Numerical simulation of the electrical charging of the ROSETTA orbiter

In the low density plasma of Wirtanen comet environment, the ROSETTA orbiter may float at a positive potential. That may prevent the collection and analysis of the very cold ions by the inboard plasma diagnostics. In order to evaluate those possible problems, we have undertaken an investigation to determine the orbiter floating potential based on a numerical modeling of its interaction with the ambient plasma. This model uses a full particle method for both ions and electrons. It was run for 2 sets of conditions corresponding to the comet at respectively 3 and 1 AU from the Sun and, in each case, for 2 distances of the orbiter from the nucleus, namely 1 R, (nucleus radius), or approximately 1 km, and 100 R-n. When far from the nucleus, the orbiter was shown to float at positive potentials of about 2 to 4 volts. When close to the nucleus the floating potential is of the order of the thermal energy of the plasma particles. The numerical model and the computed structure of the plasma sheath around the orbiter are presented in this paper.