Steve Robinson was to become the first astronaut in the 24-year shuttle program to spacewalk to Discovery's underside and, once there, the first to try to fix the spacecraft's heat shield during flight.

"See you later," Robinson radioed to his crewmates as he emerged from the spacecraft. "I'm in the great outdoors."

His task on the U.S. space agency's first shuttle mission since the 2003 Columbia disaster was to remove two strips of ceramic-covered cloth sticking out from between the heat-resistant tiles on Discovery's otherwise smooth belly.

The strips are thought to have come loose from their adhesive bond and, though protruding only an inch (2.5 cm), could change the aerodynamics enough during landing on Aug. 8 to cause dangerous heat damage to the shuttle's underside.