The Manned Maneuvering Unit (MMU) is an astronaut propulsion unit that was used by NASA on three Space Shuttle missions in 1984. The MMU allowed the astronauts to perform untethered EVA spacewalks at a distance from the shuttle.

The unit featured redundancy to protect against failure of individual systems. It was designed to fit over the life-support system backpack of the Space Shuttle Extravehicular Mobility Unit (EMU). When carried into space, the MMU was stowed in a support station attached to the wall of the payload bay near the airlock hatch. Two MMUs were carried on a mission, with the second unit mounted across from the first on the opposite payload bay wall. The MMU controller arms were folded for storage. When an astronaut backed into the unit and snapped the life-support system into place, the arms were unfolded.

To adapt to astronauts with different arm lengths, controller arms could be adjusted over a range of approximately 13 centimetres. The MMU was small enough to be maneuvered with ease around and within complex structures. With a full propellant load, its mass was 148 kilograms (326 pounds).

Gaseous nitrogen was used as the propellant for the MMU. Two aluminium tanks with Kevlar wrappings contained 5.9 kilograms of nitrogen each, enough propellant for a six-hour EVA depending on the amount of maneuvering done. Typical MMU velocity capability was about 80 feet per second (25 m/s).

There were 24 nozzle thrusters placed at different locations on the MMU. To operate the propulsion system, the astronaut used his fingertips to manipulate hand controllers at the ends of the MMU's two arms. The right controller produced rotational acceleration for roll, pitch, and yaw. The left controller produced translational acceleration for moving forward-back, up-down, and left-right. Coordination of the two controllers produced intricate movements in the unit. Once a desired orientation was achieved, the astronaut could engage an automatic attitude-hold function that maintained the inertial attitude of the unit in flight. This freed both hands for work.

Fans of Golden Age great Jack Williamson recall another example of personal rocket propulsion, the asteroid rocket from his 1931 short story Salvage in Space:

On the other side of this tiny sphere of hard-won treasure, his Millen atomic rocket was sputtering, spurts of hot blue flame jetting from its exhaust. A simple mechanism, bolted to the first sizable fragment he had captured, it drove the iron ball through space like a ship.

Through the magnetic soles of his insulated boots, Thad could feel the vibration of the iron mass, beneath the rocket's regular thrust. The magazine of uranite fuel capsules was nearly empty, now, he reflected. He would soon have to turn back toward Mars...

Thad was alone. Utterly alone. No man was visible, in all the supernal vastness of space. And no work of man--save the few tools of his daring trade, and the glittering little rocket bolted to the black iron behind him.