Some critics are casting the space shuttle’s latest setbacks as evidence it may be too costly and too risky in the twilight of its life.

But NASA and its backers are steeling themselves against any effort to retire it early.

“Maybe it’s time to consider that this is the last straw, because the shuttle is enormously expensive. It’s a risky vehicle. It’s an old technology,” said Louis Friedman, who directs the prominent space interest group known as The Planetary Society.

“The shuttle is an amazing vehicle, and it can do things in space that no other asset … can do,” countered NASA flight operations manager John Shannon. “No one is folding their tents.”

NASA managers who sent the shuttle back into space this week discovered it is still shedding big pieces of foam insulation on launch, and they suspended future flights. One chunk captured on camera was almost as big as the one that banged into the heat shield of Columbia’s wing during the previous shuttle launch 2 1/2 years ago, dooming the craft and its seven astronauts.

The shuttle Discovery and its crew of seven, which was busy this week on a supply mission to the international space station, was apparently spared any serious damage during its launch Tuesday, NASA said after viewing additional photos of the orbiter Thursday.

Astronauts Eileen M. Collins, mission commander, Stephen K. Robinson, left, mission specialist, and James M. Kelly, pilot, aboard Discovery on Thursday.

One more major inspection of the spacecraft, most likely on Friday, is needed before NASA can clear Discovery for its return home on Aug. 7.

However, frustrated NASA leaders, who have labored since 2003 to limit potential damage to the shuttle’s thermal covering, were forced to ground the fleet indefinitely to study more changes. The agency has already poured $1.4 billion into trying to make the shuttle fleet safer.

“Maybe the money would be better spent on replacing the shuttle, rather than flying it,” suggested John Pike, who directs the defense and space Web site Globalsecurity.org.

As it is, the almost 25-year-old shuttle program is supposed to end around 2010. NASA is hurrying to develop a new craft capable of going back to the moon and perhaps beyond, but that will probably take years. This same craft would go to the space station, too, at first, and be modified to go on to the moon and elsewhere.

Pike contended that the moon – beyond the reach of the shuttle – presents a bolder goal. He said the international space station might be built out by other means, without the heavy lifting of the shuttle.

But such alternatives are by no means assured. “No shuttle, no station!” snorted Doug Brown, whose brother David died on Columbia. He said the shuttle is worth overhauling, if it can be done: “They may have to take a different approach … but I wouldn’t give up so easily.”

An image from the International Space Station shows Discovery as it performs a backflip to allow detailed photography of the Shuttle's heat shield Thursday.

Others are unsettled by any prospect of retiring the shuttle now and breaking American agreements to help international partners finish assembling the station. Japanese and European research labs are waiting on the ground for shuttle missions to hoist them to the space station.

“I think there would be symbolic damage to our image as a leading and competent country that can take on challenging commitments and carry them out,” said John Logsdon, a space analyst who helped NASA investigate the Columbia disaster. “Let’s not overreact.”

Several space experts said the station is needed to keep a hand in space travel and advance understanding of how humans cope with protracted stays there.

“Our manned space program has come almost to a dead stop in the last 2 1/2 years,” complained retired astronaut Owen Garriott, who helped write a Planetary Society report last year on the future of human space flight. “The space station has been able to keep going, but it’s really just limping along.”

Other NASA supporters said the shuttle is worthwhile because it is an important step in the human journey beyond the planet’s confines. “If Galileo gave up on the development of the telescope, astronomers would not have a career, and for Americans to give up on the development of space transportation systems would end the careers and future of many, many people,” said Randy Avera, a former NASA engineer who has written two books on the shuttle fleet.

Right now, the fleet provides work for about 18,000 at NASA and its contractors. The grounding has already stirred some jitters about their future. “It’s very depressing. We’re concerned about our jobs,” said Mike Berger, an inspector at a plant near New Orleans that makes the shuttle’s insulation.

Several political leaders, including President Bush through his press secretary, backed the grounding and voiced continued confidence in NASA’s handling of the shuttle. U.S. Sen. Bill Nelson of Florida, who once flew on the shuttle himself, said “they ought to get in there and figure out the quickest fix, so we can get back flying the space shuttle.”

But James Van Allen, a retired physicist who advised NASA and did research on unmanned flights, said the grounding just reinforces his longtime view that the shuttle buys precious little scientific knowledge – for a ton of money.

“It’s a vastly difficult effort with almost no significant purpose,” he said.