When I tried to get online this AM about 11 AM I couldn't get through. I figured something was up, so I turned on the tube. Wow.
At this point it looks like tile failure on the left wing was involved, but that's incredibly speculative this early into the investigation.
Although until now NASA hadn't lost anyone on the way back, that doesn't mean reentry isn't extremely hazardous in a number of ways. Even under design conditions, the shuttle strikes the atmosphere and reaching speeds of Mach 18 or so according to the NASA speaker I just heard (Mach 18 is 18 times the local speed of sound). Under such conditions the temperatures at the leading edges of the wings reaches temperatures high enough to cause structural integrity and other heat-related problems.
To address this, the shuttle is covered with special tiles on critical surfaces. The ceramic tiles not only protect the wing surface from the high temperatures, but they also help to carry away the heat by the process of sublimation as described a few posts ago here. I don't happen to know how many of the tiles can be lost before problems arise.
That's all for now. For more on this I'll be keeping an eye on Rand Simberg and Jay Manifold. And of course Glenn Reynolds is posting up a storm.
Oh yeah, a little editorial. IMO we need to change our entire attitude to the space program and either fish or cut bait. Making NASA subject to fickle funding turns their focus away from engineering toward promotion, and IMO it's clear that there's a lot of practical engineering that remains to be mastered.