ESA GNC Conference Papers Repository

During the past few decades the population of space debris objects has grown rapidly, increasing the risk of collision and reducing the viability of future space missions. The growth of the debris population cannot be stopped only by reducing the amount of debris produced in future missions, and Active Debris Removal (ADR) plans have been proposed in order to actively reduce the amount of debris orbiting the Earth. The European Space Agency has funded several studies to remove the defunct Earth observation satellite Envisat from its orbit. The ESA e.Deorbit mission aims to use a chaser to capture, stabilize and de-orbit Envisat. However, measurements from 2013 indicate that the target has increased its angular velocity to a rate close to 3°/s, which poses a challenge for any method to capture Envisat. One of the proposed approaches consists of joining both vehicles by means of a tether, which would actuate during the stabilization and the deorbiting manoeuvres. Given the rotational state of Envisat, stabilizing the target is challenging, and there is a possibility of entanglement, rupture of the tether and impact between chaser and target. As a dynamic system of two spacecraft joined by a tether is complex and in a relatively low state of development, it is necessary to perform experiments in a microgravity environment, such as a parabolic flight or, given the time restrictions of parabolic flights, preferably inside the ISS. Using energetic considerations, it is possible to obtain representative results of such massive objects with experiments in a restricted volume. A series of experiments are proposed to test the dynamic behaviour of two SPHERES vehicles connected by a tether in a controlled environment inside the ISS, aiming to increase knowledge about these kinds of systems and reduce the risk. These experiments analyse the behaviour during the stabilization after capture and after the deorbiting manoeuvres, with the possibility of adding other external perturbations. To test the representativeness of the proposed experiments, a dynamic simulator has been set up, generating results for both the real scenario and the corresponding ISS experiment to compare the dynamic behaviour of the system and test the initial hypotheses. The simulations are performed with several tether lengths, degrees of stiffness and chaser GNC characteristics. This paper first discusses the e.Deorbit mission, including the dynamics of Envisat and the characteristics of the tether-based capture system. This discussion is followed by the method by which the dynamics are scaled to the SPHERES inside ISS. The simulator models and implementation are presented. Finally, simulation results are analysed. Simulation results indicate that it is possible to stabilize the system and perform the de-orbiting with a tether using a rather simple GNC subsystem. Furthermore, both the scaling analysis and the simulation results show that the best results are obtained using a fairly stiff tether.