Shuttle astronauts on second walk
Astronauts have gone outside the International Space Station (ISS) for the second day running to continue construction work on the platform.
Canadian Steve MacLean and US colleague Dan Burbank are completing the attachment of a new "backbone" segment fitted to the outpost on Monday.
The P3/P4 truss includes 73m-long (240ft) solar wings that will double the available power at the ISS.
The spacewalk, which began at 0905GMT, should last more than six hours.
The US space agency (Nasa) was delighted with Monday's efforts.
Heidemarie Stefanyshyn-Piper and Joe Tanner wired up the $372m (£293m) truss so efficiently they were given permission to carry out tasks scheduled for future spacewalks.
One of these tasks involved removing the launch locks from the rotary joint that allows the solar wings to move.
It was during this task that a bolt and washer floated free into space, but Nasa officials say they are confident the objects will not damage the space station or the docked shuttle.
The ISS will eventually have 11 integrated truss segments that stretch 108m (356ft) from end to end.
They will support four virtually identical solar array assemblies. They also will support radiators that will cool the station.
If all goes to plan, the new segment will be powered up and its two giant solar wings unfurled during the third spacewalk on Thursday.
Saturday's launch of Atlantis to the ISS 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.
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