New multi-fluid MHD model of comet 26P/Grigg-Skjellerup: Extrapolation to comet 67P/Churyumov-Gerasimenko

The Multi-fluid MHD simulator CASIM3D models the atmosphere of comet 26P/Grigg-Skjellerup as it was seen by the Giotto spacecraft during its flyby of the comet in July 1992. CASIM3D integrates the major chemical reactions and physical interactions to compute self-consistently over an adaptive Cartesian mesh the dynamics of ions, electrons, and neutrals within the coma. Using this model, we derived water ion velocity and density profiles and magnetic field strength along the Giotto path that are in good agreement with the recorded observations. This model is also extrapolated to the case of 67P/Churyumov-Gerasimenko for its perihelion passage of 2015 to provide a preliminary assessment of the structure of the coma of this comet. We show that the inner boundary layers are just few dozen kilometers large. This could provide an additional constraint to the orbit of the spacecraft Rosetta to maximize the science return of the mission.