Defining exactly what is an “asteroid” is somewhat problematic, as there are numerous types of celestial bodies that could fit into this category. While celestial objects strike the Earth all the time, most of them are very small. Objects between 5 and 10 meters in diameter enter the atmosphere of the Earth about once a year, but they are almost always vaporized before reaching the ground. Fifty-meter or larger asteroids strike about once every 1,000 years, the most recent example being the strike at Tunguska, Russia in June 1908. Very large objects, asteroids 1km or more in diameter, are fortunately extremely rare, striking the earth about once every 500,000 years. Generally, the larger the asteroid, the less likely it is to strike the Earth; for example, an impact of the size theorized to have been involved in the mass extinction of dinosaurs has not happened for at least 65 million years.
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