Astronomers insist that the objects pose no threat to people on Earth, or to astronauts in orbit, because friction with the atmosphere melts them and breaks them up between 15,000 and 600 miles above the planet. The Mir space station orbits about 190 miles up.
The time-lapse picture above shows 54 seconds in the life of an object about the size of a two-bedroom house which passed high over Britain on the night of 26 September last year, before breaking up more than 5,000 miles above earth.
But experts are mystified about the origins of the extraterrestrial icecubes, some weighing more than 10 tonnes. They were discovered by Professor Louis Frank, of the University of Iowa, who said yesterday: "This relatively gentle 'cosmic rain' - which possibly contains simple organic compounds - may well have nurtured the development of life on our planet."
Professor Frank's theory had been controversial since he first proposed it 11 years ago, and had been widely derided - but yesterday scientists at the spring meeting of the American Geophysical Union were forced to admit that he had been right.
"The results definitively demonstrate that there are objects entering the Earth's upper atmosphere that contain a lot of water," said Thomas Donahue, of the University of Michigan. "These certainly vindicate Lou Frank."
The objects were discovered by satellite cameras on board Nasa's Polar spacecraft. The cameras showed that the objects are not condensing within the atmosphere, but approach the Earth from beyond. Their source is still unknown.Reuse content