One of the areas of biggest interest is a chipped thermal tile near the set of doors for the nose landing gear.

If everything checks out as NASA expects, then Discovery will be free to return to Earth on Aug. 7 as planned, following an eight-day space station visit.

Shuttle managers were stunned after seeing video images of the large piece of foam shooting off the fuel tank two minutes after Tuesday's liftoff. It weighed about half as much as the piece that slammed into Columbia's left wing and was irregular in shape, at 24 to 33 inches across.

It was not until Wednesday, after viewing more video and still images from space, that managers knew where the foam came from. The foam broke off an area meant to protect cables and pressurization lines running down the length of the 15-story fuel tank, not even close to the location of Columbia's broken insulation.

Three smaller pieces of foam broke off the same vicinity of Discovery's fuel tank, including a 7-inch-long chunk that missed Discovery and the fragment that may have went into the wing.

Shuttle managers considered modifying the area after Columbia's catastrophic re-entry on Feb. 1, 2003. But they put it off because they had had little trouble with the foam there in the past, and it was a relatively easy area to check for air pockets that might cause the insulation to pop off during launch.

Shannon said that decision was based on limited flight data. Engineers had no good tank images from more than 50 of the 113 previous shuttle launches; of the remainder, only one liftoff resulted in foam loss from that area and it was attributed to a previous repair.

"As everybody who's come up here in the last two days has said, we were wrong and we missed something and we have to go figure out what it was and go fix it," Shannon said. "Whether that's just changing techniques or redesign, we don't know."

Until the problem is fully understood and resolved, NASA has decreed that no more shuttles will be launched.

The grounding cast a pall over Mission Control and the rest of the space program. Flight director Paul Hill gave his team a pep talk before Discovery started making its final approach to the space station, reminding them, "We have a job to do. We have a crew that is relying on us."

Still ahead are three spacewalks by Discovery's astronauts, supply transfers between the two linked spacecraft, the shuttle's undocking, and its descent back to Earth.

"We don't have the luxury of sitting around and thinking about what does this mean to the program, or what are we going to do after" Discovery's mission, Hill said. He added, "It's all about taking care of Eileen Collins and her crew."