SPACE CENTER, Houston (AP) -- NASA may never be able to prevent threatening chunks of insulation foam from breaking off the shuttle's fuel tank during launch, the agency's chief said Thursday, a day after future flights were ordered grounded because of the problem during Discovery's liftoff.

"We are trying to get it down to the level that cannot damage the orbiter,'' NASA administrator Michael Griffin told NBC's "Today."

"We will never be able to get the amount of debris shed by the tank down to zero," he said.

With Discovery in orbit, NASA grounded all future flights because a large chunk of foam had broken off the external fuel tank in a hauntingly similar fashion to Columbia's doomed mission. This time, NASA believes the foam missed the spacecraft, although it's being closely inspected.

"Everything that we see at this point says that the orbiter is in fact a clean bird," Griffin told ABC's "Good Morning America'' on Thursday shortly after the shuttle did a somersault maneuver to allow the crew on the international space station to photograph the shuttle's belly for signs of damage. Discovery later docked at station to deliver long-awaited supplies.

Griffin stressed in a statement late Wednesday that the current mission was a test flight and ''among the things we are testing are the integrity of the foam insulation and the performance of new camera equipment installed to detect problems."

"The cameras worked well. The foam did not," he said.

The loss of such a large chunk of debris, a vexing problem NASA thought had been fixed, shattered the euphoria from Tuesday's shuttle launch, the first in 2 1/2 years. The redesign of the fuel tank was the focal point of the space agency's $1 billion-plus effort to make the 20-year-old space shuttles safer to fly following the 2003 Columbia tragedy.

The grounding also adds to the burden on the space station, which has been relying solely on Russia's much smaller spacecraft for crew and cargo deliveries.

House Science Committee Chairman Sherwood Boehlert, R-N.Y., said NASA is handling the situation "exactly right."

"It doesn't appear that the mission is in jeopardy. Nothing is in jeopardy except the schedule. But I don't want to underestimate the seriousness of it in terms of the future,'' Boehlert said.

The three remaining shuttles are due to retire in 2010, and a new spacecraft is in the works. President Bush has a lofty plan for NASA to return astronauts to the moon by 2020 and eventually to Mars. It's unclear how the latest grounding might affect public sentiment for the space program.

The piece of foam flew off Discovery's redesigned tank just two minutes after what initially looked like a perfect liftoff, right after the booster rockets peeled away. But in less than an hour NASA had spotted images of a mysterious object whirling away from the tank.

Mission managers did not realize what the object was -- or how much havoc it would cause -- until Wednesday after reviewing video and images taken by just a few of the 100-plus cameras in place to watch for such dangers.

Shuttle program manager Bill Parsons offered no excuses, saying, ''You have to admit when you're wrong. We were wrong."

Engineers believe the irregularly sized piece of foam that came off was 24 to 33 inches long, 10 to 14 inches wide, and between 2 and 8 inches thick _ only somewhat smaller than the 1.67-pound chunk that smashed into Columbia's left wing during liftoff. The plate-sized hole let in superheated gases that caused the shuttle to break up on its return to Earth on Feb. 1, 2003.

On Discovery, the foam broke away from a different part of the tank than the piece that mortally wounded Columbia.

Atlantis -- whose own fuel tank is now suspect -- was supposed to lift off in September, but that mission is now on indefinite hold. Parsons refused to speculate when a shuttle might fly again, but did not rule out the possibility that Discovery's current mission may be the only one for 2005.

Parsons said it was unlikely Atlantis would be needed for a rescue mission, in the event Discovery could not return safely to Earth and its astronauts had to move into the space station. Discovery, fortunately, appears to be in good shape for re-entry, he said.

Wednesday's inspection of Discovery's wings and nose using a new 100-foot, laser-tipped crane turned up nothing alarming, but analysis was ongoing, Hale said.

In addition to the big chunk of foam, several smaller pieces broke off, including at least one from an area of the fuel tank that had been modified after Columbia. Thermal tile was also damaged on Discovery's belly soon after liftoff; one tile lost a 1 1/2-inch piece right next to the set of doors for the nose landing gear, a particularly vulnerable spot.

Deputy shuttle manager Wayne Hale said none of the tile damage looked serious and likely would not require repairs in orbit. Imagery experts and engineers expect to know by Thursday afternoon whether the gouge left by the missing piece of tile -- or anything else -- needs another look. The astronauts' inspection boom could determine precisely how deep the damage is, and they will probably pull it back out Friday.

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