It was the strongest statement to date on what caused the shuttle to disintegrate on re-entry, killing seven astronauts aboard.
The Columbia Accident Investigation Board has suggested in the past that the insulation may have shattered the heat shield, but before the board always called the foam impact only a "candidate" cause.
Board member Roger Tetrault said that an analysis of the tons of Columbia debris recovered from Texas and Louisiana gives strong "compelling" evidence that a wing part called panel number eight was breached in the left wing during launch when the wing was hit by foam insulation peeled off the external fuel tank.
When Columbia returned to Earth, the scorching air of re-entry flowed inside the wing and melted metal braces. Tetrault said that recovered parts were splattered with droplets of iron and nickel, probably from the melting of supports made of steel and nickel.
The distribution of the Columbia debris shows that pieces of the left wing came off first as the spacecraft streaked eastward across Texas. Tetrault said this is evident because the left wing pieces were found farther west than the pieces of the right wing.
The board continues to investigate the possible damage from the foam impact with experiments that involve shooting pieces of foam at a wing mock-up at a high rate of speed. The tests are being conducted now at the Southwest Research Center in San Antonio.
Board chairman Harold Gehman said that engineers are shooting foam at fiberglass panels this week to assure that the test apparatus is precisely set up. Later, he said, foam pieces will be fired at actual wing panels removed from the other space shuttles.
Columbia came apart while returning from space on Feb. 1. The space shuttle fleet was grounded while the accident was under investigation.
Gehman said the board is now writing its final report which is expected to be completed late in July.
NASA officials have said that needed repairs of the shuttle are expected to be completed in time for the spacecraft to start flying again late this year or early in 2004.
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