A devastating puncture that allowed hot air inside the left wing may be the reason behind the disintegration of space shuttle Columbia on February 1 that killed Kalpana Chawla and six other astronauts.
Investigators, in a first significant determination on the cause of the crash, said Columbia broke up during re-entry possibly because of the presence of super-heated air or plasma inside the left wing.
The new theory for the crash of the space shuttle effectively rules out the loss of heat resistant tiles as was initially put forth as a possible cause for the disaster.
"Heat transfer through the structure as from a missing tile would not be sufficient to cause the temperature indications seen in the last minutes of flight," NASA spokesman James Hartsfield said.
The Columbia Accident Investigation Board was now studying various scenarios that could cause a breach allowing plasma, that surrounds a spacecraft during re-entry, into the wheel compartment or elsewhere in the wing, he said.
Moments before Columbia disintegrated, mission control in Houston detected an unusually high heat build-up in the shuttle's left wing, which could have indicated missing or damaged tiles.
Officials have previously focused on an unusually large chunk of foam that broke off Columbia's external fuel tank on liftoff.
Video footage showed it struck part of the shuttle's left wing, including its toughened leading edge and the thermal tiles covering the landing gear.
Scientists, however, say the loss of a heat-resistant tile could not have produced such an unusual high temperature.
The board also dismissed suggestions on the shuttle's left landing gear being improperly lowered as it made its fiery re-entry into the Earth's atmosphere.
Earlier, the space agency had revealed that a sensor indicated the gear was down just 26 seconds before Columbia's destruction.
The landing gear is to be lowered when the shuttle is about 200 feet over the runway and flying at 555 kms per hour.
Experts say the shuttle could have disintegrated quickly if the landing gear had been lowered at a height of 64 kms during its descent at over 20,000 kms per hour.
Hartsfield revealed these details after NASA said that it had identified the remains of all the seven astronauts who perished in the disaster.
Private memorial services for the crew members will take place within the next few weeks, NASA said.
