The chance of anyone being hit by the debris of the defunct German satellite ROSAT's X-ray telescope, expected to fall to Earth this week, is one in 2,000, but what is sure is that 1.6 tons space junk will re-enter the atmosphere.

The estimate is that up to 30 fragments with a total mass of up to 1.6 tonnes might reach the surface of the Earth, says Andreas Schutz, spokesman of the German aerospace lab DLR. Because the spacecraft's mirrors had to be heavily shielded from heat, they are far more like survive re-entry, he said.

"The X-ray optical system, with its mirrors and a mechanical support structure made of carbon-fibre reinforced composite - or at least a part of it - could be the heaviest single component to reach the ground," the DLR spokeman said.

Heiner Klinkrad, head of the space debris office at the European Space Agency in Darmstadt, Germany, said ROSAT's design means more of hit will hit the surface. "ROSAT has a large mirror structure that survives high re-entry temperatures," he said.

According to space industry lawyer Joanne Wheeler of the London law firm CMS Cameron McKenna, "ROSAT does not have a propulsion system on board which can be used to manoeuvre the satellite to allow a controlled re-entry."

"And the time and position of ROSAT's re-entry cannot be predicted with any precision due to fluctuations in solar activity, which affect atmospheric drag," she added.

ROSAT was deactivated in 1999 and its orbit has been decaying since then.