On June 30, 1908, a vast fireball raced through the dawn sky over Siberia, then exploded with the force of 1,000 Hiroshima bombs. The heat incinerated herds of reindeer and charred tens of thousands of evergreens across hundreds of square miles. For days, and for thousands of miles around, the sky remained bright with an eerie orange glow--as far away as western Europe people were able to read newspapers at night without a lamp. The effect was much like that of a great volcanic eruption, yet there had been no eruption. The only objective indication of the extraordinary event was a quiver on seismographs in the Siberian city of Irkutsk, indicating a moderate quake some 1,000 miles north in a remote region called Tunguska.
Scientists did not come to Tunguska for another 19 years, apparently reluctant to travel to a site so swampy and remote. When they did finally come, they were rewarded with a stunning vision of utter devastation, with scorched trees lying in rows that stretched to the horizon. They searched for a crater but found none. They searched for fragments of a meteorite--an asteroid or a chunk of one--but found none. All they found were eyewitnesses in neighboring villages who told of a fireball streaking through the sky, horrifying noise, and a blast that knocked people off their feet. Clearly something unprecedented had occurred at Tunguska, but the trees were the only tangible proof that remained.
For 88 years the mystery of Tunguska has attracted a swarm of theories, some eminently reasonable (meteorite impact) and some considerably less so (think exploding spaceship). While the debate continues as to precisely what happened there back in 1908, compelling evidence has recently turned up that could finally put an end to most questions. Researchers have found embedded in Tunguska trees tiny particles with an extraterrestrial signature. Combined with computer simulations, the evidence points to a meteorite born of an asteroid that fragmented in the atmosphere. For many researchers, the debate is no longer whether the cause was a meteorite but exactly what kind of meteorite.
Tunguska is the kind of mystery scientists will try to solve just for the thrill of the challenge, working after hours at laboratories and paying for the travel and fieldwork out of their own pockets. Yet there’s a very practical aspect to the work. Impacts of comets or asteroids-- collectively known as bolides--are an important part of the history of the solar system. Witness, for example, the black eye that comet Shoemaker-Levy gave Jupiter in 1994. Bolides have peppered Earth as well in the past--a particularly big one probably finished off the dinosaurs 65 million years ago--and today they pose an unknown risk to human civilization. Reconstructing that fiery day over Tunguska in 1908 may give us the best picture we have of what future the sky holds for us.
The first scientist to make the journey to Tunguska was Leonid Kulik, a Russian geologist who for years had collected meteorite fragments in other parts of Siberia. When, in 1927, Kulik first saw the vast panorama of charred, downed trees, his immediate impression was that an immense fire had begun simultaneously over a wide area.
