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."

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