NASA said Wednesday it is grounding future shuttle flights because foam debris that brought down Columbia is still a risk.

The space agency said the mysterious object that came flying off the space shuttle Discovery's fuel tank during liftoff Tuesday morning was a large piece of foam insulation, which might have doomed the spacecraft had it made contact.

"You have to admit when you're wrong. We were wrong," shuttle program manager Bill Parsons said at a Wednesday evening news conference. "We need to do some work here, and so we're telling you right now, that the ... foam should not have come off. We've got to go do something about that."

Engineers believe the foam piece was 24 to 33 inches long, 10 to 14 inches wide, and just a few inches thick, only somewhat smaller than the chunk that smashed into Columbia's left wing during liftoff in January 2003.

The damage Columbia sustained that day caused the orbiter to break apart during re-entry, killing all seven astronauts aboard.

NASA expected some debris to fall off during Tuesday's launch, but officials said they won't know for a few days whether any of it will mean a risk to the crew.

The loss of a chunk of debris, a vexing problem NASA thought had been fixed, represents a tremendous setback to a space program that has spent 2½ years and over $1 billion trying to make the 20-year-old shuttles safe to fly.

Parsons said, "We won't be able to fly again" until the hazard is removed.

Discovery Astronauts Begin Checks

Discovery astronauts woke early Wednesday morning and started checking the outside of their spacecraft.

They were maneuvering a sensor-equipped robotic arm to see if any damage occurred during launch Tuesday. The examination focused on the shuttle's wings and nose -- one of the new procedures put in place since the 2003 Columbia tragedy.

At an 11 a.m. EDT Wednesday news conference, flight director Paul Hill said that much of the data from the various cameras and scans has been sent back to Earth and is being distilled into engineering reports. If any areas need additional reviews, those will likely happen on the fourth day of the flight.

Hill said the process worked well, but that it was a lot of work, requiring full-time attention from three of the seven crew members. NASA hopes not to have to do as much scanning on future flights, if it can rely on other sensors.

He noted that although some small damage was reported Tuesday, the shuttle has landed many times with some thermal protection system damage, so NASA knows what can be tolerated.

In orbit, astronauts have also tested the tools and equipment they'll use during some space walks. And they're planning to get the airlock ready for Thursday's docking with the international space station.

Engineers on the ground are analyzing images and data from the launch to see if there were other incidents, NASA said on its Web site.

The crew of the orbiting laboratory will also photograph all sides of Discovery before the two link up.

Rendezvous with the station is scheduled for 7:18 a.m. EDT Thursday. Discovery will provide a new gyroscope for the station, as well as other supplies.