All of the talk about an asteroid wiping out life on Earth next year is almost certainly overblown. Discovery Magazine's Bad Astronomy blog writes that the odds of 2012 DA14 impacting Earth this year are "essentially zero."
2012 DA14 shares the Earth's orbit with the sun, so it could come back to pose a threat in the future. But even a worst-case scenario impact from the asteroid, which is 150 feet in diameter, isn't likely to wipe out civilization. First, it would have to overcome the substantial odds of making impact with our planet. Then, it would need to survive entry through Earth's atmosphere. And finally, it would have to strike a lake, or a body of water near enough a population center to trigger a tsunami, earthquake or some other natural disaster.
NASA publicly posts data on the likelihood of 2012 DA14's impact risk and force of impact. If the asteroid did make it past all of those barriers, its ultimate impact would likely be comparable to that of a nuclear blast.
The last-known such impact occurred in 1908 in Siberia. The Tunguska blast is famous for having knocked a man off his porch who was more than 40 miles away from the point of impact. Locals kept outside researchers away from the Tunguska site for 19 years, with many of them believing the blast was a physical manifestation of God. However, Tunguska was not what people typically think of as an asteroid collision. The asteroid is believed to have exploded some five to 10 miles above the Earth's surface, sending a shock wave of destruction below.
Next February, 2012 DA14 is expected to pass within about 16,000 miles of Earth. Assuming it passes without incident, it won't travel within Earth's orbit again until around 2020.
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