The Upper Atmosphere Research Satellite (UARS) is as big and as heavy as a bus. Most of the satellite will burn up upon re-entry, but NASA estimates 26 pieces will make it all the way down.
When NASA launched the UARS from Space Shuttle Discovery 20 years ago, it was supposed to study the Ozone layer in our atmosphere.
Now, with the mission accomplished, that very same atmosphere is raising havoc with predictions, partly because of solar flares that have heated the atmosphere and expanded it, creating more drag on the satellite which is now out of fuel and out of control.
At best, NASA figures it can give Earthlings a two-hour warning.
"That's when it will really start coming down," explains San Francisco State Physicist Susan Lea, Ph.D.
It won't be the first time that space junk has fallen upon us -- space junk has previously fallen from Spacelab and the Russian space station MIR.
The problem this time is that UARS orbits over most of Earth's populated areas. Theoretically, that puts all of us at risk.
"These 26 components, which we do anticipate will survive all the way down to the surface, will be going at a moderate velocity," said NASA satellite expert Nick Johnson, Ph.D. "Tens to hundreds of miles per hour."
Would a person on the ground have much warning?
"Very little, probably not unless somebody said to you, 'It's going to hit you, run.'" Lea explained.
NASA's best estimate is that the satellite will return on Saturday. NASA says the odds of a human being hit by debris are one in 3,200. Factoring in seven billion humans, the personal odds of being hit by the falling satellite improves to one in 22 trillion.
If we dodge the bullet this time, there are still 22,000 other pieces of space junk still up there.
The satellite is expected to re-enter the Earth's atmosphere on Friday. It is not expected to land somewhere in North America.