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A surprising and unexpected phenomenon observed during Cassini's Grand Finalewas the spacecraft charging to positive potentials in Saturn's ionosphere.Here, the ionospheric plasma was depleted of free electrons with negativelycharged ions and dust accumulating up to over 95 % of the negative chargedensity. To further understand the spacecraft-plasma interaction, we perform athree dimensional Particle-In-Cell study of a model Cassini spacecraft immersedin plasma representative of Saturn's ionosphere. The simulations reveal complexinteraction features such as electron wings and a highly structured wakecontaining spacecraft-scale vortices. The results show how a large negative ionconcentration combined with a large negative to positive ion mass ratio is ableto drive the spacecraft to the observed positive potentials. Despite the highelectron depletions, the electron properties are found as a significantcontrolling factor for the spacecraft potential together with the magneticfield orientation which induces a potential gradient directed across Cassini'sasymmetric body. This study reveals the global spacecraft interactionexperienced by Cassini during the Grand Finale and how this is influenced bythe unexpected negative ion and dust populations.

Particle-In-Cell Simulations of the Cassini Spacecraft's Interaction with Saturn's Ionosphere during the Grand Finale