Space shuttle Columbia was the oldest in NASA's fleet and the first to enter the Earth's orbit in 1981. Saturday's voyage was its 28th trip into orbit.

Named after a ship that made the first American circumnavigation of the globe in 1792, Columbia flew into orbit in 1981. Four ships joined the fleet over the next decade; Challenger in 1982; Discovery in 1983; Atlantis in 1985, and Endeavour, built in 1991 to replace the Challenger after it exploded in 1986.

The shuttle went by the name OV-102, standing for Orbiter Vehicle. It weighed 178,000 pounds with its main engines installed.

The shuttle had undergone about 50 modifications, including the addition of carbon brakes, improved nose wheel steering and an enhancement of its thermal protection system. It was last refurbished in 1999.

Scientists found three cracks last July in Columbia's stainless steel liners used to direct the flow of super-cold hydrogen fuel to the main engines. Similar cracks had been discovered in other ships in the fleet. Age was not considered a factor in the cracks because they were found in NASA's oldest and newest shuttles.