By Irene Klotz

CAPE CANAVERAL, Fla. Oct 21 (Reuters) - NASA managers on Sunday cleared space shuttle Discovery for launch on Tuesday to attach a vestibule to the International Space Station that will serve as the doorway to European and Japanese laboratories.

NASA hopes to fly Europe's Columbus laboratory on Dec. 6, but its launch will depend on how quickly Discovery can begin its 14-day construction mission. The shuttle's liftoff is targeted for 11:38 a.m. (1538 GMT) from the Kennedy Space Center in Florida.

Managers wrapped up an unusually short review of launch preparations on Sunday, with no major technical issues to discuss, said LeRoy Cain, NASA's lead shuttle manager at the Florida spaceport.

"We have no issues, no constraints to launch," Cain told reporters. "We're ready to move forward to launch Tuesday morning."

Meteorologists, however, were not so sure. Florida's summer rainy season is lingering, with a fresh band of showers and thunderstorms expected sometime this week. Because of the storms, forecasters predicted a 40 percent chance of a launch delay on Tuesday.

NASA has made a few changes to launch day procedures, including cutting off about an hour of time that the shuttle's fuel tank is filled with the cryogenic propellants needed for liftoff.

Engineers believe the cold liquids contribute to the problem of the tank's insulating foam popping off during liftoff and potentially striking the shuttle.

A debris strike triggered the breakup of space shuttle Columbia and the loss of seven astronauts in 2003. Since then, NASA has made several modifications to the tanks and added instruments to survey the shuttles after they reach orbit.

NASA is counting on the in-flight scans to detect any damage to the shuttle's heat shield, which protects the ship during its return through Earth's atmosphere for landing.

Discovery's wings will face extra scrutiny to allay any lingering concerns that three of the carbon-composite panels on the wings may be slightly degraded.

Engineers using a new X-ray analysis technique warned managers that three of the 44 panels have tiny cracks in silicon-carbide coatings that cover the panels.

After a lengthy debate, managers opted to proceed with launch and assigned teams to continue investigating the discrepancies.

"We're trying to tell what the screenings are telling us," Cain said, adding that managers "have not cut any corners here."

Half of NASA's engineering and safety experts had recommended the suspect wing panels be replaced.

NASA determined the risks of flying with the questionable wing panels were less than those already posed by many other critical shuttle systems.

"If we thought otherwise, we wouldn't be flying," Cain said.

Discovery is carrying a school-bus sized module named Harmony that has six docking ports. Two hatches are reserved for Europe's Columbus lab and Japan's Kibo complex.

The crew, led by Pamela Melroy, plans to conduct five spacewalks to attach the new module, reposition a pair of the station solar wing panels and test a heat shield repair technique.

NASA needs to fly 11 more shuttle flights to complete assembly of the $100 billion orbital complex, a project of 16 nations. The agency also would like to fly two resupply missions to the station and service the Hubble Space Telescope one final time before the shuttles are permanently retired in 2010.

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