I find it hard to believe that out of the total 30,000 tiles that were on Columbia that they would find the one that is significant.

In my other post in this thread I found a link to an article citing a history of unusual amounts of damage to the tiles on Columbia after re-entry. It was stated that the problem was seen as a threat to Shuttle Endeavor as well.

If there is a history of this problem, and the "foam" impact theory is irrelevant, then logic dictates that there is a finite probability of disaster for each shuttle mission, regardless of which shuttle goes into orbit. Since it was Columbia's 28th flight the probability may be approximately 1-in-28, but if all the shuttles are considered identical w.r.t. their heat shielding tiles then the probability may be approximately 1-in-113.

If it cannot be determined that the "foam" impact theory was definitely the cause, then the "simple probability" theory for the disaster might be the preferred explanation. If the probability of a completely undamaged shuttle disintegrating on re-entry due to flawed tiles is the most important factor then the shuttles should be grounded permanently until an alternative technology can be developed IMO.