German scientists say a defunct satellite has entered the atmosphere but there is no information yet on whether any of its pieces have crashed to Earth.
Andreas Schuetz, a spokesman for the German Aerospace Centre, said there was also no indication over which country or area the ROSAT satellite had started its plunge.
Experts are now waiting for "observations from around the world," Schuetz said.
Most parts of the minivan-sized satellite will burn up during re-entry into the atmosphere, but up to 30 fragments weighing 1.7 tonnes could crash into Earth at speeds up to 450 km/h.
"The time and location of re-entry cannot be predicted precisely," the aerospace centre said in a statement on its website. "All areas under the orbit of ROSAT, which extends to 53 degrees northern and southern latitude, could be affected by its re-entry."
The falling debris could land anywhere in an 80-kilometre-wide swath spanning the satellite's orbital track above the Earth, but will not likely strike Europe, the scientists calculated.
The largest single fragment of ROSAT that could hit the Earth is the telescope's heat-resistant mirror.
During its mission, which began in 1990, the satellite orbited about 600 kilometres above the Earth's surface, but since its decommissioning in 1999 it has lost altitude, circling at a distance of only 330 kilometres in June, the German space agency said.
ROSAT, which orbits the Earth every 90 minutes, was used to study black holes, neutron stars and the universe's X-ray emissions. It does not have its own propulsion system, which is why scientists have not been able to force it into a controlled atmospheric re-entry.
The German space agency puts the odds of somebody somewhere on Earth being hurt by its satellite at one in 2,000 — a slightly higher level of risk than the figures for a NASA satellite that fell last month. But any one individual's odds of being struck are one in 14 trillion, given there are seven billion people on the planet.