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. Because populated land makes up a small part of Earth's surface, chances are UARS will land in the ocean or on some vast wildland. Still, orbital debris trackers at the space agency estimate that there is a 1-in-3,200 chance that pieces of UARS will strike someone. That stat has been erroneously reported as the odds that any particular person (say, me) will be hit by UARS debris. In actuality, my odds of being struck down by UARS on Friday are about 1 in 21 trillion, since the risk is spread across almost all of Earth's 6.7 billion inhabitants.

For a bit of perspective, I am about 14,000 times more likely to be struck dead by lightning on Friday than I am to be struck by UARS. (That estimate is based on rates of fatal U.S. lightning strikes, which show that an average American's risk of being killed by lightning on any given day is about 1 in 1.6 billion.)

And although the reentry of a UARS-size spacecraft is a relatively rare event, large pieces of debris fall back to Earth without incident on a regular basis. As NASA reports: "Since the beginning of the Space Age in the late-1950s, there have been no confirmed reports of an injury resulting from re-entering space objects. Nor is there a record of significant property damage resulting from a satellite re-entry."

NASA has set up a Web page to update the public with predictions for the reentry of the satellite. So give it a look, and then go back to planning for an ordinary weekend.