Manned Maneuvering Unit
The Manned Maneuvering Unit (MMU) is an astronaut propulsion unit
Astronaut Propulsion Unit
An Astronaut Propulsion Unit is used to move a suited astronaut in the vacuum of space. The first astronaut propulsion unit was the HHMU used on Gemini 4.-Hand-Held Maneuvering Unit:...

 which was used by NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 on three space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 missions in 1984. The MMU allowed the astronauts to perform untethered EVA
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...

 spacewalks at a distance from the shuttle. The MMU was used in practice to retrieve a pair of faulty communications satellites, Westar
Westar was the name for the fleet of geosynchronous communications satellites operating in the C band which were launched by Western Union from 1974 to 1984. There were seven Westar satellites in all, with five of them launched and operating under the Westar name.In many international recognized...

 VI and Palapa B2. Following the third mission the unit was retired from use. A smaller successor, the Simplified Aid for EVA Rescue
Simplified Aid for EVA Rescue
Simplified Aid for EVA Rescue is a small, self-contained, propulsive backpack system used to provide free-flying mobility for a Space Shuttle or International Space Station crewmember during extra-vehicular activity...

 (SAFER), was first flown in 1994, and is intended for emergency use only.


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
Extravehicular Mobility Unit
The Space Shuttle/International Space Station Extravehicular Mobility Unit is an independent anthropomorphic system that provides environmental protection, mobility, life support, and communications for a Space Shuttle or International Space Station crew member to perform extra-vehicular activity...

 (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
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 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 delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

 (velocity change) capability was about 80 feet per second (24.4 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.


In 1966, the US Air Force developed an Astronaut Maneuvering Unit (AMU), a self-contained rocket pack very similar to the MMU. This was planned to be tested during Project Gemini
Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....

 on an EVA by Eugene Cernan from Gemini 9A
Gemini 9A
- Backup crew :- Original primary crew :- Mission parameters :* Mass: * Perigee: * Apogee: * Inclination: 28.91°* Period: 88.78 min- 1st rendezvous :* June 3, 1966 - 17:45 - 18:00 UTC- Spacewalk :* Cernan...

 on June 5, 1966. But the test had to be cancelled because Cernan tired and overheated, sweated so profusely that his helmet visor fogged, before he could get to the AMU mounted on the back of the spacecraft. Astronauts did not learn how to work during EVA without tiring until the final Gemini 12
Gemini 12
-Backup crew:-Mission parameters:*Mass: *Perigee: *Apogee: *Inclination: 28.87°*Period: 88.87 min-Docking:*Docked: November 12, 1966 - 01:06:00 UTC*Undocked: November 13, 1966 - 20:18:00 UTC-Space walk:...

 mission, but no AMU was carried on that flight. Since there was no real need for self-contained astronaut EVA flight in the Apollo and Skylab
Skylab was a space station launched and operated by NASA, the space agency of the United States. Skylab orbited the Earth from 1973 to 1979, and included a workshop, a solar observatory, and other systems. It was launched unmanned by a modified Saturn V rocket, with a mass of...

 programs, the idea had to wait for the advent of the Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 program, though several maneuvering device designs were tested inside Skylab.

The MMU was used on three Shuttle missions in 1984. It was first tested on February 7 during mission STS-41-B
STS-41-B was the tenth NASA Space Shuttle mission, launching on 3 February 1984 and landing on 11 February. It was the fourth flight of the Space Shuttle Challenger. Following STS-9, the flight numbering system for the Space Shuttle program was changed...

 by astronauts Bruce McCandless II
Bruce McCandless II
Bruce McCandless II is a former naval aviator with the United States Navy and former NASA astronaut. During the first of his two Space Shuttle missions he made the first ever untethered free flight, using the Manned Maneuvering Unit.-Education:McCandless is the son of Bruce McCandless, a decorated...

 and Robert L. Stewart
Robert L. Stewart
Robert Lee Stewart is a retired Brigadier General of the United States Army and former NASA astronaut.-Personal:Stewart was born August 13, 1942, in Washington, D.C.. He graduated from Hattiesburg High School, Hattiesburg, Mississippi, in 1960...

