Stock Photo - Astronaut Robert L. Behnken, STS-123 mission specialist, participates in the mission's fourth scheduled session of extravehicular activity (EVA) as construction and maintenance continue on the International Space Station. During the 6-hour, 24-minute spacewalk, Behnken and astronaut Mike Foreman (out of frame), mission specialist, replaced a failed Remote Power Control Module -- essentially a circuit breaker -- on the station's truss. The spacewalkers also tested a repair method for damaged heat resistant tiles on the space shuttle. This technique used a caulk-gun-like tool named the Tile Repair Ablator Dispenser to dispense a material called Shuttle Tile Ablator-54 into purposely damaged heat shield tiles. The sample tiles will be returned to Earth to undergo extensive testing on the ground.