Robert L. Behnken and Mike Foreman completed their spacewalk at 04:28 GMT, successfully replacing a failed Remote Power Control Module - basically a circuit breaker - on the station's truss. Although the operation was a success, there were minor difficulties in removing a power connector. The two astronauts spent six hours and 24 minutes in open space.
The primary task of the spacewalk was the test of a heat shield repair method. The astronauts experimented with the use of a caulk-gun-like tool called a Tile Repair Ablator Dispenser to dispense a high-tech putty material into heat shield tiles that had been deliberately damaged beforehand. The sample tiles will later be tested after the shuttle returns to Earth, NASA said.
However, even if it had been available then, the experimental repair method would not have saved the Colombia space shuttle, which broke up over Texas on February 1, 2003 with the loss of all seven crew members, as the hole in its wing was too large.
The current shuttle mission - STS-123 - is the longest to the ISS and involves a record five spacewalks.
Behnken and Foreman will begin the mission's fifth and final spacewalk Saturday at 21:23 GMT. Among other tasks, they will move the shuttle's inspection boom over to the space station for use by the crew of the Discovery, the next space shuttle scheduled to visit the ISS.
The shuttle crew comprises Commander Dominic Gorie, Pilot Gregory H. Johnson and Mission Specialists Rick Linnehan, Robert L. Behnken, Mike Foreman, Garrett Reisman and Japanese astronaut Takao Doi.
