Astronauts James Reilly and Danny Olivas spent the night in the International Space Station's airlock in preparation for a Monday spacewalk to install a new 16-tonne segment to the orbiting outpost.
The two astronauts are scheduled to leave the station at 2:53 p.m. ET after the space shuttle and station's robotic arms attach the 15.9-tonne S3/S4 truss segment to the station. Reilly and Olivas will then remove bolts and constraints to prepare the segment's solar array for activation.
They might also get a chance to get a closer look at the small tear in the space shuttle's heat shield that appeared shortly after the shuttle lifted off on Friday. NASA officials on Sunday said it should be easy to repair on spacewalks, if necessary.
The gap, measuring 10 centimetres by 15 centimetres, is in the fabric that stretches across one of two rocket engines used for orbital manoeuvres. The area is not prone to problems with overheating on re-entry, officials from the U.S. space agency said on Saturday after the peeled-back fabric was discovered.
Atlantis is equipped with tools that astronauts could use to pin the fabric back in place, said John Shannon, who chairs NASA's mission-management team, at a news briefing on Sunday. They could also cover the gap with a plate.
If Reilly and Olivas don't do the work Monday, they'll get two more chances during scheduled spacewalks on Wednesday or Friday.
Thermal blankets came unstitched during flights of Discovery in 2005 and 2006 without any problems, and thermal tiles were lost in the same area where the blanket is on Atlantis on two of the earliest shuttle flights.
The spacecraft blasted off from Cape Canaveral in Florida on Friday. Engineers aren't sure whether the stitching on the thermal blanket came loose because it was hit by debris during launch.
Several minutes into the flight, onboard cameras caught an image of what appeared to be foam flaking off the external fuel tank just after the solid rocket boosters separated from the shuttle.
Damage to the heat tiles, hit by loose foam, led to the destruction of the space shuttle Columbia in February 2003, killing all seven astronauts aboard.