By BBC News Online science editor Dr David Whitehouse
Astronomers may have solved the puzzle of what it was that brought so much devastation to a remote region of Siberia almost a century ago.
In the early morning of 30 June, 1908, witnesses told of a gigantic explosion and blinding flash. Thousands of square kilometres of trees were burned and flattened.
The asteroid was probably a pile of space rubble - like Mathilde
Scientists have always suspected that an incoming comet or asteroid lay behind the event - but no impact crater was ever discovered and no expedition to the area has ever found any large fragments of an extraterrestrial object.
Now, a team of Italian researchers believe they may have the definitive answer. After combining never-before translated eyewitness accounts with seismic data and a new survey of the impact zone, the scientists say the evidence points strongly to the object being a low-density asteroid.
They even think they know from where in the sky the object came.
"We now have a good picture of what happened," Dr Luigi Foschini, one of the expedition's leaders, told BBC News Online.
The explosion, equivalent to 10-15 million tonnes of TNT, occurred over the Siberian forest, near a place known as Tunguska.
The direction of the flattened trees is a vital clue
Only a few hunters and trappers lived in the sparsely populated region, so it is likely that nobody was killed. Had the impact occurred over a European capital, hundreds of thousands would have perished.
A flash fire burned thousands of trees near the impact site. An atmospheric shock wave circled the Earth twice. And, for two days afterwards, there was so much fine dust in the atmosphere that newspapers could be read at night by scattered light in the streets of London, 10,000 km (6,213 miles) away.
But nobody was dispatched to see what had happened as the Czars had little interest in what befell the backward Tungus people in remote central Siberia.
The first expedition to reach the site arrived in 1930, led by Soviet geologist L A Kulik, who was amazed at the scale of the devastation and the absence of any impact crater. Whatever the object was that came from space, it had blown up in the atmosphere and completely disintegrated.
Nearly a century later, scientists are still debating what happened at that remote spot. Was it a comet or an asteroid? Some have even speculated that it was a mini-black hole, though there is no evidence of it emerging from the other side of the Earth, as it would have done.
What is more, none of the samples of soil, wood or water recovered from the impact zone have been able to cast any light on what the Tunguska object actually was.
Researchers from several Italian universities have visited Tunguska many times in the past few years. Now, in a pulling together of their data and information from several hitherto unused sources, the scientists offer an explanation about what happened in 1908.
They analysed seismic records from several Siberian monitoring stations, which combined with data on the directions of flattened trees gives information about the object's trajectory. So far, over 60,000 fallen trees have been surveyed to determine the site of the blast wave.
Over 60,000 fallen trees have been surveyed to determine the site of the blast wave
"We performed a detailed analysis of all the available scientific literature, including unpublished eye-witness accounts that have never been translated from the Russian," said Dr Foschini. "This allowed us to calculate the orbit of the cosmic body that crashed."
The object appears to have approached Tunguska from the southeast at about 11 km per second (7 miles a second). Using this data, the researchers were able to plot a series of possible orbits for the object.
Of the 886 valid orbits that they calculated, over 80% of them were asteroid orbits with only a minority being orbits that are associated with comets. But if it was an asteroid why did it break up completely?
"Possibly because the object was like asteroid Mathilde, which was photographed by the passing Near-Shoemaker spaceprobe in 1997. Mathilde is a rubble pile with a density very close to that of water. This would mean it could explode and fragment in the atmosphere with only the shock wave reaching the ground."
The research will be published in a forthcoming edition of the journal Astronomy and Astrophysics.