The Manned Maneuvering Unit (MMU) was a single-person propulsion system used for Extravehicular Activities (EVA). The MMU had 24 directional nozzles and was powered by pressurized nitrogen. That provided propellant for an EVA for up to 6 hours. The system, while successful in achieving untethered flight for the first time, was retired after only three shuttle flights as the risk of an emergency was deemed too high.