By now you've probably already heard that a 20-year old (obsolete) weather satellite is expected to fall back to Earth sometime tomorrow. Since it's apparently "bus-sized" and weighs 6.5 tons, the prospect that large chunks of the Upper Atmosphere Research Satellite (or UARS) will survive re-entry, damaging people and property as it falls, has generated an increasing amount of press coverage in recent days.
It struck me that all the news articles invariably lead with quotes from 'experts' and/or 'officials' assuring us that the debris will fall harmlessly. But, just as inevitably, those assurances are hedged with these equivocal adverbs. Like THIS article in the Los Angeles Times today, which begins by stating that the UARS, "is due to land Friday, most likely harmlessly, officials say." And then, if you read further into the body of these articles, they all explain that no one really knows where the debris will land, and that these assurances that no one will be harmed are based almost exclusively on generic statistical calculations about the percentage of the surface of the Earth that is populated and how small the debris field will be. "But there is plenty the trackers don't know about the satellite's reentry — namely, where those pieces will fall — and that's what has some people spooked. 'There is no modeling that predicts where it will hit the surface of the Earth,' Duncan said."
On a related note, this article also explains in passing that any object weighing more than 1,000 lbs. before re-entry may be large enough to generate a debris field on Earth. I'd always sort of wondered if this had ever been quantified. Now I know.