The solar array deploys in this image from the helmet camera of spacewalker Joe Tanner. Photo: NASA TV/Spaceflight Now

Two spacewalking astronauts bolted a $600 million solar power tower to the international space station Sunday and one of two huge solar wings was successfully unfurled after several false starts. But deployment of a second wing was put on hold after engineers determined the two solar cell blankets making up the first wing were not as taut as expected after the wing was fully extended.

While the 115-foot-long array was generating power and otherwise performing normally, flight directors wanted more time to study the unexpected slack in the array blankets before unfurling the second wing.

"We want to make sure we're in a good posture before we deploy the second wing," said lead flight director William Reeves. "The systems are very safe at this point. Even if the shuttle had to leave early, the system is self sustaining. So there are no risks to the equipment at this time. The array that is out is producing power and they're in the process of charging the batteries."

In the meantime, he said, "we're going to do some analysis on the wing that's out and when we're convinced we're ready, we'll deploy the second wing."

Slack tensioning cables may be an indication that all is not well with the starboard solar array. Photo: NASA TV/Spaceflight Now

The Boeing-built P6 solar array is the most powerful solar electric power system ever launched, capable of delivering some 20 kilowatts when both of its 115-foot-long wings are fully deployed. At least one of the wings must be deployed and operational before NASA can launch the U.S. laboratory module, Destiny, in late January.

Astronauts Joseph Tanner and Carlos Noriega began a planned six-and-a-half-hour spacewalk Sunday at 1:35 p.m. to attach the P6 array to the station and to deploy the appendages making up its two solar wings.

"Hallelujah, it is good to be back!" Tanner, a spacewalk veteran, exclaimed as he floated into Endeavour's cargo bay.

The flight plan called for the base of the array's central equipment module to be bolted to the top of a structural truss already attached to the Unity module's upward-facing, or zenith, port.

Canadian astronaut Marc Garneau, operating the shuttle's 50-foot-long robot arm with surgical precision, had no problems positioning the 17.5-ton P6 array within inches of a mechanical claw-like hook on top of the Z1 truss. Noriega then engaged the latch mechanism to pull the P6 array down onto the truss and the spacewalkers than tightened four large bolts to firmly lock the two together.

The solar array joints are extended from the top of the P6 truss. Photo: NASA TV/Spaceflight Now

Noriega then hooked up power lines from the shuttle to provide electricity to operate heaters and other systems in the event of downstream problems. The spacewalkers then made their way up the P6 tower to extend each solar wing's attachment joint into the proper position for wing deployment.

Both joints were attached to the top of the P6 tower by four pivoting bars. Before the wings could be extended, three of the four bars on the base joints had to be fully extended and locked in place. Two of the bars on the first attachment joint snapped into place after Noriega gave the mechanism a push, but he had to use a "come-along" tool to force a third bar into position.

Likewise, only three of the four bars on the second wing attachment joint engaged, but that was enough for the spacewalkers to press on.

Each wing features a central canister housing a motorized, extendable mast and two solar array blankets. As the mast is driven out of its canister, assembling itself into an open Tinker Toy-like framework as it emerges, it pulls the two solar blankets, one on each side, out of their launch boxes.

Tanner and Noriega had no problems extending the blanket boxes to the deploy position. But commands sent to open latches on one set of blanket boxes failed to get through. After a brief assessment, flight controllers ordered another set of commands using a different channel but again, the latches failed to budge.

Astronaut Carlos Noriega at work during Sunday's spacewalk. Photo: NASA TV/Spaceflight Now

Finally, engineers resolved the commanding problem and latches on the starboard-side blanket boxes opened as required. But one of 28 latch pins on the port-side array refused to budge. While assessing what to do about that, flight controllers told the crew to press ahead with deploying the starboard array wing.

Finally, at 8:23 p.m. - more than two hours behind schedule - the wing began extending. Videotape downlinked later showed the mast slowly but steadily emerging from its canister, pulling the two folded solar cell blankets out of their storage boxes.

But the motion appeared erratic, with some blanket panels bunching up only to suddenly pop loose, causing the entire blanket to ripple back and forth down its length. When the wing was fully deployed, engineers noticed the blankets on one side, at least, appeared to have more slack than expected.

"The way the mast deploys, it deploys all the way and unfurls the blankets and then they are left in a state of untensioning, they're not completely taut," Reeves said. "There's one more step you do at that point where you reclose the blanket box latches that originally held the boxes together and that same mechanism pulls some cables that tighten the blankets up.

"Before they executed that step, we noticed the two blankets had a different look to them, or a different tension, and the tension cables on one blanket appeared to be very slack. That gave us enough concern that we didn't want to proceed at that point until we understood what we were looking at."

The deployed wing "is not broken," he told a reporter. "It's a perfectly good power source and a perfectly good wing. It just doesn't have the blankets fully tensioned.

A 44-foot-long equipment cooling radiator is deployed from the side of the P6 truss. Photo: NASA TV/Spaceflight Now

"And that's why we want to understand this phenomenon so that we know all the implications behind what happened to cause these cables to be loose so we can't have a worse problem with the other wing," he said. "We don't think that's the case, but we want to be sure and since we are in a safe posture, we don't want to rush into this without thinking about it."

Tanner and Noriega, meanwhile, ended their spacewalk at 9:08 p.m., seven hours and 33 minutes after first switching their spacesuits to internal power. It was the 55th spacewalk in shuttle history and the 11th devoted to space station assembly and maintenance.

As the spacewalkers were wrapping up their excursion, commander Brent Jett sent commands to close and then reopen the latch pins on the port-side solar wing. This time around, the balky pin opened normally and the wing was ready for deployment. But flight directors told the crew to stand down pending an overnight engineering review of the blanket tension issue.

The crew ended the day on a positive note. Jett had no problems deploying a 44-foot-long radiator needed to dissipate heat generated by the P6 array's internal electronics. Unlike the solar wings, the folded radiator extended normally.