Today is the 100th anniversary of the Tunguska event which blasted hundreds of square miles of Siberia.
Don Yeomans, manager of the Near-Earth Object Office at the Jet Propulsion Laboratory tells Science@NASA:
"A century later some still debate the cause and come up with different scenarios that could have caused the explosion," said Yeomans. "But the generally agreed upon theory is that on the morning of June 30, 1908, a large space rock, about 120 feet across, entered the atmosphere of Siberia and then detonated in the sky."
It is estimated the asteroid entered Earth's atmosphere traveling at a speed of about 33,500 miles per hour. During its quick plunge, the 220-million-pound space rock heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs.
"That is why there is no impact crater," said Yeomans. "The great majority of the asteroid is consumed in the explosion."
In my 2005 column, "Earth Killers from Outer Space," I noted:
…the probability of such a catastrophic asteroid strike is very small. Researchers at NASA's Spaceguard Survey estimate that a hit similar in size to the 1908 Tunguska event in Siberia occurs once every 300 years and would take out an area of about 5,000 square kilometers (1/100,000th of the Earth's surface). Huge portions of the planet are uninhabited (ocean) or sparsely inhabited, so Spaceguard calculates that another Tunguska is apt to hit a large urban area about once every 100,000 years. Ultimately the researchers calculate that the annual probability of an individual's death from a Tunguska-type impact is 1 in 30 million.
Whole NASA story on Tunguska here.