The first untethered spacewalk was made by American Bruce McCandless II on February 7, 1984, during Challenger mission STS-41-B, utilising the Manned Maneuvering Unit. He was subsequently joined by Robert L. Stewart during the 5 hour 55 minute spacewalk. Such a self-contained spacewalk was first attempted by Eugene Cernan in 1966 on Gemini 9A, but Cernan could not reach the maneuvering unit without tiring.
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| Untethered U.S. astronaut Bruce McCandless uses a manned maneuvering unit. photo taken by Robert "Hoot" Gibson |
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 MMU was used in practice to retrieve a pair of faulty communications satellites, Westar VI and Palapa B2. Following the third mission the unit was retired from use. A smaller successor, the Simplified Aid for EVA Rescue (SAFER), was first flown in 1994, and is intended for emergency use only.
While orbiting around the Earth at a speed of 17,500 miles per hour, McCandless floated from the cargo bay into outer space, 150 nautical miles above Earth, an experience he described as "a heck of a big leap." Mission specialist Robert L. Stewart, an Army lieutenant colonel, also flew the MMU on shuttle mission 41-B.
While flying the MMU, these men were in a journalistic phrase of the time "human satellites." They checked out the equipment, maneuvered within the cargo bay, flew away from and back to the orbiter, performed docking exercises, recharged the MMU nitrogen tanks, and collected engineering data. The MMU, according to Martin Marietta's post mission report, "performed as expected and no anomalies were reported.
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 manoeuvring done. Typical MMU delta-v (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.
Yet the MMU has not been used since 1984. There are several reasons for this. First, most extravehicular activities were effective without use of the MMU. Tethers, safety grips, hand bars, and other restraints allowed astronauts to work in the open cargo bay. Furthermore, the maneuverability of the Space Shuttle itself and the utility of the shuttle's robotic manipulator arm had proved capable of rescuing satellites-the primary function for which the MMU had been designed.
For more information visit:-
http://history.nasa.gov/SP-4219/Chapter13.html
http://en.wikipedia.org/wiki/Manned_Maneuvering_Unit
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