The orbits of the vast majority of asteroids over 1 kilometer in diameter are very well known, but as asteroids get smaller, we venture into uncertain territory. "The residual impact risk - from asteroids with yet-unknown orbits - is shifting to small-sized objects," said planetary scientist Peter Brown, at the University of Western Ontario in London, Canada.
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Previously, models predicted the impact frequency of Chelyabinsk-sized meteors to be approximately one every 150 years, but as Brown points out, from the data collected around this Russian meteor event and other small recent atmospheric impacts, this estimate is likely inaccurate. The upshot from this new research is that, on average, we should expect a Chelyabinsk-sized event once every 25 years at least.
It is worth highlighting, that while interesting, this doesn't mean that we can expect a city to get hit, on average, once every couple of decades. 70 percent of the planet is ocean and the majority of the rest is unpopulated regions, so events like Chelyabinsk are rare in that metropolitan regions are a relatively small target. So, although this research adds some valuable detail to the complexities of asteroid impact risk, there's no reason to worry unduly.