Abstract: Queqiao-2 provides relay services in its overall life cycle for Chang′e-6, Chang′e-7, Chang′e-8, and Chang′e-4 missions, as well as lunar exploration missions from international cooperation. To meet the different relay needs of multiple launch windows and different flight phases of Chang′e-6 mission, the trajectory design method and process of the Queqiao-2 relay satellite to service the Chang′e-6 mission are introduced from the perspective of mission analysis. The adoption of a lunar elliptical sun-synchronous frozen orbit satisfies the requirements for communication range, coverage, launch windows, and orbital phase for the Chang′e-6 mission. Then the trajectory scheme and the control strategy adopted on each flight phase are optimized with minimum fuel constraint. A four-impulse joint optimization trajectory control strategy is employed to significantly alter the orbital ascending node, inclination and argument of perilune with few velocity increments. Finally, the status of the in-orbit flight and relay support for Chang′e-6 mission are shown respectively. For the first time internationally, Queqiao-2 has achieved a significant change of right ascension of the ascending node in lunar orbit by using a lunar low energy capture orbit, and close range and long arc relay of target on the far side of the moon by using the lunar large elliptical sun-synchronous and frozen orbit. The result shows that the in-orbit flight of Queqiao-2 is consistent with that of the design, which ensures the successful implementation of the Chang′e-6 lunar far-side sampling and return mission. The design method can provide a reference for the orbit design of deep space exploration missions.

Key words: Queqiao-2 relay satellite, lunar large-elliptical frozen orbit, sun synchronous, flight and control, lunar low energy capture orbi