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February 27, 2009 Vol. 2 Issue 2


Twenty-five years ago, astronauts Bruce McCandless II and Robert L. Stewart made the first untethered spacewalks in history during Space Shuttle mission STS-41B.

McCandless and Stewart used their nitrogen-propelled manned maneuvering units (MMUs) to make the first space walk unattached to any orbital vehicle. Until that time, astronauts were always tethered to the spacecraft.

Orbiting at a distance of 150 nautical miles, the astronauts first checked their MMU’s maneuverability within the cargo bay before flying outside of the shuttle. The entire EVA (extra-vehicular activity) lasted for a total of 5 hours and 55 minutes, during which the astronauts practiced retrieval and repair procedures to be undertaken by the next shuttle mission, STS-41C, for the disabled Solar Maximum spacecraft.

The MMUs weighed 300 pounds when fully charged with nitrogen propellant, and had 24 individual thrusters for maneuvering. The MMU is latched directly on the astronaut’s spacesuit, with all maneuvering done by hand controllers.

Learn more about STS-41B.

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