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 — YOU-arz — and it’s currently 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 reentry 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.

Polar bears and Antarctic scientists are safe.

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 reentry 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 when NASA estimates that the satellite is two hours away from hitting 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 hit. 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 UARS’s work had become redundant to that performed by other satellites, and it received its scientific pink slip.

Left alone, it would have orbited for an additional 25 years or so as a large piece of space junk in the increasingly crowded region known as Low Earth Orbit, but NASA was able to alter the satellite’s orbit to bring it to the surface sooner. In 2007, a small meteor hit UARS in orbit and knocked off four pieces but didn’t change its motion significantly.

The granddaddy of crashing NASA satellites was Skylab, which was 15 times the size of UARS and rained charred chunks on the Indian Ocean and western Australia in 1979.

About 22,000 distinct man-made objects, many of them no bigger than a softball, are currently being tracked in orbit by the U.S. military. Because space is not truly empty, but rather is a very thin soup of particles, objects in Low Earth Orbit lose speed over time and eventually fall to Earth.

Most of the small objects burn up completely as they plunge into the upper atmosphere. But if they survive that initial immolation, they will slow down and cool as they reach thicker layers of the atmosphere. They’ll hit the ground at subsonic speed.

In 1995 NASA developed a “design for demise” guideline in which new satellites of a certain size are capable of being de-orbited in a controlled manner and dropped into the vast landing zone of the South Pacific. That rule applies to the international space station, which under current policy will remain aloft until 2020 but could remain operable at least until 2028 under NASA’s projections.

Eventually the space station has to come down. NASA plans to attach a spacecraft that will slow the station in a precise manner that permits a Pacific splashdown.

Even then, it’s not an exact science. The space station is 80 times the size of UARS and its breaking apart will create a debris field nearly 4,000 miles long.

“Rest assured that we have it totally in control,” said Jeff Arend, NASA’s program integration manager for the space station.

When UARS fragments hit the surface, people should be cautious about handling them. There’s nothing toxic in the mix, but somebody could get cut on the metal, Johnson said. And, for the record, the debris belongs to the U.S. government.

Some years ago, a man in Texas tried to use the crashed cone of a rocket for a hot tub in his back yard. NASA dissuaded him, Johnson said, with the assistance of the Justice Department.