NASA said Wednesday that the mysterious object that came flying off Space Shuttle Discovery's (search) fuel tank during liftoff was a sizable chunk of foam insulation — the very thing that doomed Columbia.

But this time, fortunately, it didn't hit the spacecraft.

Space agency officials also said that a chipped thermal tile on Discovery's belly does not appear to be a danger, and it cautioned the public against overreacting to every speck of damage sustained by the shuttle during liftoff.

NASA (search) expected some debris to fall off during launch. The big question is whether any of it will mean a risk to the crew. The answer is still a few days away, NASA said one day after the ship blasted off on the first shuttle mission since the Columbia tragedy 2 1/2 years ago.

Flight director Paul Hill (search) said it is understandable that people inside and outside the space agency might be alarmed by any hint of damage to Discovery's thermal shielding.

"The last flight ended in catastrophe and we lost seven friends of ours because of damage," Hill said at a news conference. But he added: "We don't make decisions in spaceflight based on that type of emotion. We make decisions in spaceflight based on the data, and we're looking at the data."

And based on what they have seen so far, NASA engineers believe the broken tile is "not going to be an issue," Hill said.

Imagery experts and engineers expect to know by Thursday afternoon whether the gouge left by the missing 1 1/2-inch piece of thermal tile needs a second look or, in the worst case, a repair, Hill said. The astronauts have a 100-foot, laser-tipped crane on board that could determine precisely how deep the gouge is.

The tile fragment broke off less than two minutes after liftoff Tuesday and was spotted by a camera mounted on the external fuel tank. It fell off a particularly vulnerable spot, near the set of doors for the nose landing gear.

Multiple cameras also captured the chunk of foam flying off the tank but missing the shuttle. It broke away from a different part of the tank than the piece that mortally wounded Columbia by striking its wing. After the accident, the tank was redesigned to reduce the risk of foam insulation falling off.

If NASA decides to use its new inspection tool to get a 3-D view of the tile damage, the astronauts will examine the spot on Friday, a day after docking with the international space station.

On Wednesday, Discovery's astronauts spent nearly six hours using the boom to inspect Discovery's wings and nose cap for launch damage. The wings and nose are protected by reinforced carbon panels capable of taking the brunt of the searing re-entry heat.

Hill said he saw nothing immediately alarming during the laser inspection, which had been planned long before any damage to Discovery was detected. But NASA's experts have yet to fully analyze the images.

The inspection was conducted in extra-slow motion, a mere three feet per minute, to give engineers a good long look. The boom came within five feet of the shuttle's wings and nose cap.

The astronauts had to be careful not to bang the equipment into the fragile thermal panels and cause the kind of disaster the boom was designed to prevent. The task required such precision that three of the astronauts took turns performing the grueling job.

NASA should have a better grasp of the tile damage after the two space station residents photograph the approaching Discovery on Thursday. Discovery will do a slow back flip 600 feet out, so the station astronauts can zoom in on the shuttle's belly. This unprecedented maneuver was also planned long before the flight.

The photos taken from the space station should be so good that "you will almost be able to read the serial numbers on the tiles," Hill said.

After that, if the imagery experts and engineers want even more data on the broken tile, Hill said, "then by God we're going to take the (boom) down and we're going to get them more data and that data are going to look like they were sitting right there in front of the tile with their hands on it, it's going to be so good."

NASA does not expect to make a final decision until Sunday or so on whether Discovery can safely return to Earth. That is how long it will take to analyze all the data from the more than 100 cameras that tracked the liftoff, scores of sensors embedded in the shuttle wings, the laser inspection, and pictures from space.

Top NASA managers have stressed for months that they would probably see more debris than usual falling from Discovery simply because they would be looking harder this time.

Hill also reminded reporters that space shuttles have frequently landed with tile damage over the past 24 years. The seriousness depends on how deep the gouges are and how thick the tile is in the affected area, he said.

Deputy shuttle program manager Wayne Hale portrayed the current analysis as vastly superior to what took place during Columbia's mission in 2003. A chunk of fuel-tank foam insulation pierced Columbia's wing at liftoff and left a plate-size hole that proved fatal during re-entry two weeks later.

"A few people looked at the pictures, a few people ran some small analysis that wasn't grounded in much real science and came to the wrong conclusion," Hale said. This time, he said, hundreds of people are examining every frame of the video, and NASA management is focusing on whether the shuttle is safe to return.