TUNGUSKA EVENT

enormous aerial explosion that, at about 7:40 AM on June 30, 1908, flattened approximately 2,000 square km (500,000 acres) of pine forest near the Podkamennaya Tunguska River, central Siberia (60 55 N, 101 57 E), in Russia. The energy of the explosion was equivalent to that of 10 to 15 megatons of TNT. Uncertain evidence of various kinds suggests that the explosion was perhaps caused by a comet fragment colliding with the Earth. Such an object, composed mainly of ice and dust, would disintegrate in the atmosphere high above the Earth's surface, creating a fireball and blast wave but no crater. It has been estimated that the object encountered the Earth at about 100,000 km per hour (62,000 miles per hour) and weighed anywhere from 100,000 to more than 1,000,000 tons. The remote site of the explosion was first investigated from 1927 to 1930 in expeditions led by Russian scientist Leonid Alekseyevich Kulik (18831942). Around the epicentre he found felled, splintered trees lying radially for some 15 to 30 km (9 to 18.6 miles); everything had been devastated and scorched, and very little was growing two decades after the event. The epicentre was easy to pinpoint because the felled trees all pointed away from it, but at the centre there was no crater, just a marshy bog. Eyewitnesses who had observed the event from a distance spoke of a fireball lighting the horizon, followed by trembling ground and hot winds strong enough to throw people down and shake buildings as in an earthquake. (At the time, seismographs in western Europe recorded seismic waves from the blast.) The blast had been initially visible from about 800 km (500 miles) away; and, because the object (whatever it was) vaporized, gases were dispersed into the atmosphere, thus causing the abnormally bright nighttime skies in Siberia and Europe for some time after the event.