The potential trouble has nothing to do with launch debris, for a change, but rather material used to fill the spaces between thermal tiles, a common problem in the past although not necessarily to this extent.
The ceramic cloth - called "gap filler" - could create turbulence during re-entry, and cause temperatures to soar dangerously, CBS News Correspondent Mark Strassmann reports.
Flight director Paul Hill said engineers are to decide Monday whether to have the crew's two spacewalkers cut, pull out or shove back in the hanging material.
"We have a team of folks that are working aggresively on options to go and make that gap filler safe, if we decide it's an issue," Hill said.
NASA's entire approach to safety is again under scrutiny after more foam debris grounded its shuttle fleet indefinitely.
From the International Space Station, Discovery's commander said that NASA and the shuttle deserve another chance.
"I feel the shuttle is safe to fly," astronaut Eileen Collins told Strassmann. "We need to press on with the shuttle program. Until we finish the space station, and then we'll move on to the next exploration, which is to go back to the moon and on to Mars."
In light of the new development aboard the shuttle, yesterday's spacewalk to practice in-space repairs was a sound decision.
NASA astronaut Scott Parazynski told Strassman that the Columbia disaster opened everyone's eyes to the need of being able to fix things from the shuttle.
The extremely thin gap fillers are made of a felt-like material and ceramic, and are held in place with glue and by the tight fit.
It could be that it's perfectly safe for Discovery and its crew of seven to fly back with the two drooping pieces, Hill stressed, as shuttles have done on many previous flights.
One is sticking out one inch between thermal tiles, the other six-tenths of an inch.
The longest protruding gap filler seen on a returning shuttle before was a quarter-inch, but Hill cautioned that measurement was taken following re-entry, and the intense heat could have burned some of it off.
Any repair, if deemed necessary, could be performed during the third spacewalk of the mission, now set for Wednesday or a fourth unplanned spacewalk might be required, Hill said. The astronaut would have to stand on a long robotic arm in order to reach the two areas, located on the shuttle's belly near the nose.
One extreme option under consideration is to put an astronaut on the end of the brand new 100-foot inspection crane, but it could be a bouncy ride and that makes lots of experts "understandably nervous," Hill said.
He said there are strong arguments for and against most of the options.
Anything dangling from the bottom of the shuttle during re-entry will overheat the area, as well as downstream locations. The ongoing analysis is to decide whether that overheating will be within safety limits.
A hole in Columbia's left wing, left there by a large chunk of flyaway fuel-tank foam, led to the spacecraft's destruction during re-entry on Feb. 1, 2003. All seven astronauts were killed.
NASA has cleared all of Discovery's thermal tiles for landing on Aug. 8. The only remaining issues, before the final go-ahead can be given for descent, are the reinforced carbon panels that line the wings and nose cap, and the two hanging gap fillers.