In the framework of twistors, a saturated adaptive six-degree-of-freedom (6-DOF) control law for asteroid landing, which avoids the collision between the spacecraft and asteroid, is proposed with both the inertial parameters of the spacecraft and the bounds of the uncertainties unknown. First, the 6-DOF dynamics of the spacecraft relative to the desired coordinate frame is represented in a unified way by the application of twistors, and the collision avoidance constraint is established via separating the spacecraft and asteroid with an elaborately designed safety surface. Then, a saturated controller for the landing is proposed with the collision avoidance constraint considered by combining the backstepping method and artificial potential field technique. Further, the inertial parameters and the upper bounds of the magnitude of the uncertainties in the controller are substituted with their estimates obtained from the designed adaptive laws. Finally, the previous design is adjusted slightly to complete the adaptive control scheme. The stability of the closed-loop system is proven via Lyapunov theory, and numerical simulations are presented to demonstrate the effectiveness of the control scheme. The proposed adaptive control law can fully consider the orbit-attitude coupling and effectively enforce the collision avoidance constraint in the presence of unknowns in asteroid landing missions.