The orbital realm surrounding Earth is filled with millions of pieces of space junk, some of which occasionally fall back to Earth. Rarely, though, does an entire satellite or spacecraft come back uncontrolled, as NASA expects its Upper Atmosphere Research Satellite (UARS) to do sometime on Friday.The schoolbus-size UARS [ see artist's depiction at left ] weighs some 5.7 metric tons, and NASA predicts that 500 kilograms of debris will survive reentry and land somewhere on Earth...