Up to 30 pieces of an old German satellite - weighing a combined 1.7 tons - are heading to Earth and are expected to enter our atmosphere between October 20 and 25.

But the German Aerospace Center (DLR) says it is almost impossible to say where exactly the satellite will land.

"We've never found a satellite after it's re-entered Earth," said Andreas Schütz, a DLR spokesperson, in an interview with Deutsche Welle, "so we can't say for sure what happens when they do come back."

Free fall

The ROSAT X-ray observatory satellite was launched in 1990 and ended its mission nine years later.

It has no engine and has been virtually out of control since 1999.

Scientists have been tracking its course, enabling them to estimate the time of its return to earth. But the DLR says fluctuations in solar activity have raised the uncertainty.

"There's about a 20 percent chance that we're wrong about the date," said Schütz.

"But we can say that although it will hit the earth's atmosphere at a speed of 27,500 kilometers (17,000 miles) per hour, it will slow to roughly 400 kph in about 10 minutes, raising its temperature to 2000 degrees, and so anything that can burn, will burn," said Schütz.

Vanishing Point

There may be 30 individual bits that make it to earth, but the biggest is likely to be the mirror of ROSAT's telescope because it can withstand the most heat.

The heaviest fragment could weigh 1.6 tons alone - about the same as an average car. However, DLR officials say it's most likely to fall into the sea.

"Simply because most of the planet is covered in water, ROSAT is likely to land in an ocean," said Schütz. "But we can't say for sure."

"We think 60-90 tons of satellite debris falls to earth every year world wide, but we don't really know because we've never found one – and that's probably because they land in water," said Schütz.

In September, an American satellite - said to have been the size of a bus - returned to earth unexpectedly and landed in the Pacific Ocean, according to the US space agency NASA.

But experts say there were no sightings or reliable accounts of damage of the six-ton Upper Atmosphere Research Satellite.

Author: Zulfikar Abbany
Editor: Cyrus Farivar