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http://cosmiclog.msnbc.msn.com/_news/2011/10/11/8276783-satellites-doom-looms-sooner

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Satellite's Doom Looms Sooner

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Just a few weeks after a falling U.S. satellite put the world on alert, German space officials say the defunct ROSAT X-ray telescope is on course to make a fiery re-entry in the Oct. 20-25 time frame — which is earlier than previously predicted.


When NASA's six-ton Upper Atmosphere Research Satellite plunged harmlessly into the Pacific on Sept. 24, the German Aerospace Center said the 2.4-ton ROSAT probe would be the next big satellite to take a fall. But the initial projection called for atmospheric re-entry to occur in early November. The shift to a sooner date is due to updated data, and additional shifts are to be expected over the coming days.


As was the case with NASA and the UARS spacecraft, the Germans won't be able to pinpoint precisely when or where their satellite will hit — but the most likely scenario is that the debris will fall harmlessly into the sea.


ROSAT, which is a contraction for "Roentgen Satellite," was launched in 1990 from Cape Canaveral Air Force Station in Florida. The German-U.S.-British mission conducted an all-sky survey in X-ray wavelengths, cataloging more than 150,000 objects. Mission managers turned the probe off in 1999, and its orbit has been slowly decaying ever since. Because there's no propulsion system on board, the managers can't guide the bus-sized spacecraft to a controlled re-entry.


Theoretically, ROSAT could come down at any latitude between 53 degrees north and 53 degrees south, or roughly between Edmonton, Canada, and Punta Arenas, Chile. ROSAT isn't as massive as UARS was, but because of the way it was built, more of its mass is expected to survive re-entry. Experts estimate that 30 pieces totaling 1.6 metric tons could make impact, compared with 26 pieces totaling a half-ton (1,100 pounds) for UARS.


"The largest single fragment will probably be the telescope's mirror, which is very heat-resistant," the German Aerospace Center, also known by the German-language acronym DLR, said in its latest re-entry update.


The Canadian Press recently noted that ROSAT's mirror system weighs 1,700 pounds (785 kilograms), or about as much as a standard-sized polar bear. CP also quoted the University of Calgary's Phil Langill as saying that the chance of any piece hitting any person was about 1 out of 2,000, compared with 1 out of 3,200 for UARS. That statistic isn't as scary as it sounds: When you do the math, that works out to somewhere around a 1 in 13 trillion chance that a particular individual in the potential impact zone (me, for example) would be hit.


ROSAT is being tracked by the U.S. Space Surveillance Network as well as a radar facility near Bonn, Germany, and the re-entry predictions will be updated over the next week. This isn't an exact science, however. The satellite could descend more or less quickly, depending on the ups and downs in solar activity.


"Currently, the re-entry date can only be calculated to within plus/minus three days," DLR said. "This time slot of uncertainty will be reduced as the date of re-entry approaches. However, even one day before re-entry, the estimate will only be accurate to within plus/minus five hours."


Coincidentally, another kind of doomsday deadline falls around the same time — the Oct. 21 date specified by California preacher Harold Camping for the end of the world. Camping's earlier deadline for the Rapture, which was thrust into the public consciousness thanks to a multimillion-dollar marketing campaign, came and went uneventfully on May 21. I have no doubt this new date will be just as uneventful ... unless it turns out to be the day that ROSAT makes a splash.


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