Shuttle was oldest in fleet

Kennedy Space Center-AP -- Space shuttle "Columbia" was a pioneer in NASA's fleet.

The shuttle was the oldest in the fleet -- and the first to enter the Earth's orbit in 1981.

Its latest voyage was its 28th trip into orbit.

Columbia was named after the ship that made the first American circling of the globe in 1792.

The shuttle had gone through 50 modifications, including the addition of carbon brakes, improved steering and enhancement of its heat protection system.

Scientists had found three cracks last July in Columbia's steel liners, which are used to direct the flow of hydrogen fuel to the main engines.

Similar cracks had been found in other shuttles. Age wasn't considered a factor in the cracks because they were found in NASA's oldest and newest shuttles.

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