Active debris removal using double-tethered space-tug system

The double-tethered space-tug concept is proposed and the dynamics and control of the system are discussed. Investigation of the existence and stability of equilibrium solutions is carried out in the special case where the system is in a circular Earth orbit. Theoretical analyses and numerical simulations indicate that there are two horizontal equilibrium solutions when tether is short: one is an infeasible solution due to penetration of tether in debris body, whereas the other one is feasible. Both of them are locally asymptotically stable, and they become less stable with a longer tether. When the tether length is greater than a critical value, both feasible and infeasible equilibrium solution bifurcates into three solutions: two stable inclined solutions and one unstable horizontal solution. Numerical simulations are then conducted for the general case in which the system is in an elliptic orbit and the debris is spinning. Two feedback control schemes are proposed to stabilize the spinning debris, and their closed-loop performances and impact on the dynamic responses of the system are evaluated. The first control scheme can achieve the control objective of maintaining a constant attitude of debris and a constant distance between the debris and tug, but it cannot suppress the swing of the system. The second control scheme is successful in maintaining a constant attitude of debris and a constant length of one tether, and the amplitude of the swing of the system attenuates with time.