The Manned Maneuvering Unit (MMU) is a one-man propulsion backpack, used on Space Shuttle missions, that snaps onto the back of a spacesuit's portable life-support system. The MMU allows an astronaut to work outside without a tether up to 100 meters away from the Orbiter and is designed to provide EVA support for as much as six hours at a stretch. The MMU weighs 140 kilograms and is propelled by nitrogen gas fed to 24 thruster jets. Two pressurized nitrogen tanks can be filled from the Orbiter's onboard supply. All systems on the MMU are dual redundant – if one system fails, a second can completely take over. The MMU was first tested in space by Bruce McCandless in 1994.