This story was updated at 9:49 p.m. EDT.
Spacewalkers and robotic arm operators attached the final piece of the International Space Station's Japanese Kibo laboratory - an exposed platform for science experiments.
The addition completes the $1 billion Kibo complex, the station's largest lab, and allows researchers to test how different materials react to the harsh space environment.
"The Japanese Exposed Facility, or 'Jeff' as we tend to call it, is very impressive," said lead spacewalker Dave Wolf in a preflight interview. "It?s a large external porch to the space station where high quality experiments can be conducted in high vacuum of space. It?s really an exceptionally valuable piece of real estate being produced in outer space."
Wolf and Tim Kopra began the spacewalk at 12:19 a.m. EDT (1619 GMT), and concluded it about five and a half hours later. The event was the first of five spacewalks during the shuttle Endeavour's STS-127 mission. Endeavour docked at the space station Friday.
"We did an EVA and we did robotics," said lead space station flight director Holly Ridings during a briefing Saturday. "When you do all of those things simultaneously and it all works out as well as it did today, it is a great day in spaceflight."
The spacewalkers released the new 8,372-pound science deck from its berth in Endeavour's cargo bay, and cleared the way for it to be attached onto the Kibo pressurized module. Once the platform was free, astronauts inside the space station grabbed it with robotic arms.
Mission specialist Koichi Wakata and pilot Doug Hurley used the space station robotic arm to grasp the porch and move it out of the cargo bay. They handed it off to shuttle commander Mark Polasnky and mission specialist Julie Payette, who maneuvered the deck toward its new home on Kibo.
"This was a proud moment," said Tetsuro Yokoyama, Kibo deputy project manager at the Japan Aerospace Exploration Agency. "Today is a big day."
The complicated hand-off went smoothly.
"You can imagine this is space, weightlessness," Payette explained in a NASA interview. "We don?t let go until we are sure that one arm or the other is holding onto the platform itself. We call this a ?handshake?."
After their work on Kibo, the two spacewalkers moved on to the space station's backbone-like truss, where they worked to attach two spare parts platforms. One of the containers, called the Cargo Attachment System, was originally set up during the March STS-119 shuttle mission, but those astronauts had difficulty deploying it because a jammed pin refused to release.
Wolf and Kopra were able to successfully unfold the platform using a new tool built especially for the task. The cargo platform will be used to store spare parts to be dropped off by future shuttle missions to prepare the space station for when the space shuttle fleet retires, planned for 2010.
During the spacewalk the communications system in Kopra's spacesuit sounded static-y because of a slightly misplaced microphone in his helmet. As the ventilation system streamed air through the helmet the noise got picked up by the microphone, which made Kopra's communications somewhat difficult to hear. The problem wasn't serious, though, Ridings said, and the spacewalkers managed to accomplish all their goals regardless.
The issue shouldn't pose further problems because Kopra is not scheduled to make any more spacewalks during this mission. The communications systems in the other spacewalkers' helmets seem to be working fine, said lead spacewalk officer Keith Johnson.
Endeavour docked at the International Space Station Friday to begin a two-week visit, boosting the population onboard the orbital outpost to a record high of 13. The shuttle launched July 15 after more than a month of delays.
Tomorrow the crew will conduct robotic maneuvers to begin installing a set of spare supplies for the station.
Mission managers decided it was not necessary for the astronauts to conduct a focused inspection of their shuttle's heat shield to check for damage incurred during launch. NASA is confident that Endeavour is safe to return home to Earth, despite the apparent loss of an unusually high amount of foam from the shuttle's external tank during liftoff that could have harmed the orbiter if it impacted.
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SPACE.com is providing continuous coverage of STS-127 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.