A multi-stage drag de-orbit strategy and optimal commands design for a tethered system during large space debris removal

There is a serious challenge to the safe operation of orbiting satellites as the number of space debris on the geosynchronous orbit increases. Consequently, the active removal of space debris has drawn wide attention in recent years. And a tethered system is considered to be a promising method to remove debris in high orbit with its low power consumption and costs. However, the flexible tether of the system can bring the coupling of the orbit motion, the sway motion and the variation of large debris attitude, which means great danger in deorbiting phase and bring a huge challenge in later control. Hence, in this paper, aiming to de-orbit large space debris safely with a tethered system, a multi-stage horizontal drag de-orbit strategy is designed to deal with the coupling caused by a flexible tether. Based on that, optimal commands are planed using Gaussian pseudospectral method to achieve decoupling of the orbit motion, the sway motion and the variation of target attitude. Finally, numerical simulation is established to follow the planned commands by tension and tug thrusts in large space debris removal to verify the effectiveness of the proposed de-orbit strategy.