The space shuttle will come back for its final turn in the spotlight with its scheduled launch Wednesday after a 21/2-year absence, reviving America's human spaceflight program even as NASA moves to retire the venerable craft.

The shuttle Discovery stood poised at the Kennedy Space Center's Launch Pad 39B, where, barring a last-minute technical problem or a thunderstorm, it will lift off at 3:51 p.m. Eastern time for a 13-day mission.

One potential problem surfaced late Tuesday when workers found that a temporary plastic cover protecting one of the shuttle windows had fallen 65 feet and damaged heat-shielding tiles on the left on-orbit engine. Officials said, however, that engineers had quickly replaced the damaged panel and that it would not affect the launch.

"We're go for launch," NASA Administrator Michael D. Griffin said, with no other engineering problems outstanding and little left to do but "hope that the weather gods are smiling on us."

For NASA and the U.S. space program, Discovery offers the prospect of redemption after the tragic loss of Columbia -- an opportunity to restore the agency's tattered reputation, a reaffirmation of American preeminence in human spaceflight and the long-awaited opening salvo in President Bush's initiative to put humans back on the moon by 2020 and eventually send them to Mars.

Hardware and software checks at the pad continued without mishap, gantries were pulled back, and equipment was cleared away and stored. Mission commander Eileen Collins and six other astronauts, quarantined for a week from all but close family members and friends, said goodbyes and rested quietly, ready to board the spacecraft a little more than three hours before launch.

"All of the work goes extremely well," said NASA test director Jeff Spaulding, in charge of the countdown. "From my perspective, a boring countdown is a good countdown."

Weather remained the biggest unknown. With midday thunderstorms a regular feature of central Florida in July, meteorologists Tuesday gave the launch a 60 percent chance of happening on time, down from their earlier estimate of 70 percent.

Planners can try twice more in the next three days before they would have to wait for three days to service the shuttle, and perhaps even longer if Tropical Storm Emily changes course.

Weather officer Kathy Winters said the chief fear for launch day is that the afternoon sea breeze will not blow the daily midday thunderstorms inland beyond the 20-mile weather-free zone needed in case the shuttle has to return to the space center for an emergency landing. Spaulding said engineers can wait until nine minutes before liftoff before deciding whether to abort the launch.

For the space shuttle, built with 1970s technology and flown 113 times since 1981, Discovery's mission will serve as the acid test for new safety features and procedures put in place after sister ship Columbia disintegrated over Texas during reentry Feb. 1, 2003, killing all seven astronauts aboard and grounding the shuttle fleet.

In a statement Tuesday, families of the Columbia astronauts blessed Discovery's journey and praised NASA for "an exemplary job in defining and reducing the technical risk as much as possible," while urging the agency to remain "vigilant."

"I'm pleased that they're supportive of our efforts -- and it matters," Griffin said in a news conference. But, he cautioned, "there is no recovery from the mistakes we have made. The safety lessons that we who fly have learned are written in other people's blood."

Discovery will fly with an external fuel tank redesigned to minimize the shedding of ice and foam insulation, like the suitcase-size chunk that breached Columbia's heat shielding during launch, dooming the orbiter.

Independent reviewers have said that NASA made great strides in solving the debris menace but that the risk has not disappeared.

NASA has also installed more than 100 new imaging devices to check the shuttle's underside for damage during launch, a task deemed so important that Winters said cloud cover "could be a showstopper" if it blocks enough of the cameras.

"Can we be bitten by something we don't know about?" Griffin asked. "Sure. This is a very tough business, but everything we know how to do has been done."

The mission brings to life Bush's "Vision for Space Exploration," the moon-Mars initiative announced in 2004.

The vision's initial job is to complete construction of the international space station. Discovery and its sister ships, Atlantis and Endeavour, with the capacity to carry 65,000 pounds of cargo, are the trucks that deliver new components and spares to the space station and haul away its trash.

Discovery will spend most of its mission at the station, where two-member crews have spent much of their time since the Columbia disaster making interim fixes to key equipment and carefully husbanding food, water and oxygen sent up to them in limited quantities by Russian resupply spacecraft.

Space station project manager William Gerstenmaier made no secret of his eagerness to have the shuttle back, "because we can't continue assembly of the station" without it, but he insisted that an aborted launch Wednesday, or even a delay until September, would not affect the station's survival.

"It's important, but does it absolutely have to happen?" he said Monday. "No. We're still stable."

No matter when Discovery launches, the shuttle's return is only a limited engagement. On Tuesday, Griffin reaffirmed the Bush administration's wish to retire the shuttle by 2010 and replace it with a new spaceship able to travel to the moon. Discovery's flight may be historic, he said, but it is also the beginning of the end.

"The space shuttle is the most magnificent piece of transportation ever developed," just like "the clipper ships were the pinnacle of the sailing art," he said. "It's time to move on."

NASA spokesman Kyle Herring explains how a temporary plastic cover protecting one of Discovery's windows fell 65 feet and damaged heat shielding tiles on the left on-orbit engine.