Is this the crisis that ends the shuttle and throws the nation's space effort onto a new path?
Perhaps, aerospace experts say. But they emphasize that it is too early to judge the seriousness of what happened two minutes after the Discovery's liftoff: a large chunk of insulating foam broke from its external fuel tank, despite more than two years' work to prevent such a thing from happening.
Moreover, many of these experts think the remaining fleet of 1980's-era winged spaceships - Discovery, Atlantis and Endeavour - will end up muddling through, driven by decades of momentum and many billions of dollars in contracts.
In Florida alone, a politically sensitive state with blood ties to the White House, the shuttle program employs a standing army of some 14,000 technicians and engineers, managers and contractors.
"You have people speculating that this is the last flight of the shuttle," said Joseph K. Alexander, director of the Space Studies Board of the National Academy of Sciences, which advises the government on its space programs. "It seems like it's too early to tell. NASA needs more time to do more analysis of what happened before making what today would be a precipitous decision."
Even so, he added, the foam debris presents a perception of danger that raises provocative questions about the shuttle's future.
NASA's new administrator, Michael D. Griffin, and the Bush administration and Congress "realize that the sooner we can move from the shuttle to the next-generation vehicle, the better we'll be," Mr. Alexander said, adding, "So from one perspective, it's very tempting to say, 'Let's use this event as an opportunity to move on.' "
But tugging against that, he added, is all the political momentum of the shuttle program and the fact that its abandonment would force the United States to become totally reliant on Russia for access to the International Space Station. That option, he added, is quite unattractive.
Albert D. Wheelon, a former aerospace executive and Central Intelligence Agency official who helped shape the nation's early spy-satellite program, said the shuttle should be abandoned. "Its design concept is badly flawed," he said, "and the management has gotten progressively weaker with time."
The number of space-related jobs in Florida would not fall significantly, Mr. Wheelon went on, "if you start tomorrow on something new."
In Washington, Representatives Sherwood Boehlert of New York, chairman of the House Committee on Science, and Ken Calvert of California, chairman of its Space Subcommittee, congratulated the National Aeronautics and Space Administration for its "forthright way" of handling the new setback. "The foam has been the primary concern ever since the loss of the Columbia," they said in a statement. "Obviously, this problem needs to be addressed before the shuttle can fly again."
John E. Pike, director of GlobalSecurity.org, a private Washington research group on military and space topics, said the shuttle program would probably press on, hoping against more catastrophes.
The most likely assumption "is that they're going to spend a few months looking at the foam problem and decide they don't have enough time or money to get to the bottom of it," Mr. Pike said, comparing the situation to that of the faulty fuel sensor on the Discovery. After months of trying to get to the bottom of that problem, NASA finally decided to fly without understanding its origin.
"The future," Mr. Pike predicted, "will be remarkably like the past. They're up to their old tricks again in terms of fudging safety."
But Seymour C. Himmel, a retired engineer who served for more than two decades on the Aerospace Safety Advisory Panel, a group that advises NASA on the shuttle, said that the foam problem was probably fixable and that critics were too quick to foresee the shuttle's end.
"We have to learn something about this and hope we can solve it and come up with a repair that will protect us as fast as we can," he said. "Still, you've got to be cautious. You've got to not jump to conclusions. In the time they've had, I doubt they've been able to examine everything as thoroughly as it needs to be examined."
Paul S. Fischbeck, a professor at Carnegie Mellon University who warned the space agency in a 1990 report about the danger of flying debris, said the much better cameras that filmed the Discovery's ascent might be creating a false sense of danger. Perhaps in the past, he said, large chunks of foam fell off the orbiter at high altitude, unobserved.
"We've never had the camerawork that we've had on this flight," Dr. Fischbeck said. "So this might happen quite frequently. We just don't know. Not every bit of debris that comes off hits the orbiter."
He added, however, that the large piece that came off Discovery could have done real damage.
"There's only one example where a piece that size hit the orbiter, and it knocked a tile off," Dr. Fischbeck said. "I would be very concerned."