The current context of space traffic fosters the development of technologies necessary for carrying out active debris removal (ADR) missions. The increasing population of active and inactive objects in orbit poses the challenge of an efficient use of the orbital space, not only for de-orbiting but also for In-Orbit Servicing, Assembly and Manufacturing (ISAM). Some of the technologies required for ADR missions are common to ISAM. Globally, these new operational concepts and technologies will allow for a more sustainable redefinition of space logistics. 

Specifically, ADR missions must cope with uncooperative objects which translate in highly demanding requirements. Un-cooperative objects, in general, lack any kind of control and, in some situations, they are in a tumbling attitude state. Thus, capture or contact methods become much riskier undertakings than non-contact methods.  This is one of the reasons this work will focus on ADR missions using the Ion Beam Shepherded (IBS) concept. The IBS is a non-contact method that involves using the force from ion beam to de-orbit debris. It was first proposed and developed by SDG-UPM group and has now garnered considerable attention as a promising debris removal method, also finding its application in asteroid deflection. The contactless characteristics of the IBS method provide a great advantage when targeting uncooperative objects, enabling the handling of targets with varied shape, size and rotation state. The presence of both opportunities and gaps has attracted widespread attention in trying to understand and improve this method, resulting in an extensive body of literature. 

One of the key technologies to enable IBS missions is an advanced autonomous Guidance, Navigation & Control (GN&C) system able to cope with close proximity operations in a highly required environment. The target and the vehicle should follow a formation flight while exchanging momentum with the ion beam. Therefore, the dynamic non-linearities, the significant uncertainties, both in dynamics and navigation, and time-varying conditions must be addressed by the chosen methodology to cope with all the challenges. In addition, they must handle factors such as mass redistribution, and environmental perturbations, and thus have the capability of active monitoring, decision making, and adaptation. While pre-flight models remain essential for accurate design, they might be insufficient to capture in-orbit variability, motivating the need for on-board predictive models, adaptive control, and real-time reconfiguration strategies. 

In this communication, we perform a detailed analysis of the system requirements for a successful ADR mission using the IBS concept. In addition, focusing on the GNC subsystem, we describe the challenges posed by these missions and the robust and time-varying GN&C solutions that could address them. The challenges can be structured into the three following large blocks: 1) the development of a high-fidelity simulation for the representative scenarios in order to achieve V&V of the proposed methods and to highlight their limitations; 2) the system identification, including the estimation of key parameters of the dynamic models by using predictive model techniques; and 3) analyses of robust guidance and control methods for the chosen scenarios. The preliminary results of the chosen solution in the selected scenarios will be presented.