So why all the uncertainty? When you have an object zooming around the planet at 5 miles (8 kilometers) per second through a dynamic yet increasingly dense upper atmosphere, it is very hard to pinpoint how the 6-ton piece of satellite junk will act. In fact, it will likely be impossible to give an idea of reentry location until two hours before it happens.
But now, thanks to the sun, at least NASA can put some kind of constraint on when the satellite will reenter.
As reported by everyone's favorite space chicken, Camilla Corona SDO - NASA's Solar Dynamics Observatory mascot - solar activity has a huge impact on the upper atmosphere, ultimately interfering with the orbits of satellites.
"As solar activity increases, extreme ultraviolet radiation (also called EUV) heats Earth's gaseous envelope, causing it to ‘swell' and reach farther into space than normal," says Camilla in her NASA outreach blog.* "The puffed-up atmosphere causes the slow-down of orbiting spacecrafts because of this additional ‘drag.'"
This is exactly what's happening to UARS; it is encountering the tenuous atmosphere sooner than expected due to an increase in solar activity - hence the Sept. 23 deadline (plus or minus a day).
The chances of the satellite causing a public menace is 1-in-3,200. This, in itself, is very remote, but expect to see more scary graphics like the BBC's example for the next week (just don't pay much attention to them).
*Yes, I really did quote a rubber chicken.