According to findings reported at the time of this article, Earth had been struck repeatedly by relatively large asteroid fragments during the preceding ten years. The study suggested that such events were occurring roughly three to ten times more often than previously estimated. As the accompanying video illustrates, the author says that sheer luck was the reason none of these destructive objects had struck a densely populated urban area.
Researchers examining data from a global warning network used to detect nuclear explosions reported that between approximately 2001 and 2011, 26 explosions with energies equivalent to roughly 1 to 600 kilotons of TNT had occurred at locations around Earth. None, the article says, was a nuclear explosion; they resulted from large meteoroids entering or striking Earth. For comparison, the Hiroshima atomic bomb is described as having an explosive yield of only about 15 kilotons. [source]
The article states that earlier estimates had placed the chance of an impact by a sufficiently large object capable of destroying a major city at roughly one in 15,000,000, but that analysis of the warning-network data had led researchers to revise the estimate substantially, to approximately one event in every 1,500,000 to 5,000,000 impacts as described by the source.
For that reason, and to identify large objects potentially on collision courses with Earth, the B612 Foundation was working with Ball Aerospace toward launching the Sentinel Infrared Space Telescope Mission by around Solar Hijri year 1398. Its intended purpose was to survey hundreds of thousands of then-unidentified wandering objects in the Solar System and help identify potentially hazardous ones.
The original article links to the supporting material and a complete list of the impact events shown in the video; those links are preserved below. [source] [source]