A German satellite around the size of a car is speeding towards Earth, officials said Wednesday, due to re-enter the atmosphere later this month but with little idea where fragments could land.

The x-ray observatory, named ROSAT, is expected to return to Earth between October 20 and 25, travelling at a speed of around 28,000 kilometres (17,000 miles) per hour, the German Aerospace Centre (DLR) said in a statement.

"The latest studies reveal that it is possible that up to 30 individual pieces weighing a total of 1.6 tonnes may reach the surface of the Earth," the DLR said in a statement on its website.

"The time and location of re-entry cannot be predicted precisely," added the agency, citing fluctuations in solar activity for the uncertainty.

The satellite could re-enter the atmosphere three days before or after this range, the DLR said, stressing there was very little danger to humans.

Last month, a bus-sized US satellite that hurtled unpredictably toward Earth crossed over Africa and the northern Atlantic before plunging into the Pacific Ocean off California, NASA said.

There were no sightings or reliable accounts of damage as the six-tonne Upper Atmosphere Research Satellite (UARS) fell from the sky.