Overview of the Sizes and Frequencies of Meteorites Hitting the Earths Atmosphere

Meteor strikes and their implications for life on Earth have been of interest for hundreds of years, but the means to obtain and quantify detailed data have just recently become available. For as long as mankind has been interested in the topic the individual appearances in the night skies of characteristic meteor streaks, or the examination of known craters were most of what science had available to work with. Man-made satellites in earth orbit equipped with surveillance and investigatory equipment of all kinds changed the nature of the data base forever.

We have firsthand knowledge of some of the more massive strikes which have assailed the planet. Little more than 100 years have passed since the famed Tunguska explosion in Siberian Russia, attributed to a small extraterrestrial body probably no more than 50 meters in diameter. The energy released by this relatively minor intruder was sufficient to destroy the largest of today’s modern cities and in fact flattened several thousand square kilometers of forest, the scars of which are extant to this day.

Arizona boasts the vast Barringer Crater, probably the best preserved major impact site on earth. The meteor is assumed to have been little bigger than the bolide which struck Siberia, no more than fifty meters in diameter. However, this missile was structured of dense iron and survived to make physical impact with the ground. The result is a crater 4100 feet wide, nearly 600 feet deep with a rim over 100 feet above the desert floor.

Siberian style visitors are expected, on average, about every 400 years. The Barringer crater was formed about 50,000 years ago; impacts of the size which created it are expected to have a roughly 1 one million year frequency.

Probably the most universally well-known meteor impact was the one notorious for terminating the dinosaurs and many other species 65 million years ago. This strike, widely known as the Chicxulub event was a devastating kill shot, 15 kilometers in diameter and traveling at a speed 20 times faster than that of a rifle bullet. It ripped into the earth with a power exceeding that of one billion Hiroshima nuclear weapons. It is estimated that events of this magnitude occur on average once each hundred million years.

But what of the information that satellite observation brings to the table? One unlikely source is the satellites operated by the United States Department of Defense and Department of Energy. These units’ foremost task is to watch for violations of the Nuclear Arms Proliferation Treaties but in the course of doing so they frequently discover small to medium high altitude meteor explosions. These 1 to 10 meter objects seldom impact the earth, and about 40 per year were detected in the first 8 years of the study – thru 2002.

Doctor Peter Brown of the University of Western Ontario is a leader in the field of meteorite investigation, as a look at his research will quickly reveal. Pioneering the use of surveillance and detection satellites Dr. Brown is rapidly expanding our knowledge of planetary strikes, near misses and air bursts by the smaller bolides. It appears that the earth is under a more intense bombardment than had been previously believed.

It is the nature of this work that a long time must be spent gathering information before patterns in meteorite activity, if any, can be established. Recently a method of using low, long frequency sound waves as a means of detecting and qualifying larger meteoric intrusions, called infrasound detection has been added to the tool kit of Dr. Brown and others in the field.

The science is still in its relative infancy, and will never be predictive in the sense of when the planet can expect the arrival of the next species destroying kill shot. It is however, pushing back and refining our knowledge of the day to day events that take place at the upper borders of our atmosphere.