Two and a half years after the Columbia tragedy and 500 million dollars to fix the problem that caused it were not enough to prevent foam from falling off Discovery, which recently returned from space to the relief of many.

When Discovery was launched on July 26, its huge external fuel tank shed pieces of insulating foam like the one that ultimately downed the Columbia, killing all seven astronauts aboard.

On Wednesday, NASA said that the fuel tank needs to be redesigned, scratching a planned launch of Atlantis in September.

If the shuttle becomes too difficult to repair or too expensive to fly, many experts, former NASA engineers and members of Congress want to rethink current plans to retire the shuttle in 2010 -- and possibly even mothball the three surviving shuttles immediately.

"When your design stinks, Engineering 101 says admit your mistakes and go back to the drawing board," said retired NASA engineer Homer Hicham.

"The space shuttle is ... never going to be reliable no matter how much money, time and engineering careers your throw at it. Let's put the shuttle on the shelf right away and give engineers the gift of designing new ships to carry humans into space," he said.

Roger Pielke, director of the Center for Science and Technology Policy Research at the University of Colorado, Boulder agrees.

"NASA is rolling the dice with the future of the US space program by continuing to hold unrealistic expectations for shuttle performance.

"If history is any guide, then we should expect that the shuttle will fly less and cost more than we think. It is unclear what would be gained by continuing to fly the shuttle rather than moving to the next phase of US space policy sooner rather than later," he said.

He would also involve Congress in a debate on the future of space exploration.

Legislators have been raising clear warnings.

"There is going to be no room for margin of error in terms of flying again if there is not a high level of confidence that the problems we know about are solved," said Representative Bart Gordon, leading Democrat on the House Science Committee.

His Republican counterpart, Sherwood Boehlert, who chairs the committee, said the tipping point comes when problems take too long to fix.

"Then we have to rethink everything. Maybe the shuttle will be no more," he said.

The shuttle's last flight is actually slated for 2009. To have a replacement on line by 2011, NASA wants to launch a three-astronaut capsule, reminiscent of the Apollo program that reached the moon, but launched atop a rocket derived from the one that lifts the shuttle. Cargo would launch separately.

"We have ways to construct such vehicles using shuttle solid-rocket motors and external tanks and shuttle main engines," said NASA Administrator Michael Griffin.

"We think the existing components offer us huge cost advantages as opposed to starting from a clean sheet of paper. That's what I have proposed doing," he said.

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