Astronauts Jerry Ross and Lee Morin began repressurizing the space station's Quest airlock module at 5:39 p.m., officially ending a seven-hour 30-minute excursion, the second of four planned for the shuttle Atlantis' mission. The third spacewalk is on tap Sunday morning and the fourth on Tuesday.

During today's outing, Ross and Morin completed the attachment of the S0 truss by bolting two tripod assemblies in place. They also unreeled a redundant ribbon cable needed to power a mobile transporter mounted on rails on the front side of the truss. The only problem of any significance today involved a stuck bolt in a cable cutter in the reel of the transporter's redundant trailing umbilical system cable, of TUS. Engineers said the cutter could not fire on its own anyway, and that the bolt will be dealt with later in the mission.

This was the 36th spacewalk since December 1998 devoted to space station assembly. As of today, 31 NASA astronauts, one Canadian and five Russians have logged 223 hours and 23 minutes of spacewalk time building the international outpost. Eleven of those 36 spacewalks have been staged from the space station with the rest from the airlocks of visiting shuttles. This was the fourth spacewalk that utilized the Quest airlock module.

Ross, NASA's most experienced astronaut with a record seven flights to his credit, extended his spacewalk record to 51 hours and 41 minutes in eight EVAs. Steven Smith, who will venture back outside Sunday, is in second place with 43 hours and 21 minutes in six spacewalks.

Engineers monitoring today's spacewalk from mission control at the Johnson Space Center say the 27,000-pound S0 truss carried aloft by shuttle Atlantis is now permanently attached to the international space station and able to withstand any expected structural loads. Even when the truss is completed, stretching 356 feet and carrying huge solar arrays and radiators.

"It became truly a skyscraper in space," senior flight director Wayne Hale said of the station. "It was a very significant day for us as we completed the structural mating of the centerpiece of the truss for the station and set the stage for building and future expansion of the station to a complex that will extend beyond the length of a football field.

"The S0 truss is now structurally in place and it can withstand the coming and going of all visiting vehicles and reboost of the station and it's poised to accept new truss elements and new solar array power generation elements, both this year and in the coming year. So we've marked a milestone and today everything has performed flawlessly."

The $600 million S0 truss is the first of nine truss sections that ultimately will be bolted together to form a 356-foot-long beam. Huge solar arrays and radiators will be mounted on the truss to provide power and cooling for the space station.

The entire truss attaches to the station through the S0 truss, which is mounted atop the Destiny laboratory module. During a spacewalk Thursday, Steven Smith and Rex Walheim bolted down a pair of bipod struts connecting the front face of S0 to the lab module. Today, Jerry Ross and Lee Morin completed the job, bolting down two sets of struts on the back side of the truss. They also installed a second ribbon cable to provide power to a compact $190 million flatcar that ultimately will carry the station's robot arm to various work sites on the truss.

"We have completed all the module-to-truss strut attachments," said launch package manager Ben Sellari. "We are four strut groups out of four, all bolts engaged. What that means in the short term for us is that for this flight we are go for orbiter reboost, in either vernier or PRCS control, and after the shuttle leaves we'll be able to support regular station reboosts.

"It also eliminates any further concern about EVA loads while working on S0 and it opens the door for us to do the mobile transporter tests in a few days. For the long term, S0 is now a permanent fixture aboard the international space station and we couldn't be happier. It meets all the structural requirements necessary to continue the phase three of assembly so I guess in a short word, S0 is open for business."

During a spacewalk tomorrow, Smith and Walheim will remove launch locks holding the mobile transporter in place so it can be tested Monday. They also will reconfigure wiring to the station's robot arm so it can eventually operate from the top of the mobile transporter.