On the Construction of Low-Energy Cislunar and Trans-lunar Transfers   Based on the Libration Points
  There exist cislunar and trans-lunar libration points near the Moon, which are referred as the LL1 and LL2 points respectively and can generate the different types of low-energy trajectories transferring from Earth to Moon. The time-dependent analytic model including the gravitational forces from the Sun, Earth and Moon is employed to investigate the energy-minimal and practical transfer trajectories. However, different from the circular restricted three-body problem, the equivalent gravitational equilibria are defined according to the geometry of instantaneous Hill's boundary due to the gravitational perturbation from the Sun. The relationship between the altitudes of periapsis and eccentricities is achieved from the Poincar\'e mapping for all the lunar captured trajectories, which presents the statistical feature of the fuel cost and captured orbital elements rather than generating a specified Moon-captured segment. The minimum energy required by the captured trajectory on a lunar circular orbit is deduced in the spatial bi-circular model. It is presented that the asymptotical behaviors of invariant manifolds approaching to/from the libration points or halo orbits are destroyed by the solar perturbation. In fact, the energy-minimal cislunar transfer trajectory is acquired by transiting LL1 point, while the energy-minimal trans-lunar transfer trajectory is obtained by transiting LL2 point. Finally, the transfer opportunities for the practical trajectories escaped from the Earth and captured by the Moon are yielded by transiting halo orbits near LL1 and LL2 points, which can be used to generate the whole trajectories. 
Ming Xu, Yan Wei, Shijie Xu
astro-ph.EP