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September 20, 2011
Canadian Space Agency's astronaut Dave Williams (left) and astronaut Rick Mastracchio (center), both STS-118 mission specialists, participate in the mission's second planned session of extravehicular activity (EVA) as construction and maintenance continue on the International Space Station. Williams is anchored to the foot restraint on the stations' Canadarm2. During the 6-hour, 28-minute spacewalk, Williams and Mastracchio removed a faulty control moment gyroscope (CMG-3) and installed a new CMG into the station's Z1 truss. The failed CMG will remain at its temporary stowage location on the station's exterior until it is returned to Earth on a later shuttle mission. The new gyroscope is one of four CMGs that are used to control the station's attitude in orbit.

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