National Geographic News
Satellite picture: The ROSAT satellite.

An artist's impression of ROSAT in orbit.

Illustration courtesy DLR

Traci Watson

for National Geographic News

Published October 19, 2011

If you see a large glowing object plummeting from the sky late Saturday or early Sunday, duck.

A defunct European satellite called ROSAT is headed straight for Earth this weekend—and chances are even higher that a piece of space debris could hit someone than the odds placed on a NASA satellite that fell from orbit last month.

The German Aerospace Center, which led the development and construction of ROSAT, estimates that the chance of anyone being harmed by debris from the satellite is 1 in 2,000. For NASA's UARS, the injury risk was roughly a third lower, at 1 in 3,200.

ROSAT is currently estimated to make an uncontrolled reentry during the early morning hours on Sunday, Greenwich Mean Time, said Heiner Klinkrad, head of the European Space Agency's space debris office.

But Klinkrad cautions that the satellite could enter Earth's atmosphere up to 24 hours earlier or later than the estimated time.

That's because shifts in radiation from the sun aren't 100-percent predictable. If solar radiation increases, there's more heating and expansion of the atmosphere, which would increase drag on the spacecraft and cause it to hurtle downward sooner than expected.

Don't Touch the Space Junk

Unfortunately, neither Klinkrad nor anyone else can say exactly where on Earth ROSAT is headed.

Debris could come down anywhere between 53 degrees north latitude and 53 degrees south latitude, an area that includes most of Earth's land mass, the German Aerospace Center's Roland Gräve said via email.

That could be a worry, because the satellite's 1.5-ton mirror is likely to survive the superheated trip through the atmosphere all the way to the ground, where it could make a major dent in whatever it strikes.

By contrast, the biggest piece of NASA's UARS spacecraft thought to hit the planet was a 300-pound (150-kilogram) chunk of the craft's frame.

In the end, the remnants of UARS splashed down into an isolated stretch of the Pacific Ocean, disturbing no one except perhaps a few fish. (See "NASA Satellite Debris Likely Fell in Ocean, May Never Be Found.")

Despite the higher odds, ROSAT is also unlikely to hurt anyone, scientists say, given the planet's large stretches of ocean and thinly populated areas.

"We accept risks in everyday life that are many orders of magnitude higher than the risks we incur from reentering space objects," ESA's Klinkrad said.

If bits of the satellite do land in a populated area, "they will be extremely hot," added the German Aerospace Center's Gräve. "This is why we recommend not touching any satellite parts" that do make it to the ground.

And any ROSAT debris, no matter where it's found, belongs to the German government, he said.

ROSAT Worthy of a Wake

ROSAT—short for Roentgen Satellite—launched in 1990 on a Delta II rocket to measure the x-rays emitted by objects such as neutron stars, dense stellar cores left behind by some supernovae.

The mission was supposed to last only 18 months, but the satellite kept chugging for eight years. Scientists finally shut it down in 1999 after its last functional scientific instrument accidentally pointed too close to the sun, blinding the sensors.

When ROSAT was on the drawing board in the 1980s, spacecraft designers didn't plan for the end of their vehicles' lives. So ROSAT was built without a propulsion system that would've allowed for a carefully choreographed demise.

"The attitude 20 years ago was still very much, Eh, space is big, and things that reenter probably won't hit anyone, so we won't worry about it," said Jonathan McDowell, a Harvard-Smithsonian Center for Astrophysics scientist who relied on data from ROSAT.

As far as McDowell can remember, nothing as big as the ROSAT mirror has smashed into the Earth's surface since the reentry of the Soviet space station Salyut-7 in 1991.

(Space debris pictures: "Five Unexpected Objects That Fell to Earth.")

The mirror is likely to come down intact in part because it's such a massive item, added University of Leicester physicist John Pye, who worked on the ROSAT program for nearly 20 years.

What's more, he said, the glass is a special heat-resistant variety, to keep the mirror from distorting the x-rays as its temperature changed in space.

Instead of being worried, however, the CfA's McDowell is feeling so nostalgic about the satellite's imminent death that he's planning a wake. There's just one problem: When to send out the invitations.

"I'm going to wait until it looks like it's going to come down," he said, "and then I'm going to email everybody: Anybody free? ... That's the trouble with these reentry celebrations. You're never sure when they'll be."

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