One assumption too many?

At some moment during the space shuttle Columbia's last mission, when it was gliding hundreds of miles overhead on a seemingly untroubled orbit, mission control made an assumption.

They knew that a piece of foam had struck some part of the left wing during liftoff. But with no photographic evidence or on-board data to suggest serious damage, they assumed the vehicle would be OK - in part because it had survived similar incidents before.

That moment of decision, critics say, encapsulates how much the culture of NASA has changed since the days of moon shots and Mercury astronauts - and how deeply the space agency must now reform.

When the Columbia Accident Investigation Board releases its report on the causes of the Columbia disaster, perhaps as soon as this month, its most significant findings will not be about foam or bipod ramps, sources say. Rather, it will address NASA's ebbing sense of vigilance.

Sixteen years ago, the Challenger explosion exposed that NASA officials were accepting as routine various risks that might not have been countenanced by their predecessors. Earlier this week, a test that recreated the foam collision - and blew a 16-inch hole in a shuttle wing - provided the strongest evidence yet that little had changed by February.

"The question is whether Columbia and Challenger are significant enough to cause a 180 degree turn in their way of thinking," says Howard McCurdy, a NASA historian at American University in Washington. "There will be a turn, but will it be persistent enough?"

Whether that happens will become evident in coming months as NASA faces two challenges: the investigation's report and a foam problem that has existed since the first days of the shuttle. Although the foam has been a leading culprit since the hours after Columbia's disintegration over Texas on Feb. 1, there was no conclusive proof until Monday.

In a test, a chunk of foam believed to be roughly the size of the one that hit Columbia was fired at a replica wing. The damage was so severe it left investigators with little doubt - and NASA with explaining to do.

Shuttle program managers' assumption that the foam caused no critical damage was based on past experience with foam strikes and a statistical analysis. But when several engineers petitioned administrators to take satellite and telescope photos of the shuttle in hopes of gathering more data, the request was ignored.

The contrast with the past, when the prevailing culture assumed the worst and prepared for it, was stark. "In the absence of data, the assumption [this time] was that the flight is safe," says Dr. McCurdy. "It's a subtle change, but it's totally contrary to the original NASA. If you didn't have the data [then], you went out and got it."

NASA's budget has certainly played an important role in the shift. Since the days of Apollo, its share of the federal budget has shrunk by more than 80 percent, stretching the agency thin.

But NASA has also been a victim of its own rhetoric. The shuttle was originally touted as a vehicle that would make spaceflight routine, commuting to space dozens of times a year. In reality, it has turned out to be temperamental and complex technology requiring costly and lengthy maintenance. Yet, for some, the perception remains that the shuttle is far more reliable than it perhaps is.

"Properly managed, the shuttle is adequately safe," says John Logsdon, a member of the investigation panel. "But it requires constant vigilance, adequate budgets, and constant skepticism."

That skepticism, he suggests, has waned as the pressure to get shuttles on orbit frequently has grown - first to fulfill the original promises and now to supply the International Space Station. Indeed, it is the $24 billion space station - which largely depends on the shuttle for crew and equipment - that is pushing NASA to quickly resume the shuttle program, perhaps as early as December.

First, however, it must fix the foam. Since the shuttle's birth in the 1970s, foam insulation designed to keep ice from forming on the super-cooled fuel tank has been shearing off. One concerned chief project engineer went so far as to attach shuttle tiles to his car and skitter into a piece of foam at 130 m.p.h. just to see the damage.

Gradually, shuttle managers would come to see foam strikes as normal. Though engineers repeatedly tried to solve the problem, nothing worked. One estimate suggests foam has hit and damaged shuttles nearly 14,000 times since the program began in 1981. In one instance, scientists found 308 gouges after a single 1997 flight.

Such statistics are not likely to be accepted any longer. Yet some engineers wonder if the problem can be eliminated entirely.

"There's one-third of an acre of foam on that tank," says engineer James Odom, who helped develop the tank in the 1970s. "No question there can be some improvement in the big pieces coming off, but it will be very difficult to get it to zero."

Most agree that that is a good start. Large pieces a few inches in diameter, like the one that eventually brought down Columbia, are seen as the greatest threat. Some, however, go further. They wonder whether the shuttle's shortcomings are finally being exposed, and whether NASA should begin to look at its white-winged, high-tech marvel the same way it looked at the 1960s Mercury rockets that had a 50 percent failure rate.

Says Dr. Logsdon: "Organizational change is one of the hardest things to do in the world."