1. Tunguska - Thousands of square kilometers of trees were burned



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.

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.



The explosion, equivalent to 10-15 million tonnes of TNT, occurred over the Siberian forest, near a place known as Tunguska.







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.





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.



Possible orbits



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.



Trees, Luigi Foschini

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"We performed a detailed analysis of all the available scientific literature, including unpublished eye-witness accounts that have never been translated from the Russian," said Dr Foschini.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.





2. The Cando Event - A fireball in the sky



The Cando event was an explosion that occurred in the village of Cando, Spain, in the morning of January 18, 1994. There were no casualties in this incident, which has been described as being like a small Tunguska event.





Witnesses claim to have seen a fireball in the sky lasting for almost one minute. Up to 200 m3 of terrain was missing and trees were found displaced 100 m down the hill.

Opinions are divided about the causes of the explosion.Local residents, claim it was a meteor, as an object "the size of a full moon" was seen in the skies of the Spanish region of Galicia. The mystery became fertile ground for conspiracy theories that point to military or "alien activities".





3.The Vela Incident - An unidentified double flash of light



On 22 September 1979, sometime around 3:00am local time, a US Atomic Energy Detection System satellite recorded an unidentified double flash of light in a remote portion of the Indian Ocean.

Moments later an unusual, fast-moving ionospheric disturbance was detected by the Arecibo Observatory in Puerto Rico, and at about the same time a distant, muffled thud was overheard by the US Navy`s undersea Sound Surveillance System (SOSUS). Evidently something violent and explosive had transpired in the ocean off the southern tip of Africa.

Half a year later, researchers in western Australia detected increased amounts of radiation in the area. The signal appeared to come from a 3,000 mile area that included the South Atlantic, Indian Ocean, tip of Africa, and part of Antarctica. A presidential panel concluded in May 1980 that the signal was more likely an artifact of a meteoroid hitting the satellite and sunlight reflecting off particles ejected as a result of the collision.

Much of the information about the event is still classified.



4. Eastern Mediterranean Event - Calculated yield of about 2 Hiroshima bombs



The Eastern Mediterranean Event was a high-energy aerial explosion over the Mediterranean Sea, -between Libya and Crete, Greece- on June 6th, 2002.

This explosion, similar in power to a small atomic bomb, has been related to an asteroid undetected while approaching the Earth. The object disintegrated and no part was recovered. Since it did not reach the surface and it exploded over the sea, no crater was formed.

It was detected by satellites and seismographic stations, with a calculated yield of about 26 kilotons of TNT, approximately double the yield of the Hiroshima bomb, comparable to a small modern nuclear bomb.