HOUSTON -- Two spacewalkers working outside the International Space Station (ISS) shook a troublesome solar array late Saturday to help fold it away, but ultimately ran out of time during an extended spacewalk to finish rewiring the orbital laboratory.

NASA astronauts Robert "Beamer" Curbeam and Sunita "Suni" Williams spent seven hours and 31 minutes outside the ISS, apparently breezing through their planned tasks before each taking dramatic turns to shake out snags on a stubborn solar array atop the station's mast-like Port 6 (P6) truss. A fourth extravehicular activity (EVA) is slated for Monday to continue solar array work, NASA officials said.

"Beamer, you've been amazingly effective," NASA astronaut Steve Robinson told Curbeam from Mission Control here at the Johnson Space Center, and later lauded both spacewalkers. "We really commend you for a tremendous effort, an Olympian effort of our two shaking EVA members."

Curbeam and Williams spent about two hours of their long spacewalk perched just below the portside wing of the P6 solar array, which had been parked half-furled since Wednesday after folding troubles popped up during its retraction [image]. Friction between guide wires, three of which run the length of each 115-foot (35-meter) solar wing panel, and the grommets they thread through were thought to be the problem's source [image].

Using a start-and-stop method, in which astronauts inside the ISS alternately retracted the solar array slightly and stopped so the spacewalkers could shake out any snags, appeared to work well [image ].

"For awhile, we really thought that we were going to get this retracted on this EVA," said Tricia Mack, NASA's lead spacewalk officer for Discovery's STS-116 mission, adding that spacesuit consumables were the only limitation. "We all firmly believe that had we had more time, we would have been able to get it in there...we just ran out of time."

The spacewalk, the third planned for the shuttle Discovery's STS-116 mission to the ISS, began at 2:25 p.m. EST (1925 GMT).

Fourth spacewalk approved

Before Curbeam and Williams even began performing what flight controllers ultimately dubbed "Beamer shakes" and "Suni shakes," mission managers approved plans to add a fourth spacewalk to the joint docked operations between the station's  Expedition 14 crew and the shuttle Discovery's STS-116 astronauts.

"We are go for EVA 4 on Flight Day 10," Robinson told Discovery's STS-116 crew of the extra spacewalk.

Robinson said the extra spacewalk effectively adds a full day to Discovery's mission, with undocking now coming on Tuesday and landing on Dec. 22. The shuttle astronauts will still perform a late inspection of Discovery's heat shield to seek out any signs of damage from micrometeorites or other orbital debris, Robinson added.

Astronauts aboard the ISS partially retracted the six-year-old P6 array, which originally stretched over the station's port side and is designated as P6-4B, earlier this week to clear the area and allow a pair of newer solar arrays to begin rotating like a paddlewheel to track the Sun.

Attempts to jostle the guide wire free by wiggling the array, and then with an exercising astronaut causing vibrations within the ISS, failed Friday, as did work to extend the solar wing slightly and fold it back in again.

At the start of today's spacewalk, about 17.5 of the 31.5 mast segments, known as bays, supporting the solar array remained to be retracted. By the end of the activity, only 11 bays remained, NASA officials said.

"If you saw the success of today, I'm very confident that we're probably going to get that done," John Curry, NASA's lead ISS flight director during the STS-116 mission.

Station's primed power system

While their solar array shaking may not have led to a complete retraction of the unruly P6 solar array, Curbeam and Williams sped through their primary chore of completing a two-part job to overhaul the space station's power grid.

Until this week, the space station's primary power supply stemmed from a temporary electrical system fed by the two solar arrays atop the orbital laboratory's P6 truss. A successful Thursday spacewalk switched half the station's power grid to its permanent set up, with Curbeam and Williams completing that effort.

"Yes, in that sense, it is all downhill," Curry said of the rewiring work, which now leaves some cargo transfer and the solar array retraction as the only major to-do items outstanding.

With the space station now plugged into its permanent configuration and drawing power from solar arrays on the orbital laboratory's main truss, a sort of orbital backbone that will rival a U.S. football field in length when complete, it is one step closer to the arrival of more solar wings, larger six-astronaut crews and new orbital laboratories for Japan and Europe to launch in the next several years.

"The vehicle is ready to accept the international modules now, and that's a huge accomplishment," Curry said. "So that was big."

Curbeam and Williams also performed a few maintenance tasks during their spacewalk to install some spare parts for later on the exterior of the ISS. The astronauts stowed a new grapple bar on the station's exterior along with a set of debris panels arranged in a set up affectionately dubbed the "Christmas tree."

"Merry Christmas," Williams remarked as she and Curbeam delivered the new panels to the space station.

About the only drawback in today's spacewalk was a lost camera, which floated free Williams' spacesuit tool bench she arranged her tethers outside the space station's U.S. Destiny laboratory. A button designed to lock the camera in place may have failed, resulting in the camera's loss, Mack said, adding that debris analysts are studying the camera's trajectory.

Today's spacewalk marked the 76^th EVA dedicated to ISS assembly or maintenance and the 48^th staged from the orbital laboratory itself. It was the sixth career spacewalk for Curbeam, who at 13 hours and 56 minutes, now ranks 13^th among the list of all time spacewalkers, NASA officials said.

Williams made her spacewalk debut during Saturday's activity, which prompted well wishes from Mission Control as she stepped outside the space station's airlock.

"It's going to be a blast," Williams said.

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• Images: The Spacewalks of NASA's STS-116
• Images: Discovery's STS-116 Launch Day Gallery
• STS-116 Video: Power is Everything
• STS-116 Video: Building Blocks
• Mission Discovery: The ISS Rewiring Job of NASA's STS-116
• Complete Space Shuttle Mission Coverage
• The Great Space Quiz: Space Shuttle Countdown
• All About the Space Shuttle