PHEBUS UV spectrometer on board ESA-BepiColombo Mission: Instrument design & performance results

International audienceBepiColombo, a cornerstone mission of European Space Agency (ESA) in cooperation with the Japan Aerospace Exploration Agency (JAXA), will explore Mercury the planet closest to the Sun. This first European mission toward Mercury will be launched in October 2018 from the Guiana Space Centre, on a journey lasting up to six and a half years. The data that will be brought back from the two orbiters will tell us about Mercury's surface, the atmospheric composition, and the magnetospheric dynamics; it will also contribute to understanding the history and formation of terrestrial planets. Probing of Hermean Exosphere by Ultraviolet Spectroscopy (PHEBUS) is a double spectrometer that will be flown on the Mercury Planetary Orbiter (MPO) one of the two BepiColombo orbiters. This French-led instrument was developed with the purpose of detecting emission lines from Mercury's exosphere to reveal its composition and distribution, in the wide UV range 55 nm-315 nm and by recording full spectra. In this paper, we present the instrument design by focusing on the optical subsystems and giving a technical feedback of the major challenges we had to face. We also introduce the calibration philosophy and the main performance results of the instrument