Simplified Aid For EVA Rescue (SAFER) is a small, self-contained, propulsive backpack system (jet pack) worn during spacewalks, to be used in case of emergency only. If an untethered astronaut were to lose physical contact with the vessel, it would provide free-flying mobility to return to it. It is worn on spacewalks outside the International Space Station (ISS), and was worn on spacewalks outside the Space Shuttle. So far, there has not been an emergency in which it was needed.[1][2] SAFER is a small, simplified version of the Manned Maneuvering Unit (MMU), which was used for regular maneuvering.[3]