For the second time in about a month, earthlings have reason to fear a falling satellite. This time it's an old German space telescope that will plunge to Earth in the coming weeks — and Canada falls within the potential impact zone.

There are many reasons to hope ROSAT (Roentgen Satellite) steers clear of this country — 785 of them, to be exact. That's the weight in kilograms of the satellite's mirrors, equivalent to a standard-sized polar bear.

The chances of someone actually getting hit by debris from the uncontrolled satellite are hardly any different from those predicted before last month's fiery return of another satellite, which came down over the Pacific Ocean and caused no damage to humans.

NASA had calculated a 3,200-to-1 chance of that satellite causing injuries back on Earth; for ROSAT, it's calculated at 2,000 to 1.

The German Aerospace Center (DLR) advises on its website that the large X-ray observatory, the size of a mobile home, is due to re-enter the atmosphere around the end of October.

ROSAT shut down in February 1999, more than eight years after its launch, and it has no propulsion system to alter its re-entry.

The satellite's orbit covers an area between 53 degrees north and 53 degrees south, meaning it could come down anywhere between Canada and South America.

ROSAT weighs 2.4 tons in all. The latest studies suggest up to 30 individual pieces of ROSAT weighing a total of 1.6 tons may reach the Earth's surface.

Giant mirror system could survive re-entry

Holger Krag, an expert at the European Space Agency's space debris centre, is focusing mainly on the giant mirror system, which is shielded from heat and could survive re-entry.

Krag points out there are between 10 and 30 uncontrolled satellite re-entries annually. But he stresses not many are really big.

"There are only a few objects that have really high masses and with the high likelihood that parts of them will survive re-entry," he told The Canadian Press.

Krag points out it's hard to predict where any decommissioned satellites will eventually come down because of the density of the atmosphere.

He admits that, even a few hours before re-entry of ROSAT, there's no way of being able to give a precise location.

"It's just impossible to predict the behaviour of the atmosphere," he added.

But Phil Langill, a University of Calgary physics professor, says the concern that someone on the ground will get hit by pieces of ROSAT should be taken with a grain of salt.

"Most of the people live in populated areas, not out in the country," he said in a recent interview.

"The probability that this thing is going to hit a populated city is so, so small because the whole scope of the land mass over which it travels is so, so small."

Langill says there are still a few large satellites among the more than 8,000 orbiting objects that are being tracked by the United States Space Surveillance Network.

The network says about seven per cent of the space objects are operational satellites, while the rest are debris.

"All the satellites that were launched in the last 10 to 15 years are much smaller and those ones would not survive any re-entry," said Langill, who is also director of the Rothney Astrophysical Observatory.

"It's only the ones that were launched in the late '80s and early '90s that are still up there that were large in size, large in mass and could survive the fiery re-entry to make it to the ground.

"But once we get to the next few years, and these big ones come down, then the only ones that will be left up there will be the smaller-package ones."

Langill also tried to explain how scientists determined that ROSAT presents a 2,000-to-one chance of casualties.

"The simplest calculation and the one that I think they use is you just take the total surface of the Earth over which the satellite passes in its normal orbital path and divide that by the total number of people on the ground that occupy that amount of land mass," he said.

"So you simply do the number of people divided by area and you get a number like one in 2,000."

Langill admits he's disappointed the exact spot where pieces of ROSAT may come down can't be accurately predicted. "I think it would be a great opportunity to get together for a glass of wine with your neighbours and watch this thing come out of the sky. That would be a fantastic thing to see, that would be a lot of fun."