[buffyt]

So if one came off today, one could come off later during re-entry, too.

[burzum]

Possibly. But there is no evidence that one came off today. There is evidence that part of one came off. The Shuttle has landed with damaged tiles before. We certainly don't want it to land if the damage penetrates the spacecraft.

I don't want to speculate too much on what-ifs. There is a calculated risk in everything the Shuttle does (as well as what we do). When I used to go underway on submarines there were plenty of ways that I could be killed. A torpedo could explode, we could have a fire, flooding could occur, or we could ram an underwater structure. I didn't lose sleep over it (except sometimes when I slept next to the torpedoes) because the risk was small. Americans have died from fire, flooding, undersea mount ramming, and torpedo explosions on submarines. They have also died when the heat shield failed on the Space Shuttle. But we have to put it into perspective.