Marshall Space Flight Center

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The head of NASA's Marshall Space Flight Center said Wednesday engineers "got comfortable" with small amounts of foam insulation coming off the shuttle's external tank -- and they never thought the problem could lead to disaster, as investigators believe may have happened with Columbia.

In his first interview since the accident, Director Art Stephenson said engineers at the Huntsville facility knew small pieces of foam were flying off the tank and damaging the shuttle's heat-shielding tiles.

He said they developed a way to fix part of the problem, but NASA did not believe the polyurethane insulation was dense enough to cause major damage and didn't concentrate on eliminating the loss of the rigid, rust-colored insulation during launch.

Video cameras captured images of foam peeling off the external tank and striking the Columbia during liftoff on Jan. 16. The cause of the accident has yet to be determined, but NASA is assuming the foam damaged heat-resistant tiles, causing the shuttle to burn up during re-entry on Saturday.