Take cover: Another satellite is plunging to Earth

— -- It's déjà vu all over again, as Yogi Berra might have put it. This month, another defunct satellite is expected to plunge to Earth — offering us another welcome respite from worrying about our real problems.

Instead, we can worry about the fantastically slim odds of being killed by some of the debris that will be shed by Germany's ROentgen SATellite (ROSAT) X-Ray telescope on its fiery uncontrolled re-entry to Earth just before Halloween.

"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," says the European Space Agency's Heiner Klinkrad, in a statement on the re-entry. "In the final phase, ROSAT will be 'caught' by the atmosphere at which point it will not even complete an orbit around the Earth: Instead, it will go into 'free fall'."

That will make it just like the Sept. 23 plunge of NASA's UARS satellite, which the space agency says ended up in the middle of the Pacific after a month-long vigil waiting for its re-entry. The German Aerospace Center puts the odds of its falling satellite's parts hurting anyone on the planet at 1-in-2,000, a bit more dangerous than the UARS 1-in-3,500 odds offered up by NASA last month. (Perhaps hoping to goose German tourism, Klinkrad notes, "The risk of someone in Germany getting injured is to the order of about 1(-in-)700,000.")

Both UARS and ROSAT launched before the international community agreed that the odds of a person being injured need to be better than 1-in-10,000 before space-faring nations would be required to take some action to disrupt an uncontrolled re-entry of a satellite. Now nations try to steer the satellite to a targeted re-entry over the ocean, and the Defense Department went as far as actually blasting one from the sky, the malfunctioning USA-193 spy satellite, in 2008. Missile Defense Agency head Lt. Gen. Henry Obering, now retired, warned of re-entry injury odds as high as 1-in-25, as reason for shooting the satellite down.

An international mission, the 5,300-pound ROSAT headed skyward in 1990, on an 18-month mission to map, for the first time, all the sources of X-rays in the sky (The satellite's namesake, physicist Wilhelm Roentgen, won the first Nobel Prize for his 1895 discovery of X-rays). ROSAT mapped roughly 110,000 stars, supernovas and cosmic ray sources of X-rays. It also discovered that comets emit X-rays and went silent in 1999, years after the extension of its original mission.

"I don't feel the same connection to the satellite as I do to, say, Hubble, but still, it's a little sad to see it come down," said astronomer Phil Plait, on his Bad Astronomy website. ROSAT, he adds, did provide years of outstanding service to the astronomical community, and gathered a vast amount of data about the high-energy universe around us."

Bits of rocket engines, spare parts and other sizable space junk fall weekly from the sky, according to NASA space debris expert Nicholas Johnson of Johnson Space Center in Houston. Satellite plunges of the size of UARS, six tons, happen about once a year, while ROSAT, at 2.4 tons, is a more minor league event. "The re-entry of objects with mass similar to ROSAT is much more frequent," Johnson says by e-mail, "many times each year."