|Update: According to an October 25th announcement by the German Space Agency (DLR), the Rosat spacecraft reentered Earth's atmosphere at 1:50 Universal Time on October 23rd. This would have placed it over the Bay of Bengal in daylight. There are no reports of eyewitness sightings or that any pieces were recovered.|
But the situation with Röntgensatellit (usually called Rosat) is worth mentioning, because this time the doomed craft is a German-built X-ray observatory launched by NASA in 1990. Its total mass is nothing special, just 2½ tons (less than half that of UARS). But much of that is concentrated in the telescope itself, which consists of two sets of nested mirrors made of gold-plated Zerodur bonded to a carbon-fiber structure.
Zerodur is an ultralow-expansion ceramic glass, much like the shiny-smooth surface found on modern stovetops. With a melting point of 1,100°F (600°C), it's unlikely to bubble into oblivion during reentry.
An analysis by the European Space Agency's Space Debris Office suggests that the spacecraft could begin to break apart at an altitude of about 65 miles (105 km). But the entire mirror assembly — 1.6 metric tons in all — should hit the ground intact at hundreds of miles per hour. Up to 29 other fragments, totaling another 200 pounds (100 kg), might survive the white-hot descent as well.
As was the case with UARS, Rosat has no propulsion system on board and so there's no way to control where and when it falls to Earth. Rosat's orbit is inclined 53° to the equator, putting virtually all inhabited areas at some risk. According to ESA's Heiner Klinkrad, the odds of anything striking somewhere in Germany (or in a comparable target area, say, Montana) are about one in 580.
"One day before Rosat reenters, we will be able to predict its landing with an accuracy of within one orbit," Klinkrad says. That's still an hours-long window covering a wide swath of land and ocean. As of this morning, the oddsmakers had pegged Sunday morning, October 23rd, as the most likely reentry time. But that prediction is still considered uncertain by ± 72 hours.
In a way, it's sad that hard-working satellites like UARS and Rosat only make the news when they fall from the sky in pieces. They deserve better. Measurements from UARS helped unlock the secrets of how chlorine-based gases create the atmosphere's ozone holes and allowed scientists to track the spread of sulfurous gases from Mount Pinatubo's titanic 1991 eruption.
Likewise, during its eight years of service, Rosat gave astronomers an important window on the X-ray universe (Röntgenstrahlen is the German word for X-ray). Its wide-field camera, supplied by British researchers at the University of Leicester, surveyed the entire sky. More importantly, a high-resolution imager, built by a NASA-funded team at the Harvard-Smithsonian Center for Astrophysics, zeroed in on thousands of million-degree-hot cosmic phenomena such as active galactic nuclei, neutron stars, and supernovae remnants. It even discovered, astoundingly, that some comets emit X-rays.
Operations came to an abrupt, premature end in September 1998, when Rosat's orientation system malfunctioned and caused the telescope to point directly at the Sun, which damaged the high-resolution imager beyond repair. Ground controllers shut down the craft for good the following February.
Officials at DLR might not be able to pinpoint the time and location of forthcoming Rosat's demise, but I'll give them credit for putting together an informative website (much of it in English) about the reentry and the observatory's accomplishments.
One last thought: Given all the hubbub regarding these two modest scientific satellites, I can't wait to hear how NASA plans to bring down the International Space Station when its time comes a decade or so from now. At the moment, there's no specific plan, though the project's partners are "committed to a controlled deorbit when the time comes," notes Nicholas Johnson, chief scientist for NASA's orbital-debris team.