HOUSTON — The Columbia accident investigators are looking into the possibility that unusually strong wind shear, a change in boosters, or perhaps spacecraft age contributed to the shuttle's destruction.
The wind shear experienced by Columbia 62 seconds after liftoff may have weakened its left side, the investigation board said Tuesday. About 20 seconds later, foam or other material from the external fuel tank broke off and slammed into the leading edge of the left wing.
"What we're really looking at is a complex failure of a complex system," said retired Navy Adm. Harold Gehman Jr., the board's chairman. "It's possible that the foam striking a healthy orbiter would not have done enough damage to cause the loss."
But Gehman noted that Columbia may have been "unhealthy" because of the wind shear, aging or other factors.
"A normal event, which she could have survived at age 10, maybe she couldn't survive at age 21," he said.
Gehman said he and others are trying to determine "whether or not NASA was alert enough, whether or not they were doing all the right things" to detect aging as a threat to the shuttle fleet.
The leading theory is that damage to the left wing allowed hot gases to penetrate the shuttle and destroy it over Texas on Feb. 1, killing all seven astronauts. The investigators indicated the wing may have been made more vulnerable to damage from debris because it was buffeted by unusual wind shear.
The wind shear was within NASA's safety limits, but it was the strongest gust ever seen so close to the point when the shuttle is exposed to the maximum aerodynamic stress of liftoff, the board said. That point occurs around one minute into a flight.
The board also revealed that the fuel tank had been removed from its set of booster rockets last August. The two boosters were needed for another flight. In November, Columbia's tank finally was attached to another pair of boosters.
Air Force Maj. Gen. John Barry, a board member, said the removal of the boosters may have introduced problems or weaknesses in the spot where the foam ultimately came off.
A visual inspection of the suspect foam area, called the bipod, found nothing wrong, but he said more testing may have been required to find any cracks or other flaws.
"Our question really is ... is it adequate enough to just do a visual inspection," Barry said.
He said he and his colleagues are pursuing a "follow the foam" strategy in hopes of ascertaining whether that alone — or in conjunction with other things — damaged the protective heat panels on the leading edge of the left wing enough to allow the hot gases of atmospheric re-entry to penetrate 16 days later.
"You have to address the issue of aging spacecraft in an R-and-D environment," Barry said, referring to research and development. "We've never been there before."
Barry said he is ordering an analysis of the heat-resistant carbon panels that line the leading edge of the wings, to determine the thoroughness of testing between flights. It is possible that voids could have developed under Columbia's panels, because of oxidation eating away at the carbon, he said.
"Think of termites," Gehman said.
During six other shuttle flights over the past decade, the wing panels were damaged for a variety of reasons: impact, micrometeorites, scraping, bad metal seals.
One of the many questions, Barry said, is whether the panel inspections were good enough given the age of the shuttle fleet.
Columbia made the first shuttle flight in 1981.
Board member Sheila Widnall, a former secretary of the Air Force, said recent wind tunnel testing of a shuttle model shows that more than one of the wing panels would have had to be missing to explain the flight characteristics observed in Columbia's final moments. In fact, she said, four or five panels were probably missing.
As of this week, more than 28,000 pieces of Columbia had been collected and sent to Kennedy Space Center in Florida. That amounts to 39,300 pounds of wreckage, or about 18 percent or 19 percent of the descending shuttle, Gehman said.
This week, NASA also released the minutes of its mishap response team meeting that was held the morning of Feb. 1, shortly after the accident. Already, shuttle officials had requested a reanalysis of the foam debris issue and seemed to be focusing on the left wing. During the flight, engineers concluded and managers agreed that any damage caused by the debris during launch would not threaten safety.