NASA's 6.5-ton UARS Satellite will deorbit... pretty soon. When it re-enters the atmosphere, there will be a light show of immolation as parts get torn off and ignited by atmospheric friction, vaporizing the satelliteÂ… well, all but that pesky half ton remainder.
NASA launched its Upper Atmosphere Research Satellite (UARS) in 1991 on a three-year data gathering journey in 'low-Earth' orbit at an altitude of 375 miles. Although NASA decommissioned it on December 14, 2005, six of its instruments still function. However, newer satellites carry more modern and sophisticated instruments to do those jobs, and UARS' orbit was allowed to decay. Sometime in late September or early October of 2012 (within the latter part of the next six weeks), this piece of space-junk, roughly the size of four city buses stacked beside and atop each other, will come a-tumblin' down.
Lots of things affect satellite orbits, and their positions within those orbits are constantly corrected for the orbital changes induced by the Moon's gravity, the Sun's gravity, gravitational anomalies in the Earth itself and the atmosphere in which it orbits. When a satellite is decommissioned, or it runs out of fuel, those corrections are no longer made. Before long, atmospheric drag becomes the greatest part of orbital decay. Although the atmosphere at 375 miles is incredibly thin (human lungs wouldn't be able to tell the difference from deep space), it's there. And its effect changes, making an exact prediction of re-entry at any significant time interval ahead of the event essentially impossible. Thus, the UARS satellite will deorbit 'pretty soon.'
This particular event has been more difficult than most, because the Sun has thrown out some pretty hefty flares and storms of late. When it does that, the thin upper atmosphere heats up (no, that has nothing to do with global warming), and it expands. All gases do that, and the atmosphere is pretty much all gas. Anyway, Ol' Sol has both hastened the event and made it more difficult to predict. Therefore, NASA cannot yet predict either the date of re-entry or the debris field's general location.
NASA has scheduled a media teleconference for Friday, September 9, at which it hopes to be more specific. About all NASA has said so far is that the surviving components comprise "...a surviving mass of 1,170 pounds (532 kilograms) falling within a debris footprint length of some 500 miles (800 kilometers)." The agency does not yet know where that 500 mile footprint might be laid down. Recall that when SpaceLab did this same uncontrolled re-entry, most of it wound up in the Pacific, but a fair-sized chunk hit the ground in Australia. The same sort of result is not out of reason for this event.
The government asks that anyone who does encounter a piece UARS report it to law enforcement authorities and not touch it.