  A surprising and unexpected phenomenon observed during Cassini's Grand Finale
was the spacecraft charging to positive potentials in Saturn's ionosphere.
Here, the ionospheric plasma was depleted of free electrons with negatively
charged ions and dust accumulating up to over 95 % of the negative charge
density. To further understand the spacecraft-plasma interaction, we perform a
three dimensional Particle-In-Cell study of a model Cassini spacecraft immersed
in plasma representative of Saturn's ionosphere. The simulations reveal complex
interaction features such as electron wings and a highly structured wake
containing spacecraft-scale vortices. The results show how a large negative ion
concentration combined with a large negative to positive ion mass ratio is able
to drive the spacecraft to the observed positive potentials. Despite the high
electron depletions, the electron properties are found as a significant
controlling factor for the spacecraft potential together with the magnetic
field orientation which induces a potential gradient directed across Cassini's
asymmetric body. This study reveals the global spacecraft interaction
experienced by Cassini during the Grand Finale and how this is influenced by
the unexpected negative ion and dust populations.
