Free-flying robots have been recently developed to operate on-board the International Space Station (ISS) as semi-autonomous robotic assistants. Free-flyer robots can be designed as modular base for integration of a wide range of hardware and software as monitoring and maintenance tools of ISS systems. Thus, the free-flyers are the ideal platform for manual observation of the ISS by ground control, autonomous sensor readings and surveying of ISS conditions, and human-robot interaction during long duration human missions. The NASA Astrobee project aims to develop a highly capable robot that can operate for long periods of time without crew supervision or operation. The goal of this research is to realize a model and a robust attitude controlsystem for the NASA Astrobee, which is a free-flyer equipped with a 3 degree of freedom (DoF) manipulator.