Optimizing low-thrust and gravity assist maneuvers to design interplanetary trajectories

In this paper a direct method based on a transcription by Finite Elements in Time has been used to design optimal interplanetary trajectories, exploiting a combination of gravity assist maneuvers and low-thrust propulsion. A multiphase parametric approach has been used to introduce swing-bys among thrust and coast arcs. Gravity maneuvers are at first modeled with a link-conic approximation and then introduced through a full three-dimensional propagation including perturbations by the Sun. The method is successfully applied to the design of a mission to planet Mercury, for which different options corresponding to different sequences of gravity maneuvers or launch opportunities are presented.