Columbia Disaster: Why did the space shuttle burn up?

The space shuttle Columbia, which tore apart killing all seven of its crew on Feb. 1 just minutes before it was scheduled to land, may have been doomed since its liftoff. That’s when an estimated 2.7-pound chunk of insulating foam, perhaps combined with ice, came loose from the main external fuel tank and struck the underside of the shuttle’s left wing near the wheel well.