Even if the small foam fragment did hit, engineers believe the impact caused no damage of concern, deputy shuttle programme manager Wayne Hale said on Thursday.

"This is the closest to a potential hit that we have out of all the data we've got," Hale said at an evening press conference. That's why it generated "a great deal of interest," he added.

Despite the latest development, officials said Discovery still looked safe to fly home in a week, but stressed it would be another few days before the space agency could conclusively give the shuttle a clean bill of health.

Space station docking

The astronauts awoke just before midnight on Thursday, ready to continue work to unload 15 tonnes of cargo onto the space station, do some additional surveys of the shuttle and prepare for the mission's first spacewalk on Saturday.

Discovery executed a backflip
before docking

Mission Control received stunningly detailed photographs of Discovery taken by the crew aboard the international space station early on Thursday. The shuttle executed an unprecedented backflip to bare its belly to the cameras before docking with the space station.

Nasa wanted to make sure Discovery did not suffer the kind of mortal wound that brought down Columbia in 2003.

"Everything we know at this point in time, I don't see anything that would keep us from being able to re-enter," said Steve Poulos, manager of the orbiter project office.

Damage

On Wednesday, Nasa suspended all further shuttle flights after learning that a big piece of foam insulation weighing just short of a pound came flying off Discovery's external fuel tank in an alarming repeat of the problem that doomed Columbia. It missed Discovery.

The small bit of foam that may have hit Discovery's right wing came off about 20 seconds after the big piece, and was from the same general area, Hale said. None of the wing sensors detected anything unusual there, and a laser-tipped inspection boom also did not pick up any damage.

A small piece of foam broke off
the shuttle during lift-off

Camera views during lift-off 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.

"So we feel very good about this," he claimed, noting that "we're going to find the source of these problems and resolve them."

Inspection

All that remains before Nasa can clear Discovery and its seven astronauts for landing is an inspection on Friday by a new laser-tipped boom that will provide 3-D views of scraped thermal tiles on the shuttle's belly.

Nasa has stopped all launches
until the problem is solved

The 30.5 meter crane will be able to determine the depth of what looks to be surface-coating damage, said John Shannon, flight operations manager.

One of the areas of biggest interest is a chipped thermal tile near the set of doors for the nose landing gear.

If everything checks out as Nasa expects, then Discovery will be free to return to Earth on 7 August as planned, after an eight-day space station visit.

Lift-off mishap

Shuttle managers were stunned after seeing video images of the large piece of foam shooting off the fuel tank two minutes after Tuesday's lift-off. It weighed about half as much as the piece that slammed into Columbia's left wing and was irregular in shape, at 61 to 84 centimeters inches across.

"We're going to find the source of these problems and resolve them"

Wayne Hale,
Deputy shuttle programme manager 

It was not until Wednesday, after viewing more video and still images from space, that managers knew where the foam came from. The foam broke off an area meant to protect cables and pressurization lines running down the length of the 15-storey fuel tank, not even close to the location of Columbia's broken insulation.

Three smaller pieces of foam broke off the same vicinity of Discovery's fuel tank, including a 17.8 centimeters chunk that missed Discovery and the fragment that may have went into the wing.

Shuttle managers considered modifying the area after Columbia's catastrophic re-entry on 1 Febuary 2003. But they put it off because they had had little trouble with the foam there in the past, and it was a relatively easy area to check for air pockets that might cause the insulation to pop off during launch.

No more launches

Shannon said that decision was based on limited flight data. Engineers had no good tank images from more than 50 of the 113 previous shuttle launches; of the remainder, only one lift-off resulted in foam loss from that area and it was attributed to a previous repair.

"As everybody who's come up here in the last two days has said, we were wrong and we missed something and we have to go figure out what it was and go fix it," Shannon said.

"Whether that's just changing techniques or redesign, we
don't know."

Until the problem is fully understood and resolved, Nasa has decreed that no more shuttles will be launched.