A defunct German research satellite plummeted to earth at speeds of up to 280mph last night and crashed somewhere in Southeast Asia, a scientist said, but its exact whereabouts remain a mystery.

Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said two Chinese cities with millions of inhabitants each, Chongqing and Chengdu, had been in the minivan-sized satellite's projected path during its re-entry time.

'But if it had come down over a populated area there probably would be reports by now,' the astrophysicist, who tracks man-made space objects, said in a telephone interview.

Most parts of the satellite were expected to burn up as they hit the atmosphere, but up to 30 fragments weighing a total of 1.87 tonnes may have survived the re-entry, the German Aerospace Centre said.

Calculations based on U.S. military data indicate that debris of the ROSAT scientific research satellite must have crashed somewhere east of Sri Lanka over the Indian Ocean, or over the Andaman Sea off the coast of Myanmar, or further inland in Myanmar or as far inland as China, he said.

The satellite entered the atmosphere between 9:45pm to 10:15pm Saturday EDT and would have taken 15 minutes or less to hit the ground, the centre said.

Hours before the re-entry, the center said the satellite was not expected to land in Europe, Africa or Australia.

There were no immediate reports from Asian governments or space agencies about the fallen satellite, but an exact determination could take days.