Shuttle damaged on launch pad

CAPE CANAVERAL, Fla. -- NASA engineers successfully replaced two damaged thermal tiles on the space shuttle Discovery last night after a window cover came loose and fell, striking the shuttle and temporarily threatening to delay today's launch.

Agency officials said they weren't sure why a plastic and foam 2-pound covering fell off one of the windows late yesterday afternoon, dropping about 65 feet and hitting a panel near the shuttle's tail. Engineers quickly found a replacement part and, after a detailed inspection, declared the launch back on within a few hours.

''It fit perfectly," Stephanie Stilson, Discovery vehicle manager, said during a news conference last night. ''This is a minor repair for us."

The incident was the only disruption in a day when officials with the National Aeronautics and Space Administration expressed cautious optimism about today's 3:51 p.m. launch, saying no last-minute technical or safety problems had arisen.

Before the window incident, the weather seemed to be the launch's biggest obstacle, with NASA officials upping the chance of a weather delay from 30 percent to 40 percent as a high pressure system built in the western Atlantic.

''The crew is just raring to go," NASA administrator Michael Griffin said during an afternoon news conference at Kennedy Space Center, before the window problem was discovered.

''We're in good shape," Griffin said.

The launch will be the first since the shuttle Columbia disintegrated over Texas on Feb. 1, 2003, killing all seven astronauts on board. Since then, NASA scientists have worked to make the shuttle safer and to reduce the risk of debris falling off the vehicle during the climb into orbit and piercing its heat-shielding layer, as it did on the Columbia, leading to its breakup.

While many thermal tiles on the shuttle must be individually bonded -- which can take several days -- the tiles struck yesterday by the window covering were part of a prefabricated panel on the left side of the orbital maneuvering system. Because many parts on NASA's shuttles are interchangeable, engineers were able to find a replacement.

Stilson said she and others were confident the part would work, but engineers were conducting another detailed look at the shuttle to ensure that was the case and no other damage occurred.

Meanwhile, excitement was building at the Space Center yesterday as hundreds of media representatives from around the world erected tents, platforms, and staging areas to chronicle the launch.

In a bittersweet public release, family members of the Columbia astronauts wished Discovery ''Godspeed," saying they believe the sacrifice of their loved ones would aid all humankind.

''We grieve deeply but know the exploration of space must go on," the statement read.

If today's mission is delayed, NASA will try again tomorrow, when there also is a 40 percent risk of a weather delay.