An exponential increase in space traffic within the cislunar region is expected in the coming years, given the growing number of missions targeting this strategic region of space, either to exploit lunar resources or to establish an outpost for future human explorations extending far beyond current operations. Alongside the technological developments required to support this, significant attention is being given to limit the generation of a space debris problem in the region. A key priority and challenge is to establish a robust infrastructure for space situational awareness in an area of space subject to the strong gravitational influence of both the Earth and the Moon, with the aim of preventing fragmentation events and collisions involving operational or defunct orbiting objects.
A fundamental prerequisite for achieving this objective is the development of robust, low-cost End-of-Life (EoL) disposal strategies, facilitating the integration of satellite decommissioning into lunar mission design, thereby mitigating the risk of debris generation. Within cislunar space, there are four main disposal strategies: heliocentric disposal, controlled lunar impact, Earth re-entry and insertion into a graveyard orbit. The most appropriate strategy should be selected based on the characteristics of the satellite's operational orbit, the ΔV budget, the time available for disposal, and the level of safety required by regulations for current and future missions.
This work presents an analysis of the evolution of unstable manifolds, i.e. disposal trajectories, associated with a subset of periodic orbits in the Earth–Moon (EM) system. The analysis focuses on candidate orbits from the L₁ and L₂ Halo families of the EM system, which are considered of particular relevance for future mission architectures. Trajectories are propagated using a simplified coupled Circular Restricted Three Body Problem dynamical model: initially, the model accounts for the EM effect on the trajectory; subsequently, it accounts for the Sun-Earth (SE) one, once the Moon's influence on the trajectory is deemed negligible. The unstable manifold evolution from Libration point orbits around L₁ and L₂ of the EM system can result in (a) an escape from the EM system and then escape towards the SE-L₁ or L₂ point, in (b) impact with the Moon, in (c) intersection with an area of radius equal to that of the geostationary belt around the Earth, or in (d) remaining in cislunar space for the entire time considered. Each of these options can correspond to the preliminary design of a certain type of disposal and is classified as such. It is demonstrated here how disposal outcomes vary according to certain parameters, such as the relative position of the three celestial bodies at departure. This parameter is of particular importance since it is related to the influence of the Sun perturbative effect in the Earth-Moon system. Finally, it is shown how it is possible to improve the disposal design for some of the cases analysed by selecting the scenarios that best comply with existing space sustainability guidelines. This can be achieved by assessing the Zero Velocity Curves closure cost in the case of heliocentric disposal or by identifying impact points reached in the case of Earth or lunar impacts.