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 in Houston. "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.

The launch was the first shuttle mission since Columbia disintegrated during reentry on Feb. 1, 2003, a disaster that marked a technical and emotional watershed for the U.S. space program and for NASA, which spent 21/2 years redesigning much of its hardware and reevaluating its approach to shuttle safety.

-- Guy Gugliotta

Astronauts prepared to dock Discovery with the international space station in what may be the last visit for some time after NASA again grounded its fleet.