In the framework of AIDA (Asteroid Impact & Deflection Assessment), an international collaboration for planetary defense, the DART and Hera missions jointly target the binary asteroid system Didymos–Dimorphos. Their scientific objectives are to assess asteroid deflection, perform close-range characterization, 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 carry two European CubeSats—Juventas and Milani—and was launched in October 2024. Hera will arrive in the Didymos system in autumn 2026 to characterize the DART impact crater and investigate the system’s physical properties, including gravity field, internal structure, dynamical environment, global morphology, and potential dust cloud generated by the impact.
 
Hera is currently on a two-year cruise phase under ESA/ESOC operations. Once in proximity to the binary system, the Hera spacecraft will first characterize the asteroids in terms of dynamics, shape, and gravity, before releasing the two CubeSats. 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—including ejection and separation, far-range and close-range operations, landing, and disposal—each subject to payload requirements, navigation performance, safety constraints with respect to the asteroids and the mothership, and inter-satellite communication constraints.
 
This paper presents the trajectory design and flight-dynamics operations strategy of Milani, whose trajectory is significantly more complex and constrained than that of its companion CubeSat, Juventas. The study focuses on two distinct observation phases—Far Range Phase and Close Range Phase—and details how these phases were designed to fulfill the mission’s scientific objectives while satisfying all associated guidance, navigation, and control (GNC), communication, and safety constraints with respect to both the mothership and the asteroids.
In particular, the paper discusses the mission constraints imposed by Milani’s payload, ASPECT, a hyperspectral imager that requires specific geometries to map both asteroids, characterize the microstructure of their surfaces, and observe the DART impact under defined resolution and phase-angle conditions.
Furthermore, recent changes—including more stringent inter-satellite link (ISL) constraints and the earlier arrival of the Hera spacecraft within the binary asteroid system, which resulted in overlapping operational zones between Hera and Milani—required a complete redesign of these phases to ensure compliance with all safety requirements relative to the mothership. The paper also details the evolution from the baseline trajectory to the most recent one in light of these updated operational constraints.