Low-cost transfer between asteroids with distant orbits using multiple gravity assists

Low-cost transfer trajectories are significant to explore asteroids with distant orbits in a multiple targets’ mission. Methods for designing these trajectories optimally are proposed. The sequence of gravity-assists is evaluated by the Tisserand graph. Then, an optimization method combining the particle swarm optimization (PSO) and the indirect method is used to optimize the low-thrust trajectories with gravity assists. The Bang–Bang control problem in the indirect method is overcome by a smooth technique. The whole transfer trajectories solving process by the shooting method is divided into several steps to overcome the difficulty and improve the efficiency. Numerical simulations are carried out for validating the proposed method.