Why did Columbia spacecraft carrying Kalpana Chawla crash in 2003?

Space Shuttle Columbia carrying seven crew members including India-born astronaut Kalpana Chawla exploded upon reentering Earth's atmosphere on February 1, 2003. During the launch of its 28th Earth-orbiting mission, a piece of insulating foam broke off and struck the left wing. On re-entry, the damaged wing allowed superheated gases to penetrate, breaking the orbiter apart and killing the crew.