VIS-NIR Imaging Spectroscopy of Mercury´s Surface: SIMBIO-SYS/VIHI Experiment Onboard the BepiColombo Mission

The Visible and Infrared Hyperspectral Imager (VIHI) is one of the three optical heads of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) experiment onboard European Space Agency´s BepiColombo cornerstone mission to Mercury. The other two optical heads of SIMBIO-SYS are a stereo camera and a high-resolution image camera. The experiment is designed to scan the Hermean surface from a polar orbit with the three channels to map the physical, morphological, tectonic, and compositional properties of the planet. The main scientific objectives of SIMBIO-SYS are the study of Mercury´s surface geology and stratigraphy, the surface composition, the regolith properties, the crustal differentiation, impact, and volcanic processes. The VIHI experiment uses a high-performance optical layout (Schmidt telescope and spectrometer in Littrow configuration) which allows investigating the 400-2000-nm spectral range with 256 spectral channels (6.25 nm/band sampling). The instrument has an instrument field of view (FOV) of 250 μrad corresponding to a spatial scale of about 100 m/pixel at periherm and 375 m at apoherm. The instrument operates in pushbroom configuration, sampling the surface of Mercury with an FOV of 64 × 0.25 mrad. The main technical challenges of this experiment are focal-plane design (cadmium-mercury-telluride thinned to improve the efficiency at visible wavelengths), short dwell time (from about 40 ms at equator to about 100 ms at poles), thermal control, mechanical miniaturization, radiation hardening, high data rate, and compression. A description of the internal calibration unit concept and functionalities is given.