NASA said Wednesday that it will again ground the space shuttle fleet after analysts discovered that during the launch of Discovery the external fuel tank lost chunks of foam insulation similar to those that fatally damaged Columbia 21/2 years ago.

Officials at the space agency said the debris apparently did not hit Discovery, whose 13-day mission to supply the international space station remained unimpaired, but shuttle program manager William W. Parsons's decision to postpone future flights indefinitely was a stunning blow to the U.S. manned spaceflight program.

"Until we fix this, we're not ready to fly again," a grim-faced Parsons said in a news conference at Johnson Space Center. "I don't know if it's a month; I don't know if it's three months. We have a lot of work to do, and we'll do it."

For NASA, which is struggling to recover both prestige and expertise after the Columbia disaster, the foam loss was a major setback. Engineers had made reducing launch debris their priority before flying the three remaining shuttles again, and they had virtually guaranteed that the tank would shed only tiny pieces of insulation.

Instead, pictures taken by Discovery's crew and by a camera mounted on the orbiter showed that a trapezoidal chunk of foam up to 33 inches long and 91/2 inches wide had broken away from the tank about two minutes into Tuesday's launch. Deputy shuttle program manager Wayne Hale said that if that piece had fallen off at a lower altitude and struck the orbiter, "we think this would have been really bad."

The decision to ground the shuttle fleet cast immediate doubt on whether the shuttle Atlantis will be able to make a scheduled flight in September, and it ultimately could have serious consequences for completion of the space station and President Bush's ambitious plan to return humans to the moon and eventually send them to Mars.

Hale said analysts do not yet know how much the chunk of foam weighed, but he said it was too big, as were two seven-inch-long foam "divots" lost from the area where engineers made modifications to the struts that connect the external tank to the orbiter. It was foam from that area of the tank that doomed Columbia. Yet another piece of foam was lost from Discovery's tank in an area that is sprayed mechanically.

"None of the things I showed you are satisfactory," Hale said after displaying photographs of the damage. "The good news is that the orbiter Discovery appears to be in good shape."

He said a meticulous survey by Discovery's astronauts Wednesday of the hard but brittle reinforced carbon-carbon on the leading edges of the orbiter's nose and wings showed no significant damage, although part of the survey will have to be repeated.

Also deemed of apparently minor importance was a "chip" of ceramic thermal-protection tile that had ripped from the underbelly of the orbiter at the forward landing-gear door. Hale said analysts are also investigating a second chip of a similar size -- about 11/2 inches long.

"We have declared that the thermal protection system [is] not suspect," Hale said, referring to the tiles and the reinforced carbon-carbon. He said engineers would continue a "very thorough" analysis but would probably give Discovery the go-ahead for a normal reentry on Aug. 7.

Discovery, with mission commander Eileen Collins and six other astronauts aboard, lifted off Tuesday morning from Kennedy Space Center in Cape Canaveral, Fla. The crew will spend most of its time installing equipment, conducting safety tests, unloading supplies and picking up trash at the space station. The spacecraft was scheduled to dock with the station Thursday.

But Discovery's mission is also a trial run for the safety measures and new procedures implemented during the 21/2 years the shuttle fleet was grounded after Columbia disintegrated during reentry. A chunk of foam from the external fuel tank pierced that shuttle's leading-edge heat shielding during launch, allowing superheated gases to penetrate the wing as it plunged back into the atmosphere.

Although Discovery apparently avoided a similar fate Tuesday, the crew spent its first full day in orbit using the shuttle's new 50-foot Canadian-built sensing boom for an inch-by-inch examination of the reinforced carbon-carbon on the wings and nose.

Mission specialist Andrew Thomas attached the shuttle's 50-foot grappling arm to the new boom and sensor system, a 50-foot extension with a television camera and two laser imagers mounted at the far end.

The 100-foot span allowed the shuttle crew to get close-up images of virtually every spot on the shuttle. Thomas, assisted by pilot James Kelly and mission specialist Charles Camarda, took six hours to compete the inspection. The sensors produced startling images that allowed viewers on Earth to see screw holes and creases in the heat shielding and even read serial numbers on thermal panels.

The survey went so well, as did the crew's other planned activities, that flight director Paul Hill sounded almost euphoric early Wednesday, suggesting that the chipped thermal protection tile "didn't look like it was going to be a significant problem."

But as the day progressed, the mood shifted drastically as details of the foam debris began to come into focus.

A flying piece of something was readily visible in the launch footage taken Tuesday by a camera attached to Discovery's external tank. It turned out that the debris was foam from a "protuberance air load ramp," a thick ridge of insulation running down the side of the tank to provide aerodynamic protection -- the rocketry equivalent of a wind break -- to protect the tank's cables and pressurization lines.

Engineers had examined the ramp after the Columbia accident, and while they were not altogether satisfied, they cleared it for launch.

Shuttle program manager William W. Parsons, with his deputy, Wayne Hale, points to a model of the fuel tank during a news briefing.