Severed foam insulation was not considered a hazard or a high-priority problem even though it broke off during a space shuttle launch just a few months before Columbia's doomed flight, a NASA expert said Monday at a hearing into the accident.

James Halsell, an astronaut who worked as a launch manager, told the accident investigation board that the October incident did not appear to be a reason to halt shuttle flights. On this and earlier launches, foam had not struck sensitive parts of the shuttle.

In the Atlantis mission, the foam that fell during launch struck the bottom of one of the strap-on booster rockets, doing no harm to the shuttle itself. So Halsell and other managers proceeded with the shuttle Endeavour's flight in November and Columbia's in January.

"How in the world do you determine, how does the system determine that there's no safety of flight" concern, asked Harold Gehman Jr., the board's chairman and a retired Navy admiral. "Is it judgment?"

Halsell replied that NASA felt comfortable there was nothing new or generic about the problem and that no changes or weaknesses in the application or manufacturing of the polyurethane foam were made that would have introduced errors.

For 10 years before the Atlantis launch, no foam debris had fallen from this area of the fuel tank. Because of that and the fact that changes to the foam were made in the interim, engineers thought the trouble was solved.

But during Columbia's launch on Jan. 16, a 2-pound chunk of foam ripped off the fuel tank and slammed somewhere along the leading edge of the left wing. Investigators are trying to determine whether that damage could have been severe enough to doom the spaceship upon re-entry.

"Did you ever think it was possible to pop a big enough piece of foam off of this external tank to severely damage the shuttle itself?" Air Force Maj. Gen. Kenneth Hess, a board member, asked a NASA fuel tank expert.

"The answer is yes. We have large areas ... that are hard to spray," replied Lee Foster, a longtime Marshall Space Flight Center employee. "Yeah, we were always worried there's going to be a big piece that comes out."

Hess said he is perplexed about why NASA considered the foam problem, up until the Columbia catastrophe, to be a maintenance rather than a safety issue. That's because most of the pieces that came off were small, explained J. Scott Sparks, a NASA department head for fuel tank issues.

A new member of the investigation board, Sally Ride, the first American woman in space, asked Halsell if NASA would have done anything differently to address the foam problem if it had been considered an in-flight anomaly.

"Nothing," Halsell replied. "It would have made no difference," since shuttle managers had already ordered engineers to study the problem and fix it, although not before the next shuttle flight.

The problem was discussed at length at the flight review for Endeavour in November, Halsell said, and everyone felt comfortable the risks were sufficiently understood and the shuttle safe to fly.

As it turned out, no foam from this area came off Endeavour's fuel tank.

In Columbia's case, however, the foam may have caused a breach in its left wing that let in deadly hot gases, causing the spacecraft to break apart as it re-entered the atmosphere on its return home Feb. 1.

Two board members pressed Halsell on whether Columbia's flyaway foam would have been designated an in-flight anomaly if the shuttle had returned safely to Earth. "Yes, absolutely," Halsell replied. He said at that point there would have been no doubt that "what you now have in your hands is a major issue that has to be dealt with."

NASA knew from the beginning of the shuttle program that significant damage to the thermal protection system — the heat-protecting tiles and special carbon panels on the wing's edge — cannot be "fail-safe." In other words, there was no backup system and the ship and its crew could be lost.