## 1 Introduction  
In recent years, continuously increasing attention has been put on close-proximity operations (CPO) as they allow many fundamental actions for space sustainability, like refueling, inspection, and debris removal, among others. This rising interest has motivated the need for standard and internationally accepted requirements, so that the operations can fulfill their goal effectively and safely. Many activities are found within this scope: the International Rendezvous System Interoperability Standard (IRSIS) [1] was created to define standards for on-orbit crew operations, fully automated rendezvous and docking, and joint collaborative endeavors using different spacecraft; the Consortium for Execution of Rendezvous and Servicing Operations (CONFERS) [2, 3, 4, 5] focuses on technical and operations standards for On-Orbit Servicing (OOS) and Rendezvous and Proximity Operations(RPO), and the outcome can be found in the ISO 24330 document [6]; OOS was also the object of a French Space Operations Law (LOS) update issue [7,8]; and the Japan Aerospace Exploration Agency (JAXA) provided basic requirements for OOS, listed in the \"Safety Standard for On-Orbit Servicing Missions (JERG-2-026(E))\" [9]. All these documents can be compared and integrated into the European Space Agency (ESA) Guidelines on Safe Close-Proximity Operations [10]. The demonstration that the requirements and guidelines are well defined and fit for purpose is supported by the development and application of verification and validation (V&V) methods. The collection of guidelines abovementioned was thoroughly discussed, revised, and expanded in the study \"Verification and Validation of Rendezvous and Proximity Operations Safety\" [11], as a result of the collaboration of ESA and the industrial partners Thales Alenia Space, GMV, and Deimos. The aim of that study was to first derive CPO guidelines, then define verification and validation approaches, and finally demonstrate the V&V approaches for both a cooperative and a non-cooperative client. Among the different applications of Close-Proximity Operations, this paper focuses on Active Debris Removal (ADR). In the ADR field, a large number of references can be found, covering the different aspects of this particular kind of mission. An extensive state-of-the-art review of ADR technologies is provided in [12], which gives an in-depth trade-off of different capture mechanisms while presenting the progress that has been made in the field since the 1990s. State-of-the-art space technologies are described therein. In particular, the use of robotic arms is widely discussed, highlighting applications with a single arm [13, 14, 15], dual arms [16], and multi arm [17, 18]. Similar to robotic arms, the tentacles capture method is also covered [19]. Another presented capture mechanism makes use of a net shot from the active satellite that embraces the debris element, closes around it, and drags it to the desired position [15, 20, 21]. The harpoon method is also proposed due to its ease of testing and cost efficiency [15, 22]. Then, the tether-gripper method is introduced [23]. Finally, other technologies are discussed, such as magnetic capture [24], and other ways to alter the trajectory of the debris and force their re-entry through laser systems [25] or space mist [26]. Regarding programs, it is worth mentioning ESA's e.Deorbit, which was born in 2012 with the objective of removing a single large ESA-owned space debris from the LEO protected zone [15]. Within this program, different technology development studies were conducted, focusing on both capture mechanisms, and GNC and avionics aspects. A notable ADR mission is RemoveDebris: launched in 2018, this mission was a low-cost in-orbit demonstration where a microsatellite was released and then successfully recaptured two debris targets with a net and a harpoon [21]. Another successfully conducted mission is ELSA-d, in which the target was captured using a magnetic docking technology in 2021 [24].