The NASA space junk will probably crash at sea Friday, but that's a guess.
The sky is not falling. A 12,500-pound NASA satellite the size of a school bus is, however.
It's the Upper Atmosphere Research Satellite, or UARS, and it's tumbling in orbit and succumbing to Earth's gravity. It will crash to the surface Friday.
Or maybe Thursday. Or Saturday.
Out-of-control crashing satellites don't lend themselves to exact estimates even for the precision-minded folks at NASA. The uncertainty about the "when" makes the "where" all the trickier, because a small change in the timing of the re-entry translates into thousands of miles of difference in the crash site.
As of the moment, NASA says the 35-foot-long satellite will crash somewhere between 57 degrees north latitude and 57 degrees south latitude -- a projected crash zone that covers most of the planet, and particularly the inhabited parts. In this hemisphere, that includes everyone living between northern Newfoundland and the frigid ocean beyond the last point of land in South America.
It's the biggest piece of NASA space junk to fall to Earth in more than 30 years. It should create a light show. The satellite will partially burn up during re-entry, and, by NASA's calculation, break into about 100 pieces, creating fireballs that should be visible even in daytime.
An estimated 26 of those pieces will survive the re-entry burn and will spray themselves in a linear debris field 500 miles long. The largest chunk should weigh about 300 pounds.
As the Friday-ish crash gets closer, NASA will refine its estimate of timing and location, but the fudge factor will remain high.
"There are too many variations on solar activity which affect the atmosphere, the drag on the vehicle," said Nicholas Johnson, chief scientist for orbital debris at NASA. He said that with an official estimate of two hours to go before the satellite hits the Earth, there will still be a margin of error of 25 minutes.
"That equates to plus or minus 5,000 miles. That's a lot of real estate," he said.
The good news is that UARS will probably splatter into the open ocean, because Earth is a water planet. And humans, for all their sprawl, occupy a very limited portion of its surface.
NASA did a calculation of the odds that someone would be struck by UARS debris. It's very unlikely: About a 1-in-3,200 chance that one person somewhere in the world would be struck. That's not the odds for any specific person (say, a reader of this story), but for the entire human population, which is about 7 billion.
Used fuel tanks and rocket bodies fall to Earth frequently, Johnson said, "and in over 50 years of these things coming back around the world, no one has ever been hurt. There has never been any significant property damage."
The satellite was launched on the space shuttle Discovery in 1991 and spent 14 years studying the atmosphere as part of an effort to understand, among other things, the human influence on climate change.
It measured chemicals that damage the ozone layer, aerosols from Mount Pinatubo and changes in solar radiation that affect the upper atmosphere.
But NASA decided in 2005 that its work had become redundant to that performed by other satellites, and UARS received its scientific pink slip -- and a nudge into a lower orbit that would ensure its demise.