Transfers from distant retrograde orbits to low lunar orbits

The stable distant retrograde orbits (DROs) around the Moon are considered as potential parking orbits for cislunar stations that are important facilities in cislunar space. Transfer orbits from DROs to lunar orbits will be fundamental and routine for operations of the cislunar stations. This paper studies transfer orbits from DROs to low lunar orbits with inclinations between 0° and 90°. Ten DROs are selected for the construction of transfers. The planar transfer orbits from each DRO to the LLO with zero inclination are firstly obtained and compared in the planar circular restricted three-body problem (PCR3BP) to reveal basic characteristics of the transfer solutions. The planar transfers are classified into several types based on characteristics. Each type is discussed in details, especially their transfer cost and time. Based on the planar transfers, nonplanar transfer orbits are constructed in the circular restricted three-body problem (CR3BP). Some nonplanar transfers are selected and compared to show effects of the LLO inclination. Then, the planar transfer orbits are refined in the planar bicircular restricted four-body problem (PBR4BP) with the gravity of the Sun. The comparison between results in the PCR3BP and PBR4BP shows that the gravity of the Sun can increase transfer options and reduce the transfer cost. Further analysis is carried out based on the realistic results in the PBR4BP, including the ballistic capture, departure and insertion locations, transfer cost and time, etc. The results are useful for selecting parking DROs and designing transport systems to the Moon.