The ESA Comet Interceptor mission, selected within the Cosmic Vision Programme and planned for launch in 2028/2029, aims to perform the first dedicated flyby of a dynamically new or long-period comet. To maximize the probability of encountering such a target, which may only be identified shortly before perihelion, the spacecraft will be delivered to a quasi-halo orbit around the Sun–Earth L2 Lagrange point and remain there in a longterm waiting phase. From this location, transfers toward candidate objects must be initiated with limited propellant and under time constraints. Understanding the range of feasible Earth-escape conditions and the mechanisms that naturally enhance the spacecraft’s post-L2 energy is therefore a central aspect of trajectory design for the mission.

It has been shown that certain trajectories departing from L2 naturally drift toward the Earth through the unstable manifolds of the Sun–Earth Circular Restricted Three-Body Problem (CR3BP). For transfers whose perihelion lies inside 1 AU, the spacecraft inevitably approaches the Earth–Moon system, where the geometry of the invariant manifolds leads to a wide variety of Moon encounter conditions. This creates a unique opportunity: instead of treating the Earth–Moon passage as a perturbation to be mitigated, it can be exploited to provide a more favourable hyperbolic excess velocity from Earth-Moon system. In particular, fly-bys of the Moon, encountered as a consequence of the manifold-driven motion, may modify the departure Vinf and reduce the ΔV required to inject the spacecraft into the heliocentric trajectory toward the comet. Moreover, suitable fly-by conditions may have the potential to shorten the transfer time to the target, improving responsiveness and increasing the mission’s probability of success. 

This work investigates the potential benefits of incorporating Moon fly-bys into the first segment of Comet Interceptor’s departure trajectory. Building upon previous analyses of Earth-escape strategies, the study focuses on identifying the conditions under which lunar gravity assists can most effectively enhance Vinf, reduce the required propulsive cost, and potentially decrease the overall transfer duration.

The results of this investigation will support the design of Comet Interceptor’s departure strategy and will establish a foundation for future work on the complete interplanetary transfer.