After a four-hour debate, "the Flight Readiness Review board decided we were in an acceptable risk posture to go fly," shuttle program manager Wayne Hale told reporters late Tuesday.

"Which is not to say," he added, however, "that we completely and perfectly understand the problem that has been laid out and we'll continue to work very hard on it.

"If the risk grows to an unacceptable level we'd take action whether to change that hardware or to delay some flights while we do testing," Hale said, stressing the complexity of the problem.

The review board recommended replacing three thermal tiles on the leading edge of the Discovery's wings after microscopic cracks were detected on their external layers.

Temperatures on the shuttle's re-entry to the earth's atmosphere at 20 times the speed of sound can reach 1,600 degrees Celsius (2,900 Fahrenheit).

NASA has paid special attention to the shuttle fleet's thermal protection after it was determined to have been the cause of the 2003 Columbia's disintegration during re-entry that killed seven astronauts.

A piece of falling insulation from the Columbia's external fuel tank during takeoff gouged its thermal shield badly enough to cause its destruction under re-entry's high friction and temperature.

Replacing the cracked tiles would take two months, delaying all other scheduled shuttle launches, especially this year's second shuttle launch in December, as well as the construction of the International Space Station, Hale said.

Discovery and its crew of seven astronauts are due to attach to the ISS an Italian-made module during their 13 days in orbit.

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