NASA Chief Michael Griffin said
He said he has set up a "tiger team" to try to solve the problem as quickly as possible. "We don't expect this to be a long drawn-out affair," he said by telephone from Washington in a briefing with reporters in Houston.
Like other NASA officials, he said "we made a mistake" in not addressing the area of the external fuel tank where a piece of foam broke off shortly after Discovery's launch on Tuesday. That insulation chunk, which did not strike Discovery, came from a different spot on the huge fuel tank than the foam piece that ultimately brought down Columbia two years ago.
"Do I take responsibility? Absolutely," he said. "We'll fix this."
The first is Sept. 9-24, and the second is a couple of days in November. That is based on the Earth's orbit and the hours of daylight a shuttle could be launched so it could be photographed to watch for problems.
In addition, he told reporters there wasn't just one, but perhaps four, pieces of foam that were bigger than NASA was willing to allow break from the fuel tank during launch.
A large piece of foam doomed Columbia on its launch in 2003 by hitting the shuttle's wing. The announcement by NASA on Wednesday that a similar piece of debris had unexpectedly snapped off Discovery led to the grounding of future flights until the problem is fixed. It was a disturbing setback for the agency, which had spent $1.4 billion and 2½ years of work to make the shuttles safer.
Earlier Friday, Discovery Commander Eileen Collins told The Associated Press she was "quite surprised" to learn about foam debris that could have damaged Discovery.
"Obviously, we're disappointed to hear about this," she said in the first of a series of interviews from space with radio networks.
"Personally, I did not expect any large pieces of foam to fall off the external tank," the commander told CBS Radio. "I thought we had that licked."
However, Collins said she's confident Discovery will get her crew home safely.
Astronaut Andy Thomas, who also was interviewed, said he didn't think the foam problem is "a fatal blow" to future shuttle flights. ... It's an emotional disappointment. It's also an engineering disappointment."
Added Collins: "I don't think we should fly again unless we do something to prevent this from happening again. The shuttle is due to be retired eventually, but we've got more years in them. ... I'm not ready to give up yet."
Discovery's astronauts spent Friday morning unloading 15 tons of supplies onto the space station. They also began yet another inspection of the shuttle for damage - this one about three hours.
On Thursday, NASA reported that a smaller piece of foam may have hit a wing during liftoff.
So far, "no significant damage" has been found, Collins said. "We know we do have some small damage."
That damage, however, isn't any different from the beating shuttles often sustain on the way to orbit, she said.
"We are staying focused on the mission and we know we are in good hands with the people on the ground," Collins said. "I love being in space. It's magical up here."
NASA officials have said Discovery does look safe to fly home in a week, but stressed it will be another few days before engineers can conclusively give the shuttle a clean bill of health.
NASA analysts have identified several areas - including Discovery's wings, nose and belly - they want astronauts to take another look at. Problem areas were being examined Friday using the shuttle's new laser-tipped extension to its robotic arm.
Astronauts Stephen Robinson and Soichi Noguchi were to take a personal look at the other two areas - along the leading edges of the shuttle's wings - during the mission's first spacewalk Saturday. It will be the first of three orbital outings.
During the spacewalks, Robinson and Noguchi will replace a gyroscope, which helps steer the space station; try out new repair techniques for the shuttle's tiles and delicate carbon panels; and install a storage platform on the station.
In television images Friday, Robinson smiled and waved to the camera as he worked weightlessly with Noguchi about the station.
They wore yellow plastic hard hats for comic effect. Robinson, with a manual in his hand, wore a hat so tiny it didn't come close to covering his head.
The lighthearted atmosphere contrasted with days of sobering news. NASA suspended future shuttle flights earlier this week after learning about the big piece of foam insulation, which weighed less than a pound.
It was an alarming repeat of the problem that doomed Columbia in 2003, when a piece of foam knocked a hole in its left wing. The searing gases of re-entry melted the wing from the inside out, causing the spacecraft to disintegrate. All seven astronauts aboard died.
The small bit of foam that may have hit Discovery's right wing came off about 20 seconds after the large piece, and was from the same general area, deputy shuttle program manager Wayne Hale said.
An earlier inspection with the laser didn't reveal any damage. Camera views during liftoff were inconclusive because the foam tumbled out of sight.
NASA already has run tests showing that if the foam did strike the wing, it would have exerted just one-tenth of the energy needed to cause worrisome damage, Hale said.
If the astronauts do find a problem with the shuttle, however, the contingency plan is to consider untested repair techniques that were developed after the Columbia disaster or have the astronauts stay on the space station until a rescue mission can be launched.
"We have always had the option of staying on the space station," Collins said. "I don't think that is going to be the case for us."