NASA reports that UARS, an atmospheric research satellite the size of a small bus, will re-enter Earth's atmosphere on Sept. 23rd plus or minus one day. The disintegration is expected to produce a fireball that could be visible even in broad daylight. Not all of the spacecraft will burn up in the atmosphere, however; according to a NASA risk assessment, as many as 26 potentially hazardous pieces of debris could be scattered along a ground track some 500 miles long. But where? No one can say. Because of the rapid evolution of UARS's decaying orbit, the location of the debris zone is not yet known.