Attitude control with actuator limitation and constraints for LIDAR pointing in close-proximity operations

5 1. Introduction 1.1 Close proximity operations Close proximity operations in orbit involve an active space object that maintains a close distance from another one. Such operations are relevant to many scenarios like On Orbit Servicing and Active Debris removal. The distances between two spacecraft can vary depending on the mission characteristics and specificities of performed operations [1]. The execution of Active Debris Removal missions represents one of the key solutions to a critical problem of the space era: the growth of space debris. Space debris is defined as obsolete spacecraft (satellites and rockets) or fragments of spacecraft that have broken off satellites and rockets. This includes a range of objects, from miniscule paint chips, screws, etc. to whole satellites or rocket bodies, all rendered unusable or nonoperational [2]. Since the beginning of the space era, a significant amount of debris has progressively been generated and now the current Low Earth Orbit (LEO) environment contains about 3200 intact objects. Today these objects represent a threat as the presence of space debris incurs risk of collision and damage to operational satellites. Besides studies performed with long- term evolution models like DELTA have shown that a “business as usual” scenario will lead to a progressive, uncontrolled increase of object numbers in LEO, with collisions becoming the primary debris source since the consequence of a growing number of objects in space, especially LEO, is an increasing risk of objects collisions which, if happening frequently, can trigger a chain reaction of exploding space debris [1][3][4][9]. Active debris removal consists in removing debris objects by capturing them and disposing of them, for example removing the object from its current orbit and performing a controlled reentry into the atmosphere like ESA (European Space Agency) and ClearSpace SA are aiming to do with their mission ClearSpace-1, performing the first space mission to remove an item of debris from orbit [3][5][6][7]. Another solution to avoid the growth of space debris may be given by On Orbit Servicing missions, which aim at providing services to a malfunctioned spacecraft. Indeed, the traditional space missions into Earth Orbit are designed considering the achievable time of satellite operations, which is Figure 1.1 - ClearSpace-1 mission