HOUSTON — Preliminary consideration of various possibilities has not yet pointed to one likely cause for the breakup of the space shuttle Columbia, NASA engineers told the accident investigation board Tuesday.
During its third public hearing, the board heard from engineers who are reconstructing the aerodynamics and thermodynamics of the shuttle's re-entry and trying to pinpoint what damage might have occurred and when it would have happened.
"You've asked the $64,000 question," Steven Labbe, chief of the Applied Aeroscience and Computational Fluid Dynamics Branch at Johnson Space Center, told the board when asked what caused the shuttle to break apart over Texas on Feb. 1, killing all seven astronauts.
The board suspects the left wing's heat-shielding tiles were breached, possibly by insulating foam or other material falling from the external fuel tank, when Columbia launched on Jan. 16. The breach could have let hot atmospheric gases penetrate the left wing during re-entry.
Initial wind tunnel tests on various possible types of damage — including holes in the wing's leading edge, missing reinforced carbon panels on the vehicle's body and a gouge in the main landing gear door — have so far not duplicated the shuttle's catastrophic failure, Labbe said.
"We're going to be looking at multiple panels missing, where our future work will focus on. We'll do a survey of the wing leading edge and look at other scenarios," he said. "These are very preliminary results. It's premature to draw too many conclusions from these results. We're just getting started on this assessment."
Tuesday's hearing wrapped up after John Bertin, a professor of aeronautics at the Air Force Academy, told the board about the aerodynamic and thermodynamic forces a shuttle faces during re-entry.
On Monday, the board heard from shuttle officials and an expert on spacecraft re-entry, who all said a crucial clue to solving the accident could be in a piece of debris yet to be discovered in the western United States.
"If we can locate some of this (Western debris) ... that's going to make us immensely smarter on exactly how the failure started in the first place," said NASA flight director Paul Hill, who is leading debris recovery efforts in the West.
While there have been reports of the shuttle shedding debris as it flew across California to New Mexico, so far no pieces have been found west of Lubbock, Texas. Finding debris in the Southwest remains a top priority, Hill said.
Doug White, with NASA contractor United Space Alliance, told the board on Monday that sensors in the left wing failed one by one as wiring burned or simply shorted. This caused unusual temperature and pressure readings and sensor dropouts.
Yet the shuttle continued to fly "like a champ right up until the breakup," even as it was shedding debris, Hill said.
NASA has been helped in its effort to track down debris by the thousands of eyewitness reports and amateur videos shot by people who watched the shuttle on its ill-fated landing day.
Hill showed the board a short composite of 15 to 20 amateur videos. Debris could be seen coming off 16 times. On two occasions, a flash preceded the falling debris, which may have been burning.
"Without the public having taken these pictures on their own . . . we wouldn't know any of this," Hill said. "These people are definitely our heroes."
An expert in spacecraft re-entry, William Ailor of the Aerospace Corp., told the board that the debris shed the earliest by Columbia would yield the best information about what happened.
On the Net: Columbia investigation board: www.caib.us