-- 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."
So, ROSAT comes hard on the heels of UARS by chance. Starting in 1999 at an original 350-mile-high orbit, ROSAT has descended to a 168-mile-high elevation. A larger satellite with more resultant drag, UARS started 360 miles high in 2005. When satellites hit an orbit of about 70 miles high, they tend to be grabbed by the atmosphere and start their plummet to Earth.
Just like UARS, the German satellite is coming down in uncontrolled fashion. ROSAT lacks a rocket engine and all of its controlling gyroscopes and sensors are shot anyway. That prevents any effort to steer its crash by changing its orientation to create more or less drag over the ocean. Now steadily descending, ROSAT will reach a roughly 74 to 68-mile high orbit between Oct. 20 and Oct. 25, whereupon it will take a 10-minute 17,400 mile-per-hour death plunge to the surface, according to German Aerospace Center estimates.
"We are following the satellite closely, yes," Andreas Schütz of the German Aerospace Center, said in an interview in September. "We followed your satellite coming down," he added, good-naturedly. "Now you can follow our satellite."
The only real scientific mystery of the fall of ROSAT (or UARS) has been the fairly weak solar cycle that has kept the Earth's atmosphere from expanding as much as expected over the last three years. That delayed the expected ROSAT re-entry by a year, according to Klinkrad. Recent solar storms from the sun point to a slow ramping-up of the sun's 11-year solar storm cycle this decade.
About 30 parts of ROSAT could survive the re-entry, but chance, not science, will determine where. They could fall from 53 degrees North latitude to 53 degrees South latitude, anywhere from Berlin to Wellington, New Zealand (the world capitol farthest south, for trivia buffs.) Scattered over a 50-mile long path, the most worrisome bit of debris will be the probe's 880-pound high-temperature mirror, some 32-inches wide, which may survive the ride intact, at least until it lands, according to Schütz.
That's a lot of bad luck for anyone who breaks the mirror by letting it him them. It isn't very likely. But, "It ain't over till it's over," another thing Yogi Berra might have said, so it looks like we will just have to wait until the Fat Lady sings for ROSAT.