Formation flying orbits and GNC design in binary asteroid systems

Formation flying represents a promising opportunity for next space missions, due to its benefits in cost saving, redundancy and fault tolerance given by multiple small and cheap space segments exploitation, with comparable performance of a single, large monolithic spacecraft. That is even more relevant in asteroid exploration, as the harsh environment poses great risks to the probes survival. The low, irregular gravity field, the poor knowledge of shape and composition, and the possible presence of floating particles in the surroundings suggest adopting a low-risk strategy for the exploration of these bodies, delegating proximity operations to multiple nanosatellites, while keeping the main spacecraft at safe distance. As a downside, multiple cooperating spacecraft imply advanced capabilities in accurately reconstructing the relative positions and displacements and in fixing reconfiguration manoeuvres. Moreover, whenever relative distance is very close, agents’ guidance and control shall be autonomously computed by the spacecraft, as promptness of commands is not ensured relying on ground segment only. The partially unknown environment exacerbates this issue, demanding navigation and control schemes capable of withstanding potentially large uncertainties in the dynamics. If a binary system is considered, the multiple gravitational sources complicate even more the setup of the formation, requiring an accurate search and selection of operative orbits. This study will be thus defined by three different steps. First the search and development of suitable trajectories to host a reconfigurable satellite formation is carried out. Then the design of a Model Predictive on-board guidance and control law is performed to assess the capability of the spacecraft to execute successful reconfiguration transfers relying on a simplified dynamical model (namely the Circular Restricted Three-Body Problem). At the end the navigation error requirements are derived, in order to ensure the feasibility of the guidance and control scheme. Inspired by the Hera mission, this study explores the aforementioned aspects of the design of a formation for the exploration of the Didymos binary asteroid system, which comprises many complications but also challenges for many other future missions to fly in unexplored environments.