Here is a link to some of the emails published by NASA. There is one in this group that is not in the ones on MSNBC.
http://www.nasa.gov/columbia/COL_debris_email.pdfThere were several interesting points:
1) It is likely that it was a combination of insulation and ice that broke off of the external tank (ET) and struck the underside of the left wing. As NASA said before, the density of the foam alone is not sufficient to cause much damage to the tiles.
2) Ice striking the left wing has been a common occurrence because of the design of the ET. One of the engineers said in one of the emails, "On the first flights there were thousands of dings/gouges in the tiles...traced to ice impingement from launch vibrations dislodgment of the ice which builds up on the ET dump line - WHICH IS LOCATED IN PROXIMITY TO THE LEFT WING ON THE STACK...why this dump line was not repositioned to the other side of the tank away from the orbiter I do not understand...several times this [ice] damage was quite severe."
3) The shuttle's aerodynamic design is one which is meant to minimize turbulent flow. It was discovered that if air flow is turbulent ahead of the wheel well doors during peak heating the heat shield protecting the wheel well doors could/would burn through.
4) Severely damaged tiles can significantly increase the risk of laminar transition to turbulent flow.
5) If the wheel well heat shield failed and allowed burn through the resulting heat could:
a) cause the tires to explode which would yield nearly a quarter million lbs of pressure on the gear doors, blowing them off.
b) possibly ignite the explosive bungies used in emergencies to force open the gear doors and deploy the gear should they jam for some reason.
(Recall the amateur vid taken in California of the shuttle streaking along and then the sudden puff like explosion?)
6) If the gear doors opened during hypersonic flight the results would be "catastrophic". One engineer stated, "even if you could survive the heating, would the gear now deploy? And/or also, could you even reach the runway with this kind of drag?"