Explanation: At about 100 meters from the cargo bay of the space
shuttle Challenger, Bruce McCandless II was farther out than
anyone had ever been before. Guided by a Manned Maneuvering Unit
(MMU), astronaut McCandless, pictured above, was floating free
in space. McCandless and fellow NASA astronaut Robert Stewart
were the first to experience such an "untethered space walk"
during Space Shuttle mission 41-B in 1984. The MMU works by
shooting jets of nitrogen and has since been used to help deploy
and retrieve satellites. With a mass over 140 kilograms, an MMU
is heavy on Earth, but, like everything, is weightless when
drifting in orbit. The MMU was replaced with the SAFER backpack
propulsion unit. And I thought Sears Tower glass balcony was
scary!