Thursday, September 15, 2011

Satellite falls faster than forecast

NASA's Upper Atmosphere Research Satellite is now expected to fall to Earth sometime between Sept. 23 and 25, orbital experts reported today.

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.

NASA's UARS status page said the bus-sized satellite's orbit was 143 by 158 miles (230 by 255 kilometers) as of today, compared with 155 by 174 miles (250 by 280 kilometers) a week ago. Johnson said the status page would be updated again on Friday.

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