One hundred years ago today, a meteor exploded over Siberia and destroyed about one thousand square miles of forest. Imagine if today something like that happened somewhere between Boston and DC, or in Japan, or somewhere around the Benelux, or somewhere on the Indian subcontinent, or somewhere in southeastern China. Wired notes that the event came pretty close to maybe stopping communism in its tracks, and thereby altering the course of world history:
In its 1966 edition, the Guinness Book of Records concluded that, based on the Earth's rotation, had the Tunguska meteorite struck 4 hours, 47 minutes later, it would have obliterated St. Petersburg, then the capital of imperial Russia. Given the events that would shortly torment that nation – and all of Europe [Ed. note: to say nothing of Asia, Africa, and the Americas!] – for the better part of the 20th century, one is left to wonder how history might have changed in those circumstances.
Ronald Bailey of Reason puts the odds back into perspective, though:
Of course, 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.
For more on the Tunguska event, as it's called now (at the time it didn't attract much attention, though who knows what kinds of records about the event were lost during World War I, the Russian Revolution, or the ensuing Russian Civil War), see the Wikipedia article, replete with eyewitness accounts and more on the science of the event.