This paper investigates the input constrained prescribed performance control (PPC) for asteroid hovering merely using position measurements. The controller guarantees the position error converges to a prescribed residual set with desired transient performance in the absence of sensed velocities. First, the position error is transformed into a new error variable and a proper error dynamic system is established according to the PPC methodology, based on which, a backstepping prescribed performance controller is devised with the input saturation handled by an auxiliary system. Then, a state observer is developed by employing the immersion and invariance (I&I) technique, and the estimate is substituted for the velocity in the controller; meanwhile, the auxiliary system is improved to handle the input saturation of the velocity-free controller. The closed-loop systems stability is established via Lyapunov analysis. Finally, simulations are conducted to validate the effectiveness of the proposed scheme.