U.S. space agency officials are considering whether astronauts aboard the space shuttle Discovery need an extra space walk to make repairs to the craft.
Mission controllers say they are comfortable that Discovery's protective heat tiles are in good shape for landing.
But they remain concerned about the gap filler - a material between and underneath the tiles that seals them and reduces friction, protecting the orbiter from the searing heat of re-entry into Earth's atmosphere.
The gap filler is protruding in some areas, and shuttle lead flight director Paul Hill says engineers are trying to come up with some answers.
"We have a team of folks that are working aggressively at options to go and make that gap filler safe if we decide it is an issue," he said. "And we have a separate team of folks that are off looking at aero-heating and the effects of leaving those gap fillers in place and protruding. We expect to have a final results on aero-heating and those gap fillers on Monday."
NASA engineers also must confirm that Discovery's reinforced carbon wings were not damaged by debris falling during lift-off and are safe for landing.
The space shuttle Columbia disintegrated upon re-entry into the Earth's atmosphere in February 2003 after debris punctured the orbiter's insulation, allowing superheated gases to penetrate its wing. The disaster killed the shuttle's seven astronauts.
NASA extended the Discovery mission by one day to give astronauts more time to service the international space station. The next shuttle flight has been postponed indefinitely because of continuing problems with insulation falling off the shuttle' main fuel tank during launch.
The astronauts are to carry out their third and final scheduled space walk of the mission on Wednesday, during which they will continue to make repairs to the space station. If NASA decides the shuttle tile-gap filler is a problem, mission managers say the astronauts could fix it at that time or during a fourth space walk.
Meanwhile, NASA spokesman Rob Navias says the space shuttle Atlantis is standing by to perform a rescue mission if necessary.