As the saying goes, what goes up must come down, and it's time for a German satellite launched in 1990 to make the second half of that journey. ROSAT, says German Aerospace Agency DLR, is likely to fall from space between Oct. 21 and 24.

Up to 30 individual debris with a total mass of 1.7 tons could return to Earth at speeds of up to 450 kilometers per hour — likely burning from the friction of re-entry.

The idea of getting hit by a stray piece of burning space junk is terrifying. But, thanks to Earth's large swaths of water and open land, it's also exceptionally unlikely. DLR scientists put the risk of being hit by ROSAT's debris at 1-in-2,000. That's not a 1-in-2,000 chance of being hit by a satellite this weekend. Rather, the probability of someone on Earth getting injured is one for every 2,000 de-orbit events of this kind. When NASA encountered a similar debacle as its 6.5-ton Upper Atmosphere Research Satellite (UARS) tumbled toward Earth last month, it put its risk estimate at a lower one in 3,200.

Nobody was injured in UARS's descent. Nor are there many cases of humans even witnessing falling space debris — despite the phenomena being more common than you might think.

"Space debris re-enters Earth's atmosphere on an almost weekly basis," says the DLR website. "Over the last few years, the total mass of this debris has amounted to about 60 to 80 tons per year."

But even with the high frequency of falling space debris and the low risk of human injury, shouldn't scientists be doing something about a car-sized piece of metal that could potentially collide with our planet? Unfortunately, the satellite has no propulsion system that might enable a controlled landing, and the technologies for de-orbiting satellites and bringing them back to earth are still at the development stage. Since ROSAT ended its mission in 1999, communication with its aging equipment isn't possible. But scientists are doing their best despite these limitations to track where and when the satellite will re-enter Earth.

A number of international space facilities, specialists and computer programs will join the cooperative effort. If you would like to do some monitoring, you can track ROSAT's orbit at real-time satellite tracking site n2yo.com.

Image, an artist's impression of the ROSAT satellite in space, courtesy of DLR