The gap filler, which is cracked and about an inch long, remains one of the few concerns that needs resolving before mission managers give the green light for Discovery to return to Earth in over a week.

The early consensus is that it probably won't pose problems during the shuttle's re-entry into Earth's atmosphere, but engineers pulled an all-nighter to recommend what, if anything, needs to be done, NASA officials said.

Agency managers already have cleared a slew of areas on the shuttle that raised their interest after viewing images taken during Discovery's flight to the international space station and before the shuttle docked with the orbiting complex.

Markings on the shuttle's nose cap were determined to be harmless bird droppings. Marks on certain panels of the thermal blanket covering Discovery's belly didn't have crushed coating or other signs of damage. Two other gap fillers didn't appear to pose problems.

If necessary, astronauts Piers Sellers and Mike Fossum could try to pluck the gap filler during their third scheduled spacewalk, which is set for Wednesday.

During their first spacewalk, they demonstrated on Saturday that they could make repairs to the shuttle from the end of a 100-foot combination of a robotic arm and a boom. The duo will make their second spacewalk Monday to replace a cable on the space station's rail car.

"You guys proved the arm and boom are stiff enough to use for (spacewalks)," flight controllers wrote Discovery's crew in an electronic morning message, adding that engineers on the ground who work on the shuttle's protective skin "are sleeping easier tonight" because of their spacewalk.

Discovery's six astronauts planned Sunday to finish unloading almost 7,000 pounds of cargo and supplies they brought to the space station. They also planned to talk to reporters 220 miles below on Earth during a news conference with the space station's three crew members.

The shuttle crew awoke to a recording of ABBA's "I Have a Dream," a song chosen by pilot Mark Kelly's two daughters.

"It sometimes is like being in a dream up here," Kelly radioed to Houston. "The floating is a big part of that."

Engineers also were analyzing whether two thermal blankets on Discovery that appear slightly damaged pose any risk of flying off the shuttle during re-entry.

"Both of these items are very close to be cleared as safe ..." flight controllers wrote Discovery's crew in an electronic message Sunday morning.

The gap filler that concerns managers is just above an area on the shuttle's belly that is connected to the external tank during launch. Gap filler is material fitted between thermal tiles to prevent them from rubbing against each other.

Discovery had a similar piece of fabric sticking out in the same place during last year's flight of the shuttle, the first since the Columbia disaster in 2003. Two pieces of gap filler had to be removed from Discovery's belly during a spacewalk last year because of concerns they would cause problems during re-entry.