With the countdown for Discovery in its final hours, NASA (search) was dealt an embarrassing setback Tuesday when a window cover fell off the shuttle and damaged thermal tiles near the tail. But the space agency quickly fixed the problem and said it was still on track for launch Wednesday.

The mishap was an eerie reminder of the very thing that doomed Columbia 2 1/2 years ago — damage to the spaceship's fragile thermal shield.

The lightweight plastic cover on one of Discovery's cockpit windows came loose while the spaceship was on the launch pad, falling more than 60 feet and striking a bulge in the fuselage, said Stephanie Stilson (search), the NASA manager in charge of Discovery's launch preparations. No one knows why the cover — which was held in place with tape — fell off, she said.

Two tiles on an aluminum panel were damaged, and the entire panel was replaced with a spare in what Stilson said was a minor repair job.

The cover, which weighs less than 2 pounds, struck a part of the fuselage that houses one of the engines used by the shuttle to maneuver in orbit. Launch managers were still awaiting an engineering analysis on whether the blow caused any damage to the engine hardware, but Stilson said she was confident there would be no problems.

Word of the mishap came just two hours after NASA declared Shuttle Discovery (search) ready to return the nation to space for the first time since the Columbia disaster.

Up until the window cover fell, NASA's only concern was the weather. Because of thunderstorms in the forecast, the chances of acceptable weather at launch time were put at 60 percent.

Discovery and its crew of seven were set to blast off at 3:51 p.m. EDT. The last few technical concerns had been resolved Tuesday afternoon at one final launch review by NASA's managers.

"It is utterly crucial for NASA, for the nation, for our space program to fly a safe mission," NASA Administrator Michael Griffin said after the meeting. "We have done everything that we know to do."

The families of the seven astronauts killed during Columbia's catastrophic re-entry praised the accident investigators, a NASA oversight group and the space agency itself for defining and reducing the dangers.

Like those who lost loved ones in the Apollo 1 spacecraft fire and the Challenger launch explosion, the Columbia families said they grieve deeply "but know the exploration of space must go on."

"We hope we have learned and will continue to learn from each of these accidents so that we will be as safe as we can be in this high-risk endeavor," they said in a statement. "Godspeed, Discovery."

Discovery will be setting off on the 114th space shuttle flight in 24 years with a redesigned external fuel tank and nearly 50 other improvements made in the wake of the Columbia tragedy.

A chunk of foam insulation the size of a carry-on suitcase fell off Columbia's fuel tank at liftoff and slammed into a reinforced carbon panel on the shuttle's wing, creating a hole that brought the spacecraft crashing down in pieces during its return to Earth on Feb. 1, 2003.

Almost every day since then, engineers have struggled to keep foam, ice and other debris from popping off the tank. They will not know whether they succeeded until Discovery flies.

During the 12-day flight, Discovery's astronauts will test various techniques for patching cracks and holes in the thermal shielding.

The crew members will also try out a new 50-foot boom designed to give them a three-dimensional laser view of the wings and nose cap and help them find any damage caused by liftoff debris. That is on top of all the pictures of the spacecraft that will be taken by more than 100 cameras positioned around the launching site and aboard two planes and the shuttle itself.

"After this flight, we will have a much, much, much better idea of whether or not our measures we have taken ... have been effective — or not effective," Griffin said. "Now our best engineers have put their best efforts on that, and we devoutly hope that they have been effective."

The board that investigated the Columbia accident put some of the blame on the space agency's safety culture, which collapsed during the doomed flight. Shuttle managers dismissed the foam strike, and engineers did not speak up about their fears.

At Tuesday's meeting, Griffin said, there was full and frank discussion of the remaining technical concerns.

"I think we got everything that everybody knows about out on the table," he said. "Can there be something that we don't know about that can bite us? Yeah. This is a very tough business. It's a tough business. But everything we know about has been covered."

A safe and successful flight of Discovery will not vindicate the space agency, Griffin said.

"There is no recovery from mistakes we've made, whether it goes back to the Apollo fire, loss of Challenger or the loss of Columbia. Going back even further to 100 years of aviation, the safety systems that we who fly have learned and know are written in other people's blood," said Griffin, a pilot.

"The minute we say we're good enough, we start getting bad again and we need not to do that."