ESA GNC Conference Papers Repository

The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is an operational European space observatory mission. It was launched in 2002, and has exceeded its original mission duration by a long margin, and has had several mission extensions approved. In recent years several events have created interesting challenges for attitude control. In 2020 the fuel unexpectedly depleted earlier than predicted, making normal thruster-based angular momentum off-loading impossible. It was possible to devise a workaround for this, managing momentum using solar torque and "Z-flip" slews. The slew algorithm was redesigned by ESOC and industry to remove a previous constraint that can require momentum off-loading. In Sep. 2021 a major anomaly put the spacecraft into an uncontrolled tumbling state for several hours. This paper describes the successful improvised recovery of attitude control using reaction wheels, and the subsequent development of procedures and a completely new AOCS mode, a wheel-based New Safe Mode (NSM) to prevent a recurrence of loss of control. With NSM, the mission will have the ability to be safely operated without thrusters and with redundancy for any further unit failures until predicted re-entry in 2029.