Astronauts have completed another spacewalk, the second of the Atlantis shuttle's visit to the International Space Station (ISS).
Canadian Steve MacLean and US colleague Dan Burbank completed tasks that will bring into use 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 wings, packed away for launch, are now set to be unfurled on Thursday.
They will receive a remote command sent from Mission Control in Houston, Texas.
Only if the automated systems fail will astronauts Stefanyshyn-Piper and Joe Tanner - the Atlantis pair scheduled to make a spacewalk on Friday - have to move in and manually deploy the arrays.
The US space agency (Nasa) has expressed its delight with the way the first two Extra Vehicular Activities (EVAs) have gone.
The only hiccups have been the loss of two spring-loaded bolts that were allowed accidentally to drift off into space. Nasa officials are confident, however, 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.
The Atlantis shuttle is expected to leave the ISS on Sunday. It needs to be clear of the platform to allow a Russian Soyuz rocket, carrying a new ISS crew and the first female space tourist, to dock during the middle of next week.
Atlantis' mission to the station is only the third shuttle flight since the orbiter Columbia broke up on re-entry after being damaged by launch debris in 2003.
Nasa plans at least 14 more construction missions to the space station over the next four years and must complete assembly before the orbiter fleet is retired in 2010.