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Falling German satellite greater threat than UARS
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: October 12, 2011


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Less than a month after NASA's falling UARS satellite grabbed the headlines, the German space agency says one of its abandoned satellites will dive back to Earth later this month, but no one knows where it will land.


Artist's concept of ROSAT. Credit: German Aerospace Center/DLR
 
The ROSAT X-ray astronomy observatory is smaller and less massive than NASA's Upper Atmospheric Research Satellite, or UARS, which fell back to Earth on Sept. 24. But officials predict it will spread three times more debris and pose a greater threat to people than UARS.

That's because ROSAT is made of heat-resistant components, especially its primary mirror, which officials say will probably be the largest single fragment that will reach Earth.

The satellite will streak into the atmosphere at 17,000 mph, and temperatures up to 3,000 degrees Fahrenheit will burn up much of the spacecraft.

"All these forces exerted on the satellite cause it to disintegrate, which in turn means that it eventually lands in the form of a long debris trail," said Heiner Klinkrad, head of the European Space Agency's space debris office. "The lightweight objects fall to Earth first, similar to leaves from a tree. The really heavy objects land later, because they ultimately have to drill their way through the atmosphere."

But engineers expect the bulk of ROSAT to survive re-entry, littering its impact point with up to 30 pieces of debris.

The 5,348-pound satellite launched from Florida on a Delta 2 rocket in 1990. ROSAT does not have an engine or propulsion system because it used reaction wheels to point its telescope toward scientific targets in the cosmos.

Up to 3,750 pounds of the satellite could reach Earth's surface. NASA said they expected 1,200 pounds of UARS to survive re-entry.

There is a 1-in-2,000 chance someone will be struck by fragments of ROSAT on its way down, according to Germany. That equates to odds of about 1-in-14 trillion that any individual person will be hit.

The threat from UARS wasn't as high. An analysis from NASA showed there was a 1-in-3,200 chance of a collision between a human and a piece of UARS.

The remnants of UARS fell in the remote Pacific Ocean, and ROSAT will likely also end up in the sea, but its impossible to tell where it will crash until hours before.


ROSAT launched in June 1990 on a Delta 2 rocket. Credit: NASA/German Aerospace Center/DLR
 
ROSAT, which stands for Roentgen Satellite, was turned off in 1999, and its altitude has gradually dropped since then from an operational orbit more than 350 miles high. The German Aerospace Center, also known as DLR by its German acronym, says the spacecraft should re-enter the atmosphere between Oct. 20 and Oct. 25.

But the margin of error in the re-entry forecast is three days, and officials likely won't know where the satellite will come down until after it falls. Even one day before re-entry, the time of ROSAT's demise will only be known with a precision of plus-or-minus five hours, putting entire oceans and continents in the satellite's flight path.

"All areas under the orbit of ROSAT, which extends to 53 degrees northern and southern latitude could be affected by its re-entry," said a posting on DLR's website. "The bulk of the debris will impact near the ground track of the satellite."

"It will not be possible to make any kind of reliable forecast about where the satellite will actually come down until about one or two hours before the fact," Klinkrad said. "It will, however, be possible to predict, about one day in advance, which geographical regions will definitely not be affected."

ROSAT's orbit was at an average altitude of 149 miles Wednesday.

"This slow descent is due to the friction encountered by the satellite as it enters the outer fringes of Earth atmosphere, which increases the more ROSAT penetrates into our atmosphere," Klinkrad said.

Klinkrad said the major factor affecting a satellite's fall from orbit is solar activity. Energy unleashed from the sun causes Earth's atmosphere to heat up and expand, generating more drag for satellites in low orbits.

Fluctuations in solar activity can quicken or slow a satellite's re-entry. Experts initially expected ROSAT's plunge to occur last year, but solar activity turned out to be less than predicted, delaying the re-entry until this month.

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