A DEFUNCT satellite is hurtling towards the atmosphere and pieces of it are expected to crash to earth within hours, the German Aerospace Centre says.
The x-ray observatory, named ROSAT, made its re-entry between 5.45PM AEDT and 6.15PM AEDT on Sunday, the German Aerospace Centre (DLR) said in a statement.
"There is currently no confirmation if pieces of debris have reached Earth's surface," the statement added.
According to estimates cited last week, as many as 30 individual pieces weighing a total of 1.7 tonnes could reach the surface of the Earth.
But Andreas Schuetz, spokesman for the DLR, said they would have to "wait for data in the next days" to know when and where the debris could fall.Pieces of the ROSAT scientific research satellite hit before 4.30pm today (AEDT), according to the agency.
Most parts of the minivan-sized satellite were expected to burn up during re-entry into the atmosphere but up to 30 fragments weighing 1.7 metric tons could crash into earth at speeds up to 450km/h.
The satellite orbits every 90 minutes and it could hit almost anywhere along its path - a vast swath between 53-degrees north and 53-degrees south that comprises much of the planet outside the poles, including parts of North America, South America, Europe, Africa and Asia
"According to the data we currently have, we expect it not to hit over Europe, Africa or Australia
," agency spokesman Andreas Schuetz said.
"The satellite is still orbiting and we are observing the data for other parts of the world," he added.
Fluctuations in solar activity and the fact that scientists are no longer able to communicate with the dead satellite render predictions of where and when it will come down yet more difficult.
The 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.
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 an 800km 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 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-2000 - 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.