Earth has been hit by meteors many times in its history. In fact, earth is hit by meteors many times each and every day, to the tune of over 18,000 meteors larger than ten grams per year. Including space dust, the total amount of space material that is captured by the earth every year is estimated at somewhere between 37,000 and 78,000 tons. The vast majority of this material burns up completely in the atmosphere, sometimes leaving behind a brief flicker of light along their tracks. Most daytime meteors leave their impact only on the local radio waves.
To survive to the earth’s surface, a meteor has to be at least a metre in diameter when it makes contact with the earth’s atmosphere. Roughly 500 such meteors reach the earth’s surface every year, leaving behind small pit craters and meteorites ranging in size from marbles to basketballs. These meteorites have been known to cause injury and property damage, but because they have been slowed to terminal velocity, damage and injury is no worse than that caused by non-space objects falling out of the sky.
Meteors of between five and ten metres strike the earth’s atmosphere approximately once per year. In these cases, the kinetic energy striking the atmosphere often causes these meteors to explode high in the atmosphere in a giant fireball, with little if any part of the meteor surviving to hit the ground. However, the linked concussion wave often rattles windows along its path. A meteor trail for an object of this probable size was caught on videotape by the telescopes of the University of Western Ontario in March 2008.
Meteor impacts of as large as fifty metres are rare, happening only once in a thousand years. Fortunately for us, most of these meteors either strike water or detonate high in the atmosphere. When they do strike land, these meteors cause extensive, but mostly localised, damage. The object that struck Tunguska, Siberia, is believed to have been of this size. Based on eyewitness reports, the sky may glow before and during the fall, possibly as a result of sudden atmospheric ionisation in a large-scale version of our familiar fluorescent light tube. Large-scale ionisation may also have been responsible for the strange weather witnessed before the event. Even though the object would have been falling faster than the speed of sound, both crackling and snapping sounds were heard, similar to those associated with the northern lights.
References:
Bland PA et al. (1996) The flux of meteorites to the Earth over the last 50,000 years. Monthly Notices of the Royal Astronomical Society. 283:551-65.
Western News. Western astronomers on hunt for meteor. University of Western Ontario, March 7, 2008. http://communications.uwo.ca/com/media_newsroom/media_newsroom_stories/western_astronomers_on_hunt_for_meteor_20080307441590
