Sep. 15, 2011 at 6:17 PM ET
Last updated 1 a.m. ET Sept. 19:
NASA's Upper Atmosphere Research Satellite is now expected to fall to Earth sometime between Sept. 22 and 24, orbital experts reported Friday.
That's toward the early end of the original projections for UARS' fiery descent: Last week, when NASA announced that the long-defunct, six-ton satellite would crash, the time frame was given as late September to early October. That wide window of possibilities was due to the uncertainties over atmospheric conditions. Now the picture is becoming clearer, said Nicholas Johnson, head of NASA's Orbital Debris Program Office at Johnson Space Center in Texas.
"The sun has become very active since the beginning of this week, and it's accelerating the prediction," he told me.
Higher solar activity heats and expands the upper atmosphere, creating more drag for satellites in decaying orbits. The increased drag pulls down those satellites more quickly — and that's what's behind the earlier prediction.
On Thursday, the prediction was revised to say the fall would come Sept. 24, plus or minus a day. On Friday, the time frame was revised again to put the fall at Sept. 23, plus or minus a day.
As of Friday, NASA's UARS status page said the bus-sized satellite's orbit was 140 by 155 miles (225 by 250 kilometers). That compares with 143 by 158 miles (230 by 255 kilometers) for Thursday, and 155 by 174 miles (250 by 280 kilometers) on Sept. 7. The figures suggest that the decay of the satellite's orbit is accelerating.
Other parts of the prediction remain in force: The satellite, which monitored atmospheric changes between 1991 and 2005 but was then put in a disposal orbit, could fall anywhere in latitude between northern Canada and southern South America. The biggest piece to survive the fall is expected to weigh about 300 pounds (150 kilograms), or roughly the weight of a refrigerator. Johnson says the chance that any piece of the satellite will hit anybody at all is 1-in-3,200, and the chance that you specifically would be hit is 1-in-20 trillion. (Unless you live in, say, Finland. Then there's zero chance.)
The minuscule chance that someone will be hit is largely due to the fact that most of our planet's area is empty of people.
When word of the revised prediction got out over Twitter, Astro Guyz blogger David Dickinson noted that UARS would make a series of nighttime passes over the southeastern U.S. starting Sept. 20 — which means it's possible Americans might see debris streaking through the skies when UARS falls. But at this point, there's no way to predict when that might be, so we'll just have to wait and watch.
Where's UARS now? This satellite-tracking page shows you its location in real time.
Update for 1 a.m. ET Sept. 19: NASA's prediction still calls for re-entry on Sept. 23, plus or minus a day. As of early Sunday, UARS' orbit was at 133 by 149 miles (215 by 240 kilometers).
More about space debris:
Check NASA's UARS status page for updated information about the satellite's whereabouts, all the way to the end.
Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.