This paper addresses an impedance control for a free-floating space robot in the grasping of a tumbling target with model uncertainty. Firstly, the operational space dynamics for a free-floating robot is derived with a novel, computationally efficient formulation. Then, considering the grasped target as a disturbance force on the end-effector, the proposed control method is completely independent of the target inertial parameters and the end-effector can follow a given trajectory in the presence of model uncertainty. To verify the effectiveness of the proposed method, a three-dimensional realistic numerical simulation is carried out