The DART and Hera missions jointly target the Didymos-Dimorphos binary asteroid system in the framework of AIDA (Asteroid Impact & Deflection Assessment), an international collaboration for planetary defense. Their objectives are to perform close-range characterization, evaluate asteroid deflection capabilities and demonstrate key technologies for future planetary-defense missions. NASA’s DART spacecraft impacted Dimorphos in September 2022. Hera, the ESA follow-up mission, will arrive in the Didymos system in autumn 2026 to characterize its physical properties and investigate the DART impact crater, by studying the system gravity field, internal structure, dynamical environment and hypothetical dust cloud generated by the impact.

The Hera spacecraft is currently on a two-year-long cruise phase under ESA/ESOC operations. Once in proximity to the binary system, it will perform a primary characterization of both asteroids in terms of dynamics, shape, and gravity, before releasing two European CubeSats, Juventas and Milani, which embark their own instruments and possess distinctive scientific objectives. CNES has been entrusted with the responsibility for close-proximity flight-dynamics and mission-planning operations for both CubeSats, from their separation from the mothership until the completion of their respective scientific objectives. These operations will be conducted from the FOCSE (French Operation Center for Science and Exploration) in Toulouse, as part of the CMOC (CubeSat Mission Operation Center, ESEC, Belgium), in coordination with the HMOC (Hera Mission Operation Center, ESOC, Germany).

Each CubeSat’s close-proximity sequence consists of a series of phases, which include ejection and separation, far-range and close-range operations, landing, and disposal. As such, maneuver plans are engineered to perform various transfer arcs for the CubeSats to travel from one point in the Didymos binary system to another. Trajectory design needs to fulfill the mission’s scientific objectives while satisfying all associated guidance, navigation, and control (GNC), inter-satellite communication, and safety constraints with respect to both the mothership and the asteroids.

The Lambert’s Problem classically provides the required velocity needed to reach a certain target in a specified time from a known departure point. However, because of the relatively low mass of the primary asteroid, trajectories in the dynamical environment of the Didymos system will be heavily influenced by both the Solar radiation pressure and the attraction of the secondary asteroid. In this situation, the maneuvers obtained from the solution to the Lambert’s Problem are quite inadequate because only applicable to a Keplerian environment.

Furthermore, many uncertainties remain today about Juventas and Milani forthcoming mission. The first characterization of the system by Hera may reveal a completely different dynamical environment from the one currently assumed. Consequently, the Hera spacecraft operational trajectory strategy may be adjusted at the last minute. Those factors indicate that severe changes on the CubeSats trajectory design are to be anticipated. There is a critical need for the ability to rapidly design trajectories that are compliant with mission’s scientific objectives and all safety requirements.

With this mindset, several methods have been imagined, investigated and implemented by CNES flight dynamics team in order to possess a generic approach to compute suitable transfer arcs and solve the Lambert’s Problem in a heavily perturbed dynamical environment. This generic approach involves several steps: the necessity to find a proper first initialization, a convergence on a local scale by iterating using the information provided by the final state transition matrix, and finally taking into account the two previous steps on a more global scale by using intermediary stages to guide the convergence and discard inadequate solutions such as those passing by the unwanted side of the primary asteroid. Different intermediary stages’ natures have been assessed (intermediary trajectory points, dynamical models, or durations), leading to a large range of possible resolution strategies, ensuring a suitable maneuver computation even in complex cases.

This paper presents the studied methods principles and characteristics as well as their performance on some phases of Juventas and Milani currently planned mission. The suggested generic approach will be used for the upcoming updates to the Juventas and Milani operational phases but may also be helpful for other asteroid-related missions in the future