Investigators say they are close to explaining what caused the U.S. space shuttle Columbia to disintegrate three months ago.
The probe of the shuttle disaster is entering its final phase. The independent board of experts looking into the calamity says it will offer its hypothesis on the cause in seven to 10 days.
Columbia broke apart February first after it re-entered Earth's atmosphere and fell in thousands of pieces over Texas and neighboring Louisiana, killing all seven crewmembers.
The chairman of the investigating board, retired Admiral Harold Gehman, says the hypothesis is the result of what he calls mountains of data and shuttle debris.
"The test that we have is that not all the data have to agree with our hypothesis, but there can be no conflicts," he said. "We can't have any evidence that says that our hypothesis is no good."
The board found early in its probe that something caused a break in the front edge of the shuttle's left wing, allowing extremely hot atmospheric gases to enter the orbiter and destroy it. Admiral Gehman says the hypothesis will finally suggest what caused the break.
The leading theory has been that the opening was caused by a piece of hard insulating foam that broke off the shuttle's external fuel tank during launch and slammed into the wing at high speed. Admiral Gehman would not confirm that theory, although a separate investigation by the U.S. space agency, NASA, has accepted it.
Board investigator Roger Tetrault says the technical analysis is closing in on the location of the left wing break. He believes it was in one of several reinforced carbon seals that fit between protective front edge panels. Such a seal is a leading candidate as the piece of debris seen floating away from the shuttle on the second day of its mission. In the board's view, this loss might have caused the wing opening.
Over the past few weeks, Mr. Tetrault and his colleagues have altered their view about where the break occurred, gradually moving it closer in to the shuttle's body as new evidence has appeared.
"We've had this oscillating pattern of where it is, but it's getting closer and closer, and I feel that we're probably within 30 inches [less than a meter] of where the actual breach occurred," he explained.
As the investigators reach their conclusions, the search for shuttle debris ends Wednesday. It has turned up 78,000 pieces, amounting to nearly 40 percent of the shuttle by weight. The remainder is thought to have been tiny fragments that burned up during Columbia's fiery demise.