Cape Canavarel, Fla. ? With the countdown for Discovery in its final hours, NASA 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 today.

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, 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 Discovery 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 Space Shuttle Discovery sits on Pad 39B Tuesday at the Kennedy Space Center at Cape Canaveral, Fla. A window cover fell from the shuttle on Tuesday, damaging several tiles on a leading edge of the tail. NASA officials say they can have the damage prepared in time for the scheduled launch today.

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.

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.”