Optimal low-thrust escape trajectories using gravity assist

Electric propulsion provides a spacecraft with continuous steering capabilities, which can be used to approach a planet with hyperbolic excess velocity that enhances the gravity assist. Low-thrust trajectories to escape from the solar system are considered in the present paper, which searches for the strategy that maximizes the spacecraft energy for assigned payload and engine operating time. The optimal conditions to escape using electric propulsion and gravity assist are presented for the cases of free-height and minimum-height flybys. Optimal trajectories that exploit Jupiter or Venus flybys have been computed for constant exhaust power with either constant or variable specific impulse; the procedure is also able to determine the optimal power level and to suggest when it is convenient to switch the engine on and off. The benefit that system performance can receive by increasing the number of controls, i.e., by adding the possibility of coast arcs and engine throttling to the thrust direction control, is also noted.