The space shuttle Discovery has been carrying astronauts into orbit for nearly seven years, Atlantis for almost six. Next month, Columbia will celebrate the 10th anniversary of its first flight.

As NASA prepares to haul Discovery back to the hangar because of cracked door hinges, many are questioning the viability of the aging fleet and wondering if it shouldn't be put in mothballs. It's the third time in less than a year the space agency has had to pull a shuttle off the launch pad for repairs.Discovery, NASA's most traveled spaceship, was to have made its 12th trip on March 9, but the military mission is now targeted for late April or early May. Atlantis, whose door hinges seem fine, is to blast off in early April.

"Nobody has ever flown a shuttle 12 times, and nobody knows what's going to come next," said former NASA historian Alex Roland. "I'd be inclined to expect more surprises rather than less." He said he had been skeptical from the start about NASA's goal of having each shuttle last 100 flights.

Columbia's aluminum door hinges also are cracked but not so badly as those on Discovery.

The hinges on two doors on the belly of the orbiter that cover fuel lines are original shuttle parts. There are no spares because no one thought they would be needed. NASA plans to replace Discovery's cracked hinges with parts from the nearly finished shuttle Endeavour, while making more pieces.

Design, more so than age, produced the fault, said Keith Hudkins, director of NASA's shuttle orbiter division. The doors are opened and closed in the hangar more than anyone envisioned, he said.

Launch director Bob Sieck said it's inevitable "some hardware is going to require periodic replacement" though NASA would prefer to make replacements "on a planned basis as opposed to having to do it because it fails or breaks at the wrong time."

Discovery's cracks were detected Feb. 18, three days after the shuttle arrived at the launch pad. They were missed in the hangar because the two hinges on each door are not normally checked, officials said.

Engineers determined wear and tear caused small stress cracks on the lugs of the 4-foot-square doors' hinges. Those gave way to two 2-inch-long crevices, one on each door.

Not knowing exactly when or why the large cracks occurred prompted officials to order repairs Thursday, even though tests indicated the doors probably would work in space.

Both doors must shut tightly once the external fuel tank drops off shortly after liftoff. Otherwise, the shuttle could be destroyed during its fiery plunge to Earth.