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By Irene Klotz

CAPE CANAVERAL, Fla., Oct 23 (Reuters) - Space shuttle Discovery blasted off on Tuesday for a construction mission at the International Space Station, raising hope that NASA will be able to add laboratories for European and Japanese partners later this year.

The shuttle bolted from its seaside launch pad on time at 11:38 a.m. (1538 GMT), defying forecasts of poor weather and a potential delay from an ice buildup on its fuel tank.

It quickly disappeared from view behind low-lying clouds before reaching orbit 8.5-minutes later.

"This is a great start to a challenging mission," Bill Gerstenmaier, NASA's associate administrator for space operations, told reporters after launch.

The seven-member crew, led by retired Air Force colonel Pamela Melroy, 46, plans to spend 10 days at the station preparing it for the arrival of Europe's Columbus laboratory in early December.

Five spacewalks are planned during Discovery's visit to move and reattach a pair of the station's solar wing panels, install a vestibule so Columbus and Japan's Kibo module can be attached and other tasks.

While the work is similar to previous station assembly missions, NASA has never attempted to put so much into a single flight.

"It's the combination of all these things," that make this flight complicated, Gerstenmaier said.

NASA already has accomplished one of the most challenging aspects of the mission -- getting into space.

A 4-inch chunk of ice that formed on the outside of the shuttle's fuel tank triggered a round of debate among engineers trying to determine if it could break off and damage Discovery's heat shield during liftoff.

Engineers believed most of the ice would shake off in the six seconds after the shuttle's engines start but before its twin booster rockets ignite to lift the ship off the launch pad.

Their predictions proved correct, although NASA will not be able to determine definitively the condition of Discovery's heat shield until after a series of on-orbit inspections.

"It looked like a pretty clean ascent," astronaut Terry Virts from the Mission Control Center in Houston told Melroy.

"Great news," she replied.

Video of the shuttle's liftoff showed about six pieces of the tank's foam insulation flying off the tank, though the timing was after the critical period when atmospheric forces can slam debris into the vehicle.

NASA lost the space shuttle Columbia and seven astronauts during a landing attempt in 2003 due to undetected heat shield damage from a launch debris impact.

Discovery is due to arrive at the station on Thursday. Its return to Florida is scheduled for Nov. 6.

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