Two US astronauts have completed a six-and-a-half hour spacewalk outside the International Space Station (ISS).
Astronaut Heidemarie Stefanyshyn-Piper working on the ISS
Heidemarie Stefanyshyn-Piper and Joe Tanner wired up a new $372m (£293m) addition, which will provide power, data and communication services.
A bolt, spring and washer floated free during the work, and Nasa is examining whether it could cause problems.
The second of three spacewalks planned during the 11-day mission is due to take place in 24 hours.
The first task was to connect up the P3/P4 truss, a 17-tonne segment of the space station's "backbone", which includes a huge set of solar arrays.
Shortly before the spacewalk, astronauts Steve MacLean and Jeff Williams manoeuvred it into place using a robotic arm from inside the ISS.
Spacewalk veteran Tanner then stepped out of the airlock into space, tethered to the space station, followed by novice spacewalker Piper.
"Welcome to the world of EVAs," he told his colleague, using the Nasa slang for spacewalks.
"Aaah. Wonderful," replied Piper.
Piper and Tanner carried out a long list of mundane but essential construction tasks.
Towards the end of the spacewalk, Tanner was working with a bolt when it sprang loose and floated towards the truss.
ISS: ORBITING OUTPOST
Construction work has been on hold for four years
16 nations contribute to the ISS, including the US, Russia, Japan, Canada, Brazil and European Space Agency states
The ISS will eventually be the size of a football field
The washer went into space, but the astronauts were concerned that the bolt and spring might have got caught up in the wiring and tubing they were fitting.
"Not a good thing," said Tanner. "Let's hope it doesn't end up somewhere in the mechanism."
Earlier in the spacewalk, Nasa's Mission Control Center in Houston informed the Atlantis crew that no detailed inspections of the shuttle's heat shield were needed.
Nasa saw some debris from the fuel tank hit the orbiter during its launch, but no damage was detected from ground observations.
This allows astronauts Dan Burbank and MacLean to press ahead with the second spacewalk, planned for Wednesday.
If all goes to plan, the new segment will be powered up and two giant solar arrays unfurled during the third spacewalk on Thursday.
Saturday's launch was just the third shuttle mission since the orbiter Columbia broke up on re-entry after being damaged by launch debris in 2003.
The six astronauts, one woman and five men, arrived at the ISS on Monday. After docking with the station, they greeted its three occupants, and started work on the first construction mission since November 2002.
Nasa says the next few days will be some of the most complex and busy in shuttle history.