U.S. astronauts have mounted a giant girder to the back of the international space station, part of the continuing expansion of the orbiting research outpost. The segment will form the base to support future power and cooling equipment.

With the space shuttle Atlantis docked at the station, shuttle and station astronauts hoisted the 13-ton girder from Atlantis with the station's crane and moved it to the outpost's U.S. laboratory.

The 13-meter long truss is the first of nine segments that will eventually span 100-meters, the longest structure in space. It will support massive solar energy panels and cooling radiators for European and Japanese research laboratories that will be mounted in later years.

After the girder's placement, shuttle crewmen Rex Walheim and Steve Smith ventured outside the station to unfasten temporary latches and begin the process of bolting the section firmly into place and making some of its power and cooling connections.

The final twist on the last of eight bolts they tightened elicited their cries of joy and approval from U.S. mission control in Houston.

Walheim: Got it! Smith: Got it! Walheim: Hot dog! Smith: Good job, Rex! Walheim: Thanks! Mission control: Houston copies the success, and we're cheering loud[ly] down here.

But the outing soon ran into trouble when the two crewmen struggled to release a tray containing power and fluid lines. This delayed the spacewalk more than an hour, forcing astronaut Smith to return to the station to replenish his oxygen supply. Mission directors also postponed some electrical tasks to a later spacewalk.

Nevertheless, the mission official overseeing the launch of the truss, Ben Sellari, says the spacewalk was a success.

"The truss in orbit right now is in an excellent posture to be ready to go into the remainder of this flight. To say we accomplished all of our major objectives is something to be proud of. It wasn't an easy task at all. We knew there would be bumps along the way," he said. "We're just in great shape for the remainder of the mission."

The truss segment positioned Thursday has 475,000 parts, making it the second most complicated piece of hardware on the station after the U.S. laboratory. Three more spacewalks are required during this shuttle visit to finish bolting it to the station and running its power and coolant lines.

The U.S. space agency will send up two more truss segments later this year.