[meteorite-list] ROSAT's Upcoming Reentry Greater Threat Than UARS

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 13 Oct 2011 09:31:08 -0700 (PDT)
Message-ID: <201110131631.p9DGV8gp028959_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/news/n1110/12rosat/

Falling German satellite greater threat than UARS
BY STEPHEN CLARK
SPACEFLIGHT NOW
October 12, 2011

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.

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, 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.
Received on Thu 13 Oct 2011 12:31:08 PM PDT