Columbia's control system struggled to keep the craft at the proper descent angle, investigators noted.
SPACE CENTER, Houston (AP) -- NASA is casting a wider net in the space shuttle investigation now that it has essentially ruled out a theory that a breakaway piece of foam may have caused Columbia to rip apart.
Other possibilities abound, from an accidental triggering of explosive devices on board to a collision with a piece of space garbage, or perhaps a flaw in a wing that caused the spacecraft to swing out of control and disintegrate moments before it was to land.
Space shuttle program manager Ron Dittemore said every theory was being examined.
"Was it something that happened after launch? Was it something that happened during the entry? Or was it something that happened during ascent [launch] and we didn't see it? Those are all possibilities," Dittemore said at a news conference Wednesday.
For days, the investigation has centered on a 21/2-pound chunk of foam insulation that peeled off the external fuel tank during launch and smashed into the underside of Columbia's wing. The theory was that the collision damaged the thermal tiles that keep the craft from burning up during re-entry into Earth's atmosphere.
But Dittemore said a rigorous analysis concluded that the impact would not be strong enough to doom the shuttle.
"It just does not make sense to us that a piece of debris would be the root cause for the loss of Columbia and its crew," said Dittemore. "There's got to be another reason."
In one of several factors being examined, investigators have noted that during Columbia's descent, its automatic control system struggled to maintain the craft at the precise angle required for a safe return to Earth.
The autopilot caused the craft to rapidly move its control surfaces -- devices like flaps and rudders -- and eventually fired small rockets in a vain attempt to keep Columbia in proper alignment.
Dittemore said there was something on the left wing -- "what, we don't know" -- that caused it to catch the wind and drag the craft toward the left.
The autopilot added "more and more flight control muscle" in an attempt to keep Columbia pointed straight, he said, but just before the last signals from the spacecraft reached Earth, it was clear "we were beginning to lose the battle."
He suggested the drag may have been the result of a rough surface, missing tiles or something wrong with the wing.
Rise in temperature
Dittemore said engineers are also exploring the possibility that increases in temperature detected on Columbia's left side may have somehow caused the loss of control and breakup of the shuttle.
Just minutes before Columbia fell apart, a rise of 30 to 40 degrees was detected in the left compartment that houses landing gear. There was also a 60-degree temperature rise on the left side of the fuselage, just above the wing. Temperatures were lower on the right side.
Dittemore mentioned that the shuttle's landing gear compartment contains small explosives designed to deploy the gear if the normal system fails. If heat accidentally triggered the explosives, it could be catastrophic, but Dittemore suggested this was unlikely.
"Even though we have pyrotechnic devices in the wheel well, 30 to 40 degrees rise does not constitute cause for concern," he said.
Another scenario involved orbiting garbage, perhaps parts from other spacecraft. According to the theory, a collision from even a small object could have hit the shuttle like a bomb.
Dittemore said small pieces of junk have hit and damaged space shuttles in orbit, although it is a rare occurrence.
"We have seen it happen," he said. "Is it a likelihood in this case? I'm not sure. It's certainly possible."
The NASA engineer said important clues may come from the growing collection of space shuttle debris collected from Texas and Louisiana. The search for bits and pieces of Columbia has extended to California.
Investigators have created a list of pieces that would be particularly useful in solving the mystery of the disaster. They include pieces of the left wing, tiles, and voice or data recorders. None of these "red tag" items have been found, Dittemore said, but searchers are using helicopters and other aircraft to scour the thousands of miles where Columbia debris could have fallen.
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