HOUSTON -- A spray of molten aluminum found on much of Columbia's debris and burst tires from the left landing gear are providing concrete clues for shuttle accident investigators a month into their probe.
They are not yet ready to say, however, whether the scorching gases of re-entry entered the left wing at the vulnerable leading edge or through the landing gear compartment.
"Everybody has their own theory," Roger Tetrault, an investigation board member, said Tuesday. "I'm going to be patient and not express my theory at this time."
During the board's weekly news conference, Tetrault and other members revealed that molten aluminum was found on Columbia's thermal tiles and even inside the leading edge of the left wing, bolstering the theory that the shuttle was destroyed by hot gases that penetrated a damaged spot on the wing.
Tetrault said he suspects the melting of the spaceship's aluminum frame occurred because of the intruding gases and also because of the intense heat of falling through the atmosphere.
The melted aluminum, or slag, looks like black soot, and is present on both the right and left sides of the spacecraft, especially the left, he said.
"Many of the tiles on the left side have a thin, black deposit on them, and that deposit has never been seen on any previous flight," he said.
Tetrault said both tires from the left main landing gear also show evidence of extreme trauma from the Feb. 1 disaster: They are flat with torn fabric, possibly from a rupture in the final seconds of the spaceship's flight.
"I would not speculate that it blew out the door or blew down the landing gear and that caused the accident. It is the result, not the initiating event," said Tetrault, a retired corporate executive with experience in nuclear submarines. He noted that rupturing tires "could have been the ultimate breakup event - but we don't know that."
Investigators have theorized that foam or other debris that broke off the shuttle's big external fuel tank during liftoff Jan. 16 damaged the wing - perhaps the leading edge, perhaps the area around the wheel well - and allowed hot gases to penetrate the wing and destroy the shuttle.
All seven astronauts were killed when their ship shattered over Texas, just minutes before their planned Florida landing.
More than 32,000 pounds of Columbia wreckage has been collected so far, representing about 13.7 percent of the returning spaceship. None of the debris is from the left landing gear door; most of the right gear door, however, has been recovered.
The board's chairman, retired Navy Adm. Harold Gehman Jr., said the tires provide a true comparison of both sides of the shuttle.
"Right now, we've got all these random pieces and we're seeing all of these marks and chars and destruction," Gehman said. "It will be useful to us when we get an identical piece from the right wing and the left wing, and we can see that there's a difference in how they look. ... That's why the tires are so important."
What is particularly intriguing, Tetrault said, is that deposits of stainless steel were found along with molten aluminum on the inside of one of the carbon panels that protected the leading edge of the left wing. "How do you get that stainless steel and the aluminum up onto the back edge ... when, in fact, that stainless steel is behind the area," he said, adding that maybe it had something to do with the tires rupturing.
A sizable hole - 4 inches by 2 inches - was found burned into a left inboard elevon actuator, likely the result of the heat from re-entry, Tetrault said. Traces of hydraulic fluid that leaked from that hole surprisingly showed no significant overheating, he added.
"We have more questions than answers right now. But we're getting smarter fast and I believe that there's a very good chance that we will, in fact, be able to localize the breach that occurred in the left wing," Tetrault said. "Until we have determined that location of the breach, every postulated cause of the accident is really just a theory."
He added: "What we have to do is follow the heat."
Gehman said the board will delve into what role NASA management and the agency's institutional culture played in the tragedy. But he said it is more important, for now, to find out what caused the accident.
"You've got to remember that at this point in the Challenger investigation, they knew what went wrong and so the review of who did what to whom and who did his job well and who didn't do a job well was relatively fairly focused," Gehman said. "I'm really not interested ... without any particular focus or without any particular reason, for just casting about and casting some big chill over NASA."
The investigation board will hold its first public hearing on Thursday, at a college campus just a few miles from Johnson Space Center. Among the witnesses will be NASA's shuttle program manager, Ron Dittemore.
Tuesday's news conference also was held off NASA property, for the first time, in an attempt by the board to distance itself from the space agency.
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Columbia accident investigation board: http://www.caib.us