  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, treated as coast
phases between two thrusted or coasting trajectory arcs. Gravity maneuvers are
at first modeled with a linked-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.
