A defunct satellite entered the atmosphere early Sunday and pieces of it were expected to crash into the earth, the German Aerospace Centre said.
There was no immediate solid evidence to determine above which continent or country the ROSAT scientific research satellite entered the atmosphere, agency spokesman Andreas Schuetz said.
Most parts of the minivan-sized satellite were expected to burn up during re-entry, but up to 30 fragments weighing 1.87 tonnes (1.7 metric tonnes) could crash into Earth at speeds up to 280 mph (450 kph).
Scientists were no longer able to communicate with the dead satellite and it must have traveled about 12,500 miles (20,000 kilometers) in the last 30 minutes before entering the atmosphere, Schuetz said.
Experts were waiting for "observations from around the world," he added.
Scientists said hours before the re-entry into the atmosphere that the satellite was not expected to hit over Europe, Africa or Australia. According to a precalculated path it could have been above Asia, possibly China, at the time of its re-entry, but Schuetz said he could not confirm whether the satellite actually entered above that area.
The 2.69-tonne (2.4 metric tonne) scientific ROSAT satellite was launched in 1990 and retired in 1999 after being used for research on black holes and neutron stars and performing the first all-sky survey of X-ray sources with an imaging telescope.
The largest single fragment of ROSAT that could hit into the earth is the telescope's heat-resistant mirror.
During its mission, the satellite orbited about 370 miles (600 kilometres) above the Earth's surface, but since its decommissioning it has lost altitude, circling at a distance of only 205 miles (330 kilometres) above ground in June for example, the agency said.
Even in the last days, the satellite still circled the planet every 90 minutes, making it hard to predict where on Earth it would eventually come down.
A dead NASA satellite fell into the southern Pacific Ocean last month, causing no damage, despite fears it would hit a populated area and cause damage or kill people.
Experts believe about two dozen metal pieces from the bus-sized satellite fell over a 500-mile (800 kilometer) span of uninhabited portion of the world.
The NASA climate research satellite entered Earth's atmosphere generally above American Samoa. But falling debris as it broke apart did not start hitting the water for another 300 miles (480 kilometres) to the northeast, southwest of Christmas Island.
Earlier, scientists had said it was possible some pieces could have reached northwestern Canada.
The German space agency puts the odds of somebody somewhere on Earth being hurt by its satellite at 1-in-2,000 — a slightly higher level of risk than was calculated for the NASA satellite. But any one individual's odds of being struck are 1-in-14 trillion, given there are 7 billion people on the planet.