km already and will offer even more detailed close-up views at shorter distances (1.5 cm/px at the 1.2 km closest approach), complementary to the Narrow and Wide Angle Cameras (on board RAMSES), as well as MapCam (on board OSIRIS-APEX) images.
Beyond possible major changes in the shape and main surface features of Apophis, RAMSES should detect and monitor possible very subtle changes in the regolith coverage and properties. CHANCES will use a broadband Vis-NIR imaging sensor (0.4 - 1.7 μm) together with a 12-positions filter wheel (9 bandpass filters, 2 polarisers and one blank for darks, derived from the CaSSIS rotation mechanism). This will allow us to characterise both the visible red slope of the asteroid and the near-infrared features including the 1-μm absorption band and reflectance maximum at 1.6 μm, which are the most prominent for this spectral class of asteroids and sensitive indicators of physical and compositional changes in the surface material. The near-infrared dataset will ideally complement Vis data from CHANCES and the other cameras and the longer wavelength thermal infrared data from TIRI.
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