According to sensors that detect nuclear explosions, our planet has been hit by 26 asteroids since the year 2000.
For a brief period in the late 90s, pop culture got really concerned about cataclysmic asteroid strikes, to the point of making two Hollywood blockbusters about them. Over the past decade we've calmed down somewhat; asteroid detection technology advanced to the point that we can see civilization-killers years in advance, and the chances of being hit in the immediate future are incredibly small. But perhaps the space rocks which drive species to extinction aren't our most immediate problem; it's the smaller ones with enough energy to wipe out a city before you can say "I don't want to close my eyes". Oh, and on average, these asteroids seem to hit Earth twice a year.
At least that's what what the B612 Foundation is saying, based on findings from the Nuclear Test Ban Treaty Organization. From 2000-2013, the organization operated a worldwide sensor network to monitor nuclear explosions, but instead picked up several detonations that didn't fit with nuclear weaponry. It turns out that the sensor network was doing a great job of picking up atmospheric asteroid impacts, uncovering 26 explosions within this time frame.
The good news is these explosions occurred in the atmosphere, far above the point where they'd cause extensive ground damage. What's disconcerting is that these detonations aren't tiny, as each impact unleashed anywhere from 1 to 600 kilotons. For comparison, the Hiroshima atomic bomb exploded with 15 kilotons of force, similar to the 1908 Tunguska Event. Meanwhile, the near miss asteroid that injured hundreds when it burst above Chelyabinsk, Russia emitted at least 500 kilotons.
"While most large asteroids with the potential to destroy an entire country or continent have been detected, less than 10,000 of the more than a million dangerous asteroids with the potential to destroy an entire major metropolitan area have been found by all existing space or terrestrially-operated observatories," said Dr. Ed Lu, former astronaut and B612 CEO. "Because we don't know where or when the next major impact will occur, the only thing preventing a catastrophe from a "city-killer" sized asteroid has been blind luck."
It's these middle-range asteroids, somewhere in-between civilization-destroying behemoths and shooting stars, that B612 hopes will be detected by the in-development Sentinel telescope. In the meantime, this data should go a long way towards understanding the frequency of asteroid impacts, although hopefully we won't need Bruce Willis to prevent them just yet.