This article presents impact dynamic analysis of a free-floating space robot, subject to a force impulse at the hand. We study the joint and the base reactions in terms of finite velocity changes and clarify their role for the post-impact motion behavior of the robot. The analysis makes use of a joint-space orthogonal decomposition procedure involving the so called reaction null space. The article focuses on the specific case of a nonredundant arm and a reaction null space in terms of base angular motion. We further show that with proper post-impact control it is possible to transfer the whole angular momentum from the base toward the manipulator, and in the same time to reduce the joint velocity.