Low energy transfer to earth-moon DRO via Lunar Gravity Assists

This paper discusses an approach to design transfer trajectories to a lunar distant retrograde orbit (DRO) from a low-altitude Earth parking orbit by using one lunar gravity assist or two lunar gravity assists. The spacecraft is placed on the Earth-Moon free return orbit by the first impulse, which is assumed parallel to the velocity of the parking orbit. Later, the second impulse, which is again parallel to the velocity of the spacecraft, enable the spacecraft to arrive at perilune. Thus, the third impulse applied at the perilune achieves a powered lunar gravity assist and promotes the spacecraft to fly to the lunar DRO. Finally, the spacecraft enters DRO by the last impulse whose direction is not restricted. However, the lunar gravity assist associated with the third impulse can place the spacecraft into a transfer orbit having the apogee distance more than two Earth–Moon distances. In this case, it is possible that the small third impulse, combined with the perturbation of the Sun, result the spacecraft to approach the Moon again from the exterior. So that the second lunar gravity assist is possible, which is also achieved by one impulse, and take the spacecraft enters DRO by another impulse. The total delta-v and flight time required to transfer from LEO to various phase points in a certain DRO was examined in both cases. The results showed that certain regions where the delta-v is low exists; and the flight time varies considerably depending on the targeted DRO phase point and the aforementioned two case. The transfer orbit using two lunar gravities needs lower cost (the total delta-v is less than 100m/s except the first impulse) but longer time. An applicable launch window with these features was proposed, which showed the potential to widen the option to transfer to a DRO.