CAPE CANAVERAL, Fla. -- The International Space Station grew 13 tons heavier Thursday with the addition of a $600 million truss segment, but it took some old fashioned muscle power by a pair of spacewalkers to prevent the possibilty of the hardware freezing beyond repair.

STS-110

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Loaded with computers, electronic black boxes and other sensitive gear, the Boeing-built structure had to be hooked up to a source of heat-generating power within 16 hours of it being lifted clear of Atlantis' cargo bay by the frontier outpost's robot arm -- although officials said later the clock could stretch to 56 hours.

Two trays of power cables and other plumbing packed on the side of the S-Zero truss had to be unbolted and pulled free of the beam so spacewalkers Steve Smith and Rex Walheim could connect the lines to the proper sockets on the side of the U.S. Destiny science laboratory.

Those lines, and many others yet to be connected, eventually will feed electricity and coolant to the rest of the station from giant power-generating solar arrays and heat-dispelling radiators to be added to a fully assembled truss -- a rigid beam that will span the length of a football field.

After first bolting two structural support legs from S-Zero to the science module, Smith and Walheim each claimed an umbilical tray to unfasten and pull free. Walheim's came loose as expected, but Smith's wouldn't budge despite several hefty tugs.

As the sense of anticipation -- even drama -- grew in space and on the ground, flight controllers quickly rearranged the spacewalk and eventually ordered Walheim to abandon his pre-planned timeline to assist Smith in releasing the stuck tray.

The first two attempts resulted in a little but nevertheless promising movement. Then Smith counted out for the third try: "OK ready, hold on. One, two, three," Smith said. What sounded next like a grunt was heard on NASA TV, followed by a rapid series of euphoric hoots and howls.

"Got it!" Walheim called out a split-second before Smith exclaimed the same words.

"Hot dog," Walheim added to underscore the sense of relief.

"All right guys, proud of you. Stanford and Cal, working together," mission specialist Jerry Ross said, recalling the rivalry between the two spacewalkers' colleges: Stanford and the University of California at Berkeley.

"Houston copies the success and we're cheering loud down here," added astronaut Barbara Morgan from Mission Control.

The extra effort required to free the stuck tray delayed the overall spacewalk timeline, prompting managers to move around some tasks scheduled for the three remaining spacewalks planned during Atlantis' week-long stay at the station.

Three more sorties outside, all to be staged from the outpost's Quest airlock, are still on the books. Ross and Lee Morin will be the next two spacewalkers on Saturday, followed by Smith and Walheim again on Sunday and then Ross and Morin will take a final turn next Tuesday.

Thursday's spacewalk, which lasted seven hours and 48 minutes, was Walheim's first, and his more experienced colleagues made sure he stole a moment or two to enjoy the view.

"Look at that sunrise," Walheim said at one point as the orbital complex moved out of Earth's shadow. "The moon's just coming over the horizon at the same time as the sunrise. Spectacular."

"Enjoy these opportunities Rex, and take some mental pictures," said Ross, NASA's most experienced spacewalker.

"It's amazing," Walheim replied. "Just amazing."

The long day began with mission specialist Ellen Ochoa using the station's robot arm to maneuver the S-Zero truss into place over the Destiny science module as Expedition Four flight engineer Dan Bursch looked over her shoulder to help.

A metal claw on cradle atop the module then grabbed hold of the truss to firmly hold it against the station. The claw will be released after another set of struts are bolted in place during Saturday's spacewalk.

Once free of the truss, the Canadarm2 was used as a cherry picker work platform for Walheim, whose feet were locked into a brace attached to the end of the arm. A long-standing mode of operation with the shuttle's robot arm, this was the first time the station's arm was used in this manner.

Despite a hardware failure on the arm that required reliance on a hastily written but thoroughly proven new set of software, the Canadarm2 set a new record for the amount of use during a single day.

"Dan and I just wanted to give a big thanks to the whole robotics team... for giving us some great procedures and great training," Ochoa said after the truss was released from the end of the arm.

"Ellen, Houston, we copy all. The entire team, we concur, did a great job getting S-Zero installed. And hats off to you and Dan on some nice flying," was the reply from Mission Control.