A method to determine the interaction between the plume of a magnetoplasma spacecraft engine and the adjoining spacecraft surfaces using a 3D axisymmetric numeric model is presented. Traditional methods use a continuum model to simulate plasma even when the plume's Knudsen number > 1 and thus the continuum assumption is not valid. Therefore, the new method uses kinetic particle theory to numerically model a subset of individual ions to determine their trajectories and kinetic energies after ejection. The extrapolated data is used to predict how many ions will remain trapped by the engine's magnetic field and interact with the spacecraft in the form of surface erosion. The quantity and kinetic energy of all ions impacting spacecraft surfaces are then used to predict erosion of aluminum and silicon surfaces. It was determined that 0.012% of ejected particles remain trapped in the engine's magnetic field under nominal operation with an erosion rate of 1.386 nm per month Al and 0.611 nm per month Si but that these amounts can vary significantly by changing the average exit velocity, magnetic field strength, and field geometry. Electrical charging rates can be determined using this method and will be presented in another paper.