. Two months later during mission STS-41-C
STS-41-C was NASA's 11th Space Shuttle mission, and the fifth mission of Space Shuttle Challenger. The launch on 6 April 1984 was the first direct ascent trajectory for a shuttle mission...

, astronauts James van Hoften
James van Hoften
James Dougal Adrianus "Ox" van Hoften is a former NASA Astronaut.-Personal data:Van Hoften was born June 11, 1944, in Fresno, California. He was active in the Boy Scouts of America where he achieved its second highest rank, Life Scout. He considers Burlingame, California, to be his hometown. ...

 and George Nelson
George Nelson (astronaut)
George Driver "Pinky" Nelson is a former NASA astronaut.Nelson was born in Charles City, Iowa, but considers Willmar, Minnesota, to be his hometown. His wife Susie is from Alhambra, California. They have two daughters, Aimee and Marti....

 attempted to use the MMU to capture the Solar Maximum Mission
Solar Maximum Mission
The Solar Maximum Mission satellite was designed to investigate solar phenomenon, particularly solar flares. It was launched on February 14, 1980....

 satellite and to bring it into the orbiter's payload bay for repairs and servicing. The plan was to use an astronaut-piloted MMU to grapple the SMM with the Trunion Pin Attachment Device (TPAD) mounted between the hand controllers of the MMU, null its rotation rates, and allow the Shuttle to bring it into the Shuttle's payload bay for stowage. Three attempts to grapple the satellite using the TPAD failed. The TPAD jaws could not lock onto Solar Max because of an obstructing grommet on the satellite not included in the blueprints for the satellite. This led to an improvised plan which nearly ended the satellite's mission. The improvisation had the MMU astronaut use his hands to grab hold of an SMM solar array and null the rates by a push from MMU's thrusters. Instead, this attempt induced higher rates and in multiple axes; the satellite was tumbling out of control and quickly losing battery life. SMM Operations Control Center engineers shut down all non-essential SMM subsystems and with a bit of luck were able to recover the SMM minutes before total failure. The ground support engineers then stabilized the satellite and nulled its rotation rates for capture with the orbiter's robotic arm, the Shuttle Remote Manipulator System (SRMS
The Shuttle Remote Manipulator System , or Canadarm , was a mechanical arm used on the Space Shuttle to maneuver a payload from the payload bay of the orbiter to its deployment position and then release it. It can also grapple a free-flying payload, maneuver it to the payload bay of the orbiter...

). This proved to be a much better plan. Their successful work increased the lifespan of the satellite.

The final MMU mission was STS-51-A
STS-51-A was the second flight of Space Shuttle Discovery, and the 14th flight of NASA's Space Shuttle program. The mission launched from Kennedy Space Center on 8 November 1984, and landed just under eight days later on 16 November....

, which flew in November 1984. The propulsion unit was used to retrieve two communication satellites, Westar
Westar was the name for the fleet of geosynchronous communications satellites operating in the C band which were launched by Western Union from 1974 to 1984. There were seven Westar satellites in all, with five of them launched and operating under the Westar name.In many international recognized...

 VI and Palapa B2, that did not reach their proper orbits because of faulty propulsion modules. Astronauts Joseph P. Allen and Dale Gardner
Dale Gardner
Dale Allan Gardner is a former NASA astronaut who flew two missions for NASA in the early 1980s.-NASA experience:...

 captured the two satellites and brought them into the Orbiter payload bay for stowage and return to Earth.

After the Space Shuttle Challenger disaster
Space Shuttle Challenger disaster
The Space Shuttle Challenger disaster occurred on January 28, 1986, when Space Shuttle Challenger broke apart 73 seconds into its flight, leading to the deaths of its seven crew members. The spacecraft disintegrated over the Atlantic Ocean, off the coast of central Florida at 11:38 am EST...

, the MMU was judged too risky for further use. NASA also discontinued using the Shuttle for commercial satellite contracts, and the military discontinued the use of the Shuttle, eliminating the main potential uses. Although the MMU was envisioned as a natural aid for constructing the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...

, with its retirement, NASA developed different tethered spacewalk approaches. Additionally, the MMU gas ejecta was considered too damaging for the only other ongoing task, upkeep of the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...


NASA transferred the flight article #3 to the National Air and Space Museum in 1998. The remaining MMU are stored at NASA (location not known).

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.