David Wolf and Piers Sellers shouted, "Yippee!" and "Excellent!" when it was time to get to work on the 45-foot-long, 14-ton girder that arrived with them aboard space shuttle Atlantis on Wednesday.
Wolf connected power and data cables, as Sellers released the locks on the three folded-up radiators mounted to the girder.
Sellers, a first-time spacewalker, marveled at the view more than 240 miles below.
"Where am I?" Sellers asked. When told he was flying over the Pacific and coming up on South America, he observed, "Wow - it's too beautiful for words - unbelievable." After a two-second pause, he said, "That's it, back to work."
Station astronaut Peggy Whitson and shuttle astronaut Sandra Magnus began the 240-mile-high construction work shortly after waking up. They used the station's robot arm to remove the 45-foot-long, 15-foot wide girder from Atlantis' payload bay.
Three hours later, the women hoisted the structure into place on the station. A mechanical claw secured the girder to a similar frame that was launched earlier this year.
That was the cue for the spacewalk to begin.
Sellers and Wolf ran into minor problems deploying and then locking down an S-band antenna on the S1 solar array truss, reports CBS News Space Correspondent William Harwood. As a result, the spacewalk will last a bit longer than planned.
Wolf encountered some stiff bolts; one broke into five pieces, and he managed to catch them before they floated off.
Sellers, meanwhile, worked on his own to unlatch launch locks that held a set of folding ammonia radiator panels firmly in place along the top of S1. Eighteen radiator beam launch locks must be unlatched, most during today's spacewalk, before the station crew can send commands later in the mission to deploy one of the three folding radiator wings mounted on the trailing face of S1.
It now appears they will forego using the station's robot arm to assist them installing equipment for an external camera group, says Harwood. The astronauts will simply free-float while doing the procedure. A final set of electrical and data cables also must be connected betwen S1 and the S0 truss it's attached to.
This is the 44th spacewalk devoted to space station assembly and maintenance, the 19th staged from the station itself and the 10th to use the U.S. Quest airlock module. Going into today's excursion, 32 U.S. astronauts, one Canadian, one Frenchman and seven Russian cosmonauts had logged 265 hours and 44 minutes building the international outpost.
When completed, the station's 11-segment solar array truss will stretch longer than a football field and carry two huge sets of solar panels on each end.
Atlantis arrived with the girder on Wednesday, along with a fresh supply of salsa.
After four months of bland, canned food, Whitson, the space station's lone American, had requested some spice.
"We've got your salsa," Atlantis' skipper, Jeffrey Ashby, radioed as the shuttle drew near.
"OK, we'll let you in then," Whitson replied.
The docking took place more than 240 miles above central Asia. Mission Control congratulated Ashby and his crew on "an impressive, flawless rendezvous and the smoothest docking sequence we've seen in quite a few flights."
Less than two hours later, the hatches between the two spacecraft swung open and Whitson and her two Russian crewmates embraced their first guests. Cheers, shouts and laughter filled the orbiting complex. "You guys look great!" one of Atlantis' crewmen said.
Whitson shared an especially long hug with Magnus, a fellow classmate and good friend who lugged a big brown bag, presumably filled with goodies.
Besides salsa, the six shuttle astronauts brought onions, garlic, fresh fruit and a pecan pie.
"We hope you're enjoying the company," Mission Control radioed.
The $390 million girder delivered by Atlantis is equipped with three radiators and 15 miles of wiring. It also has more than one-third mile of fiber optic cable and 426 feet of stainless steel tubes for ammonia, which will serve as a coolant, and nitrogen, which will maintain the ammonia pressure.
The cooling system will not be activated until next year.
Another girder with another set of radiators will be delivered next month by space shuttle Endeavour, which will also be the ride home for Whitson and cosmonauts Valery Korzun and Sergei Treschev. The three moved into the space station in June.
Atlantis is set to return to Earth Oct. 18.
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.