Evolving Systems Approach to the Attitude Control of a Space-Debris Removal Spacecraft

Abstract

The unexpected loss of contact with Envisat, the European Space Agency's largest environmental spacecraft, has added one more item to the growing list of debris in space.ESA plans to retrieve Envisat by attaching a chaser spacecraft to it with the help of a robot arm and tentacles.This paper investigates the stability and controllability of the system before, during, and after docking operations.A new approach, called Evolving Systems, is applied to the problem by modelling the connection between Envisat and the chaser as an increasingly stiff (rotational) spring-damper system.The stability of the system is assessed by a linear stability analysis and a nonlinear system analysis.For the nonlinear analysis, a simulator is used consisting of the two spacecraft, the connection model, a Linear Quadratic Regulator, a reaction-control system, and a control allocator.The linear stability analysis showed that the system remains stable during its evolution; however, if the debris would have the same size as the chaser, instability can occur.Further research could focus on docking with smaller space debris.The nonlinear analysis showed that a simple Linear Quadratic Regulator can stabilise the system before, during, and after docking, even if the initial conditions of the target are uncertain.Moreover, the docking phase has little effect on the motion of Envisat and the stability of the system, because of the large difference in size between the chaser and Envisat.