Posted: Tue, Aug 14, 2007, 9:40 AM ET (1340 GMT)

Spacewalkers replaced a faulty gyroscope on the International Space Station during a spacewalk on Monday while shuttle managers continued to study whether on-orbit repairs of a gouge in the shuttle Endeavour's tiles are required. Astronauts Rick Mastracchio and Dave Williams spent nearly six and a half hours outside the ISS on Monday, starting at 11:32 am EDT (1532 GMT), on an EVA to replace a Control Moment Gyroscope (CMG) on the station that failed last year. The two successfully replaced the faulty CMG with one brought up on the shuttle; the old one will be returned to Earth on the shuttle. The EVA was the second of three planned for the STS-118 mission, with the third scheduled for Wednesday. Shuttle managers are still considering whether to add a fourth EVA to repair a tile gouge on the underside of Endeavour. Officials said tests confirmed that the gouge does not pose a safety risk to the shuttle during reentry, although if left unrepaired it is possible heating could cause some minor damage. A final decision on whether to repair the tile is expected by Wednesday.