By Deborah Zabarenko
WASHINGTON (Reuters) - The extreme heat observed on the shuttle Columbia's left side during its fatal re-entry could have been caused by hot plasma penetrating the craft's wheel well, independent investigators said on Thursday.
 Reuters Photo Plasma is the super-heated gas that surrounded the shuttle as it streaked toward a landing at Kennedy Space Center (news - web sites) on Feb. 1. Plasma typically envelops a fast-descending space shuttle, but this time, preliminary analysis indicates it may have gotten inside the spacecraft's protective surface.
"Preliminary analysis by a NASA (news - web sites) working group this week indicates that the temperature indications seen in Columbia's left wheel well during entry would require the presence of plasma," the Columbia Accident Investigation Board said in a statement forwarded by NASA.
However, the board said the heat was so excessive that it could not have been caused by the absence of just one missing tile in the last minutes of flight.
This is significant since questions have centered on the possibility that some of Columbia's heat-shielding tiles were knocked off by a piece of foam insulation that fell off the shuttle's external fuel tank about 80 seconds after launch, apparently striking the left wing.
The board said investigators were looking at other ways the shuttle's skin might have been breached to let plasma into the wheel well area or elsewhere in the wing.
They also discounted fears that a problem with the landing gear on the left side of the spacecraft might have caused the shuttle to disintegrate over Texas, as a NASA engineer suggested in an e-mail two days before the shuttle's demise.
PROBLEMS ON COLUMBIA'S LEFT SIDE
"Other flight data including gear position indicators and drag information does not support the scenario of an early deployment of the left gear," the board said.
The board's statement was distributed by NASA, which has already come under criticism for failing to keep its distance from the independent inquiry that was appointed just hours after Columbia broke apart.
After a bruising four-hour congressional hearing on Wednesday with just one witness -- NASA Administrator Sean O'Keefe -- the space agency amended the investigative board's charter to address concerns about its independence.
The search for debris from Columbia continued in Texas, where a preliminary analysis of low-frequency sound wave recordings indicated the shuttle exploded between the cities of Amarilla and Lubbock, about 330 miles west of Dallas.
The data from sensitive devices that record infrasound, or low-frequency sound waves inaudible by humans, was sent to NASA, said Eugene Herrin, a geophysicist at Southern Methodist University in Dallas.
Herrin said data indicates a single explosion over Texas. A monitoring station in New Mexico showed the spacecraft as being intact as it passed over that state, he said.
Scientists operate a worldwide system of infrasound detectors that record items such as sonic booms. The devices look for any disturbance in the atmosphere such as a nuclear blast, meteor flights or even hurricanes.
Herrin said the infrasound likely recorded the moment when the shuttle blew apart, but the findings were preliminary.
The search on the ground shifted to Anderson County, west of Nacogdoches, where most of the reported shuttle debris fell. Officials said they have likely found more parts from the shuttle's wings and are checking serial numbers on recovered parts to see if they came from the left wing.
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