ESA GNC Conference Papers Repository

The Rosetta mission was launched in March 2004 on a 10 years journey towards a rendezvous with the comet Churyumov-Gerasimenko. Once reached the comet nucleus in summer 2014, Rosetta will orbit it for about 1.5 years down to distances of a few kilometers and deliver a lander, named Philae, onto its surface. Rosetta has exited as planned a hibernation mode with almost no active function the 20th January 2014. Before entering hibernation mode, two out of the four reaction wheels on-board were experiencing anomalous behaviour in terms of friction torque. This unforeseen behaviour may question the performances of the current Normal mode, which relies on three reaction wheels to ensure a fine three-axis attitude control. As a risk mitigation action, Airbus Defence and Space (formerly Astrium) has been requested by ESOC to develop a new mode using only two wheels while still ensuring a fine three-axis attitude control compatible with the mission pointing requirements: this new AOCS (Attitude and Orbit Control System) mode, called Hybrid Mode, is to be considered as a back-up mode of the Normal Mode to handle a double reaction wheel failure. Based on positive outcomes of feasibility studies, carried out in close cooperation with ESOC flight operations team, the challenge of the SoftWare development and validation of this new mode by Airbus Defence and Space, 10 years after the Rosetta launch, has been decided with the objective to deliver a SW version beginning of 2014. The main objectives of the Hybrid Mode are to ensure that the spacecraft can perform scientific operations with the required pointing accuracy using only two wheels, while taking into account the SW implementation constraints (necessary trade-off between achievable performances and acceptable complexity). This paper describes this original concept combining both wheels and thrusters as actuators: two spacecraft axes being wheel-controlled and the third spacecraft axis being thruster-controlled at pre-defined times.