30 June

The Tunguska event was a large explosion that occurred near the Stony Tunguska River, in Yeniseysk Governorate (now Krasnoyarsk Krai), Russian Empire, on this day in 1908.

The explosion over the sparsely populated Eastern Siberian Taiga flattened 2,000 km2 (770 sq mi) of forest (it caused no known casualties among humans). The explosion is generally attributed to the disruption in mid-air of a superbolide (comet or asteroid). It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of 5 to 10 kilometres (3 to 6 miles) rather than hit the surface of the Earth. Different studies have yielded varying estimates of the superbolide’s size, on the order of 60 to 190 metres (200 to 620 feet), depending on whether the body was a comet or a denser asteroid. It is the largest known impact event on Earth in recorded history.

Early estimates of the energy of the air burst range from 10–15 megatons of TNT, to 30 megatons of TNT, depending on the exact height of burst estimated when the scaling-laws from the effects of nuclear weapons are employed. However, modern supercomputer calculations that include the effect of the object’s momentum estimate that the air-burst had an energy range from 3 to 5 megatons of TNT, and that more of this energy was focused downward than would be the case from a nuclear explosion.

The 15 megaton (Mt) derived estimate is an energy about 1,000 times greater than that of the atomic bomb dropped on Hiroshima, Japan; roughly equal to that of the United States’ Castle Bravo (15.2 Mt) ground-based thermonuclear test detonation on 1 March 1954; and about one-third that of the Soviet Union’s later Tsar Bomba (at 50 Mt was the largest nuclear weapon ever detonated).

It is estimated that the Tunguska explosion knocked down some 80 million trees over an area of 2,150 square kilometres (830 sq mi), and that the shock wave from the blast would have measured 5.0 on the Richter magnitude scale. An explosion of this magnitude would be capable of destroying a large metropolitan area, but due to the remoteness of the location, no fatalities were documented. This event has helped to spark discussion of asteroid impact avoidance.