In its first significant determination, the accident investigation board announced that heat damage from a missing tile would not have been sufficient to cause the unusual temperature increases detected inside Columbia minutes before it disintegrated. Sensors picked up an unusual heat buildup of about 30 degrees inside the wheel well before the accident.
The board did not specify whether a breach could have been the result of a structural tear in Columbia's aluminum frame or a hole from debris striking the craft. The board also did not indicate when the breach might have occurred during the shuttle's 16-day mission.
Officials have previously focused on an unusually large chunk of foam that broke off Columbia's external fuel tank during liftoff. Video footage showed that it struck part of the shuttle's left wing, including its toughened leading edge and the thermal tiles covering the landing gear.
The announcement focused renewed attention on possible catastrophic failures inside the wheel compartment that might have contributed to the Feb. 1 breakup that killed seven astronauts.
Officials are not sure where a breach might have opened in Columbia's skin, said NASA spokesman James Hartsfield. But he said the leading edge or elsewhere on the left wing, the fuselage or the left landing gear door were prime candidates.
"Any of those could be potential causes for the temperature change we saw," Hartsfield said. "They do not and have not pinpointed any general location as to where that plasma flow would have to originate."
In an unusual plea for assistance, NASA officials urged Americans yesterday to share with them any photographs or videotapes of Columbia's descent from California to eastern Texas. Some members of the public have already handed over images, "but more material will help the investigation of the Columbia accident," the space agency said.
The board's announcement did not surprise those experts who have long believed that a mysterious failure of sensors within Columbia's left wing indicated that super-hot plasma had penetrated the shuttle.
"I think there was a substantial hole in the wing," said Steven P. Schneider, an associate professor at Purdue University's Aerospace Sciences Laboratory. "That would not be at all surprising. All the sensors in the wing failed or gave bad readings" by the time ground controllers lost contact with Columbia, he said.
The board dismissed suggestions that Columbia's left landing gear was improperly lowered as the spacecraft raced through Earth's atmosphere at more than 12,000 miles per hour. NASA disclosed earlier yesterday that a sensor indicated that the gear was down just 26 seconds before Columbia's destruction.
If Columbia's gear was lowered at that speed - and in those searing temperatures as the shuttle descended over Texas from about 40 miles up - the heat and rushing air would have sheared off Columbia's tires and led quickly to the spacecraft's tumbling destruction, experts said.
Officials said they were confident that the unusual sensor reading was wrong. Tires are supposed to remain raised until the shuttle is about 200 feet over the runway and flying 345 miles per hour.
Two other sensors in the same wheel compartment indicated the gear was still properly raised, they said.
While Columbia's piloting computers began almost simultaneously firing thrusters struggling to keep the wings level, officials said a mysterious disruption in the air flow near the left wing was not serious enough to suggest that the shuttle's gear might be down.
The investigating board concluded that its research "does not support the scenario of an early deployment of the left gear."
NASA also confirmed that searchers near Hemphill, Texas, about 140 miles northeast of Houston, recovered what is believed to be one of Columbia's radial tires. A spokesman was not immediately sure which of the shuttle's six tires was found.
The tire was blackened and had sustained a large split across its tread, but it was impossible to know whether the tire was damaged aboard Columbia or when it struck the ground, the NASA spokesman said.
The board's announcement came one day after NASA released e-mail showing that midlevel safety engineers in Virginia and Houston had considered the risks of tires bursting inside Columbia's belly from heat damage.
Robert H. Daugherty, responding to an inquiry from the Johnson Space Center in Houston, cautioned in one e-mail message that damage to insulating tiles near the landing gear door could cause one or more tires inside to rupture, perhaps ending with "catastrophic" failures that would place the seven astronauts "in a world of hurt."
Harold Gehman, the retired admiral who heads the panel investigating the Columbia accident, called Daugherty's concerns "one of the many, many interesting leads that we have."