UARS, Upper Atmosphere Research Satellite, is about to fall from the sky. This is the most exciting event that NASA has been able to stage in years. But, it is actually Mother Nature who is doing the orchestration.

UARS is a large, expired NASA spacecraft that will plunge into the atmosphere in its final moments, predicted to occur sometime in the next few weeks. However, with all of its expertise, NASA and the USAF are unable to tell us exactly when and where this reentry will take place. Only Mother Nature knows and she is not talking.

The fact is that the uncertainties in atmospheric density over time and location are sufficient to prevent an accurate time and place for the event. This uncertainty in any prediction is roughly 20% of the prediction. In other words, if NASA predicts a reentry in, say, 20 days, then the actual event could occur sometime between 16 and 24 days.

Thus, we will not have a good idea of which orbit will be its last until the last few hours. Even then, the best we can do is to predict the orbit of reentry, but not the exact locations of any debris pieces that will reach the ground.

The good news is that most of the Earth's surface is covered with water. Even the dry land masses are generally sparsely populated. Take the Skylab reentry in mid-1979.

The last orbit of the 76,000-kg spacecraft took it over the Atlantic Ocean from north to south, around the southern tip of Africa and into the Indian Ocean. A few heavy pieces did make it to Australia, but no one was hurt.

UARS is the size of a school bus and current NASA predictions can only tell us that the expected reentry will occur sometime between late September and early October.

Many pieces of the 5,600-kg spacecraft are expected to reach Earth's surface, leaving a debris field that may be 800 km long somewhere between the latitudes of 57 deg North and 57 deg South, which covers everything from northern Canada and southern Africa.

Not to worry. The probability of any casualty or property damage is about the same as being hit by a meteorite. So, if you are approached by an insurance agent selling UARS debris insurance, run for cover.

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