As the construction continued on the International Space Station (ISS), STS-118 astronaut and mission specialist Rick Mastracchio participated in the second session of Extra Vehicular Activity (EVA) for the mission. Assisting Mastracchio was Canadian Space Agency representative Dave Williams (out of frame). During the 6 hour, 28 minute space walk, the two removed a faulty control moment gyroscope (CMG-3) and installed a new CMG into the Z1 truss. The failed CMG will remain in its temporary stowage location on the exterior of the station 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 orbital attitude of the station.