Unveiling the true plasma behaviour around comets by spacecraft charging simulations

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In 2019, Comet Interceptor was selected by ESA as the first F-class mission. This mission will make a flyby of a long-period comet and for the first time make multi point measurements in the cometary environment using three spacecraft: Spacecraft A (ESA), Probe B1 (JAXA) and Probe B2 (ESA). All spacecraft will carry plasma instruments, allowing, for the first time, a threedimensional study of the cometary plasma environment. The plasma measurements are, however, expected to be affected by the spacecraft potential, as already observed by other cometary missions (e.g. the Rosetta mission, Bergman et al., 2020). Throughout the flyby, the spacecraft will pass by several plasma regions with different characteristics, and hence the spacecraft potential is expected to vary. In this study, we estimated the spacecraft potential of Probe B1 of Comet Interceptor throughout the cometary flyby using Particle-In-Cell (PIC) simulations and study the influence on the plasma measurements. The main results are as follows:(1) At a low relative flyby velocity, a more effective cooling of the electrons in the inner coma results in less negative spacecraft potentials for a Halley-type comet.(2) Secondary electron emissions from neutral impacts charge the spacecraft positive in most environments at a high relative flyby velocity.