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1st untethered spacewalk is taken, February 7, 1984

-February 07, 2018

During NASA Space Shuttle mission STS-41-B, astronauts Bruce McCandless II and Robert L. Stewart made the first untethered spacewalk using the manned maneuvering unit (MMU). On February 7, 1984, just four days after launching on Space Shuttle Challenger, the MMU was taken out for its first extravehicular activity (EVA). In practice, it would be used to retrieve faulty satellites.

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 and to adapt to astronauts with different arm lengths (see NASA photo below of McCandless using the MMU).



The MMU used gaseous nitrogen as its propellant based on 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 unit's two arms.

This was not the first attempt at an independent maneuvering unit. In 1966, the US Air Force developed an astronaut maneuvering unit (AMU), a self-contained rocket pack very similar to the MMU. The AMU was to be tested during Project Gemini on a spacewalk by Eugene Cernan in June 1966, but that test was cancelled. Cernan was so tired and overheated that his helmet visor fogged.

After the Space Shuttle Challenger disaster in 1986, the MMU was deemed too risky for further use. NASA determined many of the activities planned for the MMU could be done effectively with manipulator arms or traditional tethered EVAs. NASA also discontinued using the Space Shuttle for commercial satellite contracts, and the military discontinued its use, eliminating the MMU’s main potential use for rescuing wandering or damaged satellites.

Although the MMU was expected to be of help in constructing the International Space Station, NASA developed different tethered spacewalk approaches by that time. A smaller MMU successor, the simplified aid for EVA rescue (SAFER) unit, was first flown in 1994. It is intended for emergency use only.

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For more moments in tech history, see this blog.
EDN strives to be historically accurate with these postings. Should you see an error, please notify us.

Editor's note: This article was originally posted on February 7, 2013 and edited on February 7, 2018.

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