This research area forms the basis of every analysis or tool that aims to predict the evolution of the debris environment, the danger of an actual impact with an asteroid, the consequences of a deflection or removal action.
Kartik Kumar, Nicolas Thiry, Massimo Vetrisano
Recent studies have highlighted the need for active removal of debris. Recent works  have provided some understanding of the requirements of such active debris removal under a variety of scenarios. Similarly for NEOs, a number of studies have investigated mission scenarios to deflect orbits for hazard mitigation or exploitation. Many missions to asteroids have been studied. For example, the Don Quijote mission  was planned to demonstrate the requirements of an impact mission, the Marco Polo mission will unveil new science on asteroids. Mission analysis and design for active debris removal is instead still partially unexplored. In both cases, different scenarios need to be analysed, designing optimal trajectories minimizing launch mass and operation time while achieving the primary mission goals: maximum deflection or maximum removal rate. This translates into a multiobjective/multicriteria optimisation problem that combines a combinatorial component (sequence of debris to be removed) with an optimal control problem. Large search spaces, the mixed nature of the problem and the proven existence of many optima represent a significant challenge that requires advanced optimization and analysis tools .
Objectives. The main objectives are: to train future specialists on classical space trajectory design techniques and introduce them to the most advanced global optimization methods. Starting from mission requirements, the researcher will be able to design space transfers to Earth orbiting objects (debris) and interplanetary targets (asteroids, planets), exploiting gravitational and /or impulsive manoeuvres, and low-thrust propulsion.
Massimo Vetrisano, Marko Jankovic
The manipulation of both asteroids and space debris and the modification of their orbits require close proximity operations. Proximity operations for both asteroids and space debris present a number of conceptual commonalities: formation flying dynamics and control, measurements (e.g. using LIDAR, optical flow, landmarks), filtering and state estimation techniques, level of actuation, autonomy, data fusion, etc. Starting from these conceptual commonalities, specialist hardware and operational requirements can be derived to address specific missions to asteroids or space debris. Recent work on LEO spacecraft proposed vision based solutions for formation flying satellites  and real time robust control methods; more recent activities have targeted asteroid operations using multi-agent systems . Classical approaches to proximity operations assume a cooperative target. These include autonomous path planning with real-time collision avoidance, for example using artificial potential field methods, and manoeuvring along a fixed line-of-sight to a tumbling target requiring control laws transformed to a rotating frame of reference. Many solutions to proximity operations around NEOs have been proposed for missions such as Don Quijote, Marco Polo, PROBA-IP, which exploit the peculiarities of the dynamic environment around asteroids: retrograde stable orbits, photo-gravitationally stable orbits, trailing orbits, etc. In application to such scenarios both the controlled and uncontrolled motion has been analysed including collision avoidance .
Objectives. To develop autonomous systems for proximity operations to asteroids and space debris, to estimate the state of motion of an non-cooperative target from in-space measurements, to investigate optimal proximal motion solutions for single and multiple spacecraft and integrate these aspects with the model of the removal technologies.
Active Removal/Deflection Control Under Uncertainty
Chiara Tardioli, Kartik Kumar
This young area of research aims at developing optimal strategies to control the motion of objects under uncertainty. Both system and dynamic uncertainties need to be modelled, quantified and inserted in either an open or closed control loop. To date, a study on the feedback deflection action under uncertainties for an non-cooperative space target is still missing. Advancements are required in the quantification of uncertainty in asteroid/debris dynamic and physical models. Besides dynamics and physical uncertainty, a crucial point is the uncertainty in the technology used either to actively remove debris or to deflect asteroids. Once an array of possible strategies is formulated and modelled, a set of uncertain variables has to be identified, together with an uncertainty model, quantification and a set of possible constraints that have to be respected during the mitigation manoeuvre. Standard optimal control techniques cannot solve the problem. Common closed loop control problems in space flight dynamics are often treated with linear methods whose main advantage is the simplification of the problem. Their accuracy drops off rapidly for highly nonlinear systems under uncertainties. Nonlinear controllers or the use of high order expansions are possible solutions. The use of imprecise probabilities  is another and can address epistemic uncertainty in system design. Other techniques include the use of stochastic optimal control.
Objectives. To find robust approaches to active removal/deflection control under uncertainty, to investigate the use of imprecise probabilities for robust coupled system and control design under uncertainty and integrate these aspects in the removal/deflection technology models.
Active Removal/Deflection Technologies
Nicolas Thiry, Natalia Ortiz, Marko Jankovic, Kartik Kumar
Various technologies have been proposed for both the removal of space debris and deflection of asteroids. Some techniques require direct contact with the target , while others are contactless; some are conceived to be installed on-board future spacecraft while others target the current debris population. For space debris, accelerated de-orbit through drag enhancement devises (e.g., sails, balloons, foam), tethers, ion beams, lasers or enhanced radiation pressure have been proposed. Sails, balloons, tethers require some attachment and are ideal to be installed on future spacecraft  while ion beams, lasers  are contactless and can remove existing debris. None of these techniques, however, are yet at the level of maturity to be implemented in a real mission. For large objects, the control of the deorbiting and re-entry phase is still an open issue. Different techniques apply better to different types of targets in different orbits and a complete picture, with a full comparative assessment is still missing. Even for asteroids, a plethora of concepts have been proposed in the last few decades. Many comparative assessments exist but the high degree of uncertainty on the physical nature of NEOs makes some conclusions questionable. Some deflection techniques imply an instantaneous transfer of momentum (explosive techniques, impact techniques) along with a low level of control but a sizeable effect, whereas others imply a controlled but fainter effect on the asteroid trajectory (gravity tractor, ion beam shepherd, laser ablation). Moreover, the problem of deflecting asteroids or satellites not originally designed for being captured, and/or tumbling in space with no attitude control capabilities, is an extremely difficult problem requiring extensive research to advance the enabling technologies, including robust guidance, navigation, and control in highly nonlinear and uncertain scenarios.
Objectives. To model existing deflection/removal techniques, to produce a comparative assessment of active debris removal techniques, to investigate new conceptual approaches and include them into a consistent evaluation tool integrated with existing damage and risk prediction tools.
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