A new direct adaptive failure compensation approach is developed for satellite attitude control systems in the presence of uncertain failures of redundant actuators. The adaptive failure compensation controller is designed via a backstepping design, which can accommodate uncertainties in actuator failure time instants, values, and patterns. The failure uncertainties are estimated directly by adaptive laws and the adaptive satellite attitude control system with actuator failures is analyzed, to show its desired stability and asymptotic tracking properties. Finally, simulation results of a satellite attitude control system with redundant reaction wheels are presented to demonstrate the effectiveness of the proposed adaptive failure compensation scheme.