Wayne Hale quotes:

The Columbia accident made us realize that we had been playing Russian roulette with the shuttle crews -- that we had been very, very fortunate in the past that the foam did not cause critical damage.

Having three operational vehicles in the fleet affords the shuttle program great schedule flexibility as we move toward flying safely and completing the international space station.

The bottom line is there is large uncertainty because nobody has a very good handle on the aerodynamics at those altitudes and at those speeds. Given that large degree of uncertainty, life could be normal during entry or some bad things could happen.