Many people think heat tiles which protect the shuttle may have been damaged by some insulation foam which peeled off and hit the wing during the launch.
But some scientists say this wouldn't cause much damage. They think Columbia may have been hit by space debris while it was still in orbit.
Junk in space
But there are problems with the theory.
Firstly, the astronauts on Columbia would have noticed if something hit their shuttle.
And secondly, Nasa and the US air force have such good radars that they know where all the orbiting rubbish is.