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© NASA
NASA reports that UARS, an atmospheric research satellite the size of a small bus, will re-enter Earth's atmosphere on Sept. 23, plus or minus one day. Not all of the spectacularly-disintegrating spacecraft will burn up in the atmosphere; debris could be scattered along a ground track some 500 miles long. Because of the rapid evolution of UARS's decaying orbit, the location of the debris zone is not yet known.

A NASA risk assessment places the odds of a human casualty at 1:3200. For last-chance sightings of UARS, check the Simple Satellite Tracker or download the Satellite Flybys app for your smartphone.