Reduced Order Spacecraft Charging Models for Electrostatic Proximity Operations

Spacecraft charge in orbit due to naturally occurring electric currents from the ambient plasma emitted from the surface, as well as artificial currents produced by devices such as an electron gun. This results in electrostatic forces and torques between two spacecraft in close proximity that can perturb the relative motion and attitude during on-orbit servicing, assembly, and manufacturing (OSAM) operations. The forces and torques may also be utilized to remove or detumble dysfunctional satellites. In prior work on electrostatic proximity operations, typically charging models based on spherical spacecraft were used to compute the electrostatic potential, and that potential was prescribed to be constant in relative motion simulations. In this work, several charging models are compared for spacecraft with nonspherical shapes, including simple sphere models and faceted models. The faceted model is promising because its total surface area is more accurate, and it allows for the consideration of a time-varying sunlit area and ram-facing area. It is shown that the orientation of the spacecraft with respect to the Sun can significantly affect the equilibrium potential and the resulting force and torque. This is demonstrated by a charged attitude motion simulation for the electrostatic tractor (ET) debris removal concept. The effect of differential charging on the electrostatic force and torque is investigated. It is shown that differential charging can lead to significant force differences with respect to a fully conducting spacecraft, including a switch from repulsive to attractive force components.