Comet Interceptor, ESA’s first F-class mission of the Science Program, aims to perform the first close-up, multi-point investigation of a Dynamically New Comet, or even an interstellar object. Suitable targets are typically discovered at most a few years before the perihelion passage. Consequently, a launch-before-target mission concept is envisaged: the spacecraft will wait in a parking orbit around the second libration point of the Sun-Earth system (SEL2) until a suitable comet with a viable transfer trajectory is identified.  

An innovative dual-launch strategy has been analysed in detail by the Mission Analysis team at ESOC. The standard GTO launch profile is used for the co-passenger, typically a GEO satellite. Ariane 64 upper stage reignition capability is exploited afterwards to boost Comet Interceptor into the desired near-parabolic orbit. The probability of finding a suitable co-passenger is maximized when considering the standard “midnight” GTO launch window. However, this results naturally in an apogee directed towards the Sun and away from SEL2, which complicates the transfer to the SEL2 parking orbit. Hence, the aim of this research is to assess the feasibility of employing this novel launch strategy followed by an indirect transfer to reach SEL2 orbit. The strategy is not only relevant to Comet Interceptor, as it could be potentially applied to other missions as well. 

Feasible transfers are found using the common bisection method for libration point orbit mission design.  In this case, however, the Weak Stability Boundary effect and the heteroclinic pathways between the SEL1 and SEL2 regions need to be exploited in order to insert the spacecraft into a stable manifold that converges toward a periodic solution around SEL2, with minimal deterministic manoeuvring required. By setting appropriate bisection bounds and mapping the design space with a fine grid in the impulsive ΔV, different solutions can be obtained that are later filtered to satisfy mission constraints.  

For this case, the analysis yielded a remarkably wide and highly populated year-long launch window with few problematic epochs, which are mainly due to perturbation of the trajectory by the Moon.  Furthermore, in general, several solutions are found for each epoch, offering versatility to suit different mission characteristics, requirements, and drivers. As such, depending on the figure of merit for which to optimise, the launch window could be populated with very different transfers. When minimising the transfer time, for instance, 99% of trajectories reached the SEL2 parking orbit within six months, with the fastest requiring roughly 100 days. Aside from the expected diversity in transfer geometries because of the sensitive dynamics of the problem, the variety of obtained periodic solutions is noteworthy, ranging from Lissajous orbits of very different in-plane and out-of-plane amplitudes, to even northern and southern quasi-Halo motion in some portions of the launch window. The feasibility of navigating the resulting trajectories was confirmed via a dedicated navigation analysis of several transfers of interest.  

Overall, the results suggest that the investigated launch strategy is a promising alternative to direct transfer for missions that target, as parking or final orbit, a libration point orbit around SEL2. The bisection method allows to account for very diverse mission constraints due to its generality and constitutes an efficient way to explore a broad range of solutions.