A plastic and foam cover fell off one of space shuttle Discovery's window Tuesday, causing some damage less than 24 hours before a planned return to flight.

The agency said the cover hit the front cover of an orbital maneuvering rocket, damaging thermal tiles that protect the spacecraft when it re-enters the atmosphere.

A hole in thermal protection on Columbia caused its destruction 2½ years ago.

No workers were nearby when the window cover fell off and dropped about 60 feet. It's not clear why the cover -- which was held by tape -- came loose.

A NASA manager said that the panel with the damaged tiles could be replaced in about an hour and should not delay the launch.

Word of the mishap came just two hours after NASA declared that if the weather cooperates, Discovery is ready to go for the first flight since the Columbia disaster.

The agency's administrator, Michael Griffin, said Tuesday the last few technical concerns have been resolved.

"We're go for launch tomorrow, pending the weather," he said.

The shuttle is set to take a crew of seven to the international space station, where it will install a new gyroscope, among other tasks.

In the aftermath of Hurricane Dennis, thunderstorms have been in the forecast along Florida's Atlantic coast. NASA earlier Tuesday said there was a 60 percent chance the weather would allow an on-time launch.

That was down from a 70 percent chance the agency offered Monday.

Liftoff is scheduled for 3:51 p.m. Eastern time. foxreno.com plans to offer live video of the launch.

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Video: NASA Ready For Discovery Launch

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The shuttle test director said earlier the shuttle and its crew are ready.

"Our nation is ready," Jeff Spaulding added.

The problems concerned fuel tank sensors that didn't read properly during a recent test and a balloon monitor that gathers data on upper-level wind speed.

NASA officials also wanted to review some of the tools that will be used during a spacewalk to test the shuttle's thermal protection system. The answers to those problems are expected to be finalized at a readiness meeting Tuesday.

Discovery is outfitted with a redesigned fuel tank and has dozens of motion and temperature sensors embedded in the wings to detect any blows from fuel-tank foam or other debris. That's what officials blame for Columbia's demise.