Pieces of broken bolts that release the external fuel tank from the rocket after liftoff could damage the orbiter, jeopardizing the mission and the astronauts aboard.

"It has the potential of being catastrophic in the future," board member John Barry said.

The three remaining shuttles have been grounded since the Columbia accident.

NASA wants to be sure the craft are safe before flying them again, perhaps by early 2004.

But "it's a possible return-to-flight issue that we are examining," Barry added.

The culprit is a catcher, designed to prevent the pieces from hitting the craft after the bolts explode. That explosion allows the separation two minutes after liftoff.

The bolts split into two pieces weighing about 20 kilograms (44 pounds) each.

"The bolt catcher is not as robust as we would want," chairman Harold Gehman said.

The board has already found that a piece of insulating foam that fell from Columbia's fuel tank on takeoff led to the ship's breakup February 1.

However, the board members said, the bolt problem is not linked to the Columbia accident.

That was caused by a hole the foam opened in the left wing on takeoff. On reentry, the fissure allowed superheated gases to enter and weaken the structure as the craft hurtled toward Earth. Seven astronauts were killed.

SPACE.WIRE