Despite a few bumps along the way, the Atlantis astronauts and their space station colleagues successfully attached a 27,000-pound truss to the orbiting outpost today, accomplishing the primary goal of the 109th shuttle mission.

Atlantis astronaut Ellen Ochoa and station engineer Daniel Bursch used the lab's Canadian-built robot arm to mount the S0 truss atop the Destiny laboratory module, clearing the way for two spacewalkers - Steven Smith and Rex Walheim - to lock it down and wire it up during a seven-hour 48-minute spacewalk.

As the spacewalk was concluding, engineers at the Johnson Space Center in Houston began directing power to the truss, activating computers, heaters and other equipment needed to bring the massive component to life. To everyone's delight, the systems powered up normally, without any significant glitches.

"The truss on orbit right now is in an excellent condition, an excellent posture to be ready to go into the remainder of this flight," said Ben Sellari, the STS-110 launch package manager. "I couldn't be happier. We accomplished all of our major objectives on EVA-1.

"For almost a year now we've been advertising to management and here recently to the public that we knew this was going to be a difficult flight, we knew EVA-1 was going to be a difficult hill to climb," he added. "To say tonight that we accomplished all of our major objectives on that EVA is something to be very proud of. It wasn't an easy task at all, we knew there would be bumps along the way. But it was excellent work by everybody and a real pleasure to see it operate."

Smith and Walheim deployed two massive bipods on the front face of the S0 truss and bolted them to plates in Destiny's hull, providing enough structural rigidity to hold the truss solidly in place during shuttle dockings and undockings. Two large tripods assemblies will be deployed from S0's aft face during a spacewalk Saturday to complete the beam's attachment.

S0 is the backbone of a truss that eventually will span some 356 feet, anchoring huge solar arrays on each end and ammonia radiators closer in. The bipods and tripods on S0 provide the structural support necessary to anchor the completed truss and keep it rigidly attached during station reorientation maneuvers and the comings and goings of space shuttles and Russian spacecraft.

"The S0 is structurally sound," Sellari said. "Our activations are underway. All of the S0 components on the truss itself are going through the final throes of activation, the mobile transporter will be next. So shortly, we'll be able to turn off our LTA clocks, or launch-to-activation clocks, on the S0 truss element and we're just in great shape for the remainder of the mission.

Dina Barclay, lead spacewalk officer for mission STS-110, was equally thrilled at the success of today's excursion by Smith and Walheim.

"A lot of people were watching today as Ellen installed the truss and then as Steve and Rex began its assembly," she said. "We're pleased with the crew's performance, and that's to put it mildly. We have a lot of smiling engineers in mission control right now."

The astronauts will take a bit of a break Friday, transferring oxygen and nitrogen from the shuttle to tanks on the Quest airlock module, transferring equipment and supplies to the station and participating in round-robin media interviews.

On Saturday, Lee Morin and Jerry Ross plan to stage the mission's second spacewalk to install the aft-facing tripod assemblies needed to complete S0's structural attachment. A third spacewalk, this one by Smith and Walheim, is on tap Sunday to rewire the Canadarm2 spacecrane for its eventual mounting on a mobile transporter that will run the length of the completed truss.

After tests of the mobile transporter Monday, Ross and Morin plan to stage a fourth and final spacewalk Tuesday to perform a variety of "get-ahead" tasks for upcoming assembly missions.