The MSA Instrument (Mass Spectrum Analyzer) Onboard Bepi Colombo MMO (Mercury Magnetospheric Orbiter)

International audience; The launch of the Bepi Colombo spacecraft, MPO (Mercury Planetary Orbiter) and MMO (Mercury Magnetospheric Orbiter), will take place in the fall 2018 for an arrival and orbit insertion at Mercury at the end of 2025. The objective of the MMO spacecraft that is under responsi- bility of JAXA, is to thoroughly investigate the magnetized environment of Mercury. On this purpose, MMO payload includes in particular the MPPE (Mercury Plasma Particle Experiments) consortium that is an instrumental suite dedi- cated to particle measurements. MSA is one of these instru- ments that focuses on measurements of the magnetospheric plasma composition. Taking advantage of MMO spin (4s period), MSA will measure three-dimensional ion distribu- tions over a large range of masses (1-60 amu) and energies (from 1 eV/q up to 38 keV/q). MSA consists of a spherical top-hat analyzer for energy selection, followed by a Time-Of- Flight chamber for mass identification. With its "reflectron" design, MSA offers a high mass resolution (typically, m/∆m > 50) that will allow for instance to distinguish between 39K and 40Ca ions. This capability will be of particular impor- tance to identify the various species that populate Mercury’s magnetosphere and to characterize the interaction between the planet surface and the solar wind.