A German satellite the size of a car is expected to re-enter the Earth's atmosphere over the weekend, officials said on Wednesday, adding they had little idea where the fragments were likely to land.

The x-ray observatory, named ROSAT, is expected to return to Earth between October 22 and 23, travelling at a speed of around 28,000 kilometres (17,000 miles) per hour, the German Aerospace Centre (DLR) said in a statement.

The DLR had previously been banking on re-entry between October 20 and 25 but said they could be slightly more accurate as the satellite nears.

Nevertheless, the space debris could arrive two days before or after the currently expected time, the DLR said.

"This time slot of uncertainty will be reduced as the date of re-entry approaches," it said.

According to the latest estimates, as many as 30 individual pieces weighing a total of 1.7 tonnes could reach the surface of the Earth.

"The largest single fragment will probably be the telescope's mirror, which is very heat resistant," the centre said.

However, statistically speaking, there is very little danger to humans from space junk, the experts said. The debris will almost certainly fall in the sea or on an uninhabited piece of land.

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.