The Manned Maneuvering Unit was a backpack propulsion device that facilitated astronauts' mobility for extravehicular activities. It enabled them to maneuver inside the shuttle's payload bay or fly a distance away without requiring safety tethers anchored to the vehicle. The MMU had twenty-four small gaseous nitrogen thrusters operated by controllers on the arms of the unit. When not being used, the MMU arms would fold down to be flush with the side of the unit. A lock, such as this one, prevented the arms from collapsing while in flight.