NASA still needs to find where a bit of foam that broke off in July's launch came from.

And, X-rays have revealed small cracks in the insulating foam.

The shuttle external tank program's manager said the cracks could have contributed to the problem.

"It would be premature to say that the cracks played a factor in that, but they might have," John Chapman said.

He said nine fine cracks were found in the foam of a fuel tank attached to the Discovery before its July flight, but which was replaced for other technical reasons.

Discovery's tank still lost a piece of foam, which struck the delicate ceramic tiles that make up the craft's re-entry heat shield.

Wayne Hale, deputy manager of the Space Shuttle Program, also said that in any case, the pace of the tank repairs would determine the launch schedule, which is tentatively set for May.

"We have not set (May) as the flight date. We have rather set our schedule to work toward that date," Hale said.

"The technical progress will drive the schedule not the other way around... We are committed to do it safely, returning to flight as soon as is practical to do so but in a safe manner," he said.

He added that the shuttle program was set back three months because of Hurricane Katrina, which flooded its Michoud site used to build the external tanks.

NASA decided to ground its three remaining shuttles after the Discovery's July flight. That was the first flight since the 2003 downing of space shuttle Columbia, killing all seven aboard.

NASA earmarked 1.5 billion dollars to fix the crumbling foam that prevents the build-up of ice on the outsides of the gigantic tanks of liquid oxygen and hydrogen, which fuel the takeoff.

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