The S6 truss (lower left) rests at the end of Canadarm2, the International Space Station's robotic arm as astronauts deliver it during NASA's STS-119 mission in March 2009.
This story was update at 8:17 p.m. EDT.
Two spacewalking astronauts floated outside the International Space Station Thursday to help attach the last pair of solar wings for the orbital lab?s power grid.
Clad in their NASA-issue spacesuits, astronauts Steven Swanson and Richard Arnold II spent just over six hours installing the new solar arrays and their 16-ton support girder on the space station?s starboard side.
?It?s a lot bigger than when you left it, you guys were outstanding,? station commander Michael Fincke told the spacewalkers as they began wrapped up their work. ?Thanks for the hard work.?
Delivered by the shuttle Discovery, the $298 million solar wings are due to be unfurled on Friday. Until then, they?ll stay folded up like oversized maps inside their storage boxes.
The station currently has six solar wings, two for each of its three deployed arrays. When the new arrays are deployed, the station will have eight wings, with four on each side giving it a symmetrical look.
Astronauts inside the space station used the orbiting lab?s robotic arm to gently ease the solar wing segment in place, making tiny adjustments based on verbal cues from Swanson and Arnold, who had a ringside seat to the delicate attachment.
The spacewalkers secured the 45-foot (14-meter) solar array girder in place with four bolts and hooked up a power and data cables, at times wrestling with stubborn clasps.
?It wasn't quite as smooth as we had hoped, but those guys did a great job,? said Discovery astronaut Joseph Acaba, who choreographed the spacewalk from the shuttle and - like Arnold - is a former schoolteacher.
Thursday spacewalk began at 1:16 p.m. EDT (1716 GMT) as the docked shuttle and space station flew 220 miles (354 km) above the Pacific Ocean near New Guinea. While Swanson and Arnold worked outside, their crewmates inside the station successfully repaired Discovery?s broken exercise bike.
?It was a great team effort out there today,? Swanson said. ?Wonderful.?
Space station power up
Each of the space station?s new solar wings will extend about 115 feet (35 meters) when deployed and, together, will boost the outpost?s power grid by 25 percent. When complete, the four U.S. solar arrays will generate enough electricity to power 42 average sized homes, NASA has said.
That power is vital for the space station, where astronauts hope to increase the amount of science performed inside its U.S., Russian, European and Japanese modules. The extra power will also be needed once the space station doubles its crew size up to six people in late May.
The new starboard solar wings are moored to a 31,000-pound (14,061-kg) girder known as the Starboard-6 (S6) truss. It is the last major American-built piece of the $100 billion International Space Station and the final part of the outpost?s 11-segment main truss.
With the S6 girder?s installation, the space station now weighs nearly 1 million pounds (453,592 kg), with its backbone-like main truss extending more than 300 feet (91 meters), long than a football field. The space station can be easily spotted from Earth by the naked eye on a clear night.
Thursday?s spacewalk marked the 121st excursion dedicated to space station construction and the first of three planned for Discovery?s astronauts.
It was the third career spacewalk for Swanson, who finished with 19 hours and 52 minutes, and the first for Arnold, who is making his first spaceflight.
Discovery?s mission initially included four spacewalks and one extra day. NASA shortened the flight after a series of launch delays so the shuttle crew could complete the construction work and leave the station before the arrival of a previously scheduled Russian Soyuz spacecraft.
That Soyuz is due to launch a new crew and American billionaire Charles Simonyi, who is paying about $35 million for his second space tourist flight, to the station next week.
SPACE.com is providing continuous coverage of STS-119 with reporter Clara Moskowitz and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.
- New Video - Discovery?s STS-119 Night Launch
- Video - Discovery's Mission: Space Station Power Up!
- New Show - Inside the International Space Station