Meteor Science:  Overview

Every day thousands of tons of solid debris enter the Earth's atmosphere at speeds in the range of 7 to 25 kilometers/second. Most of this material is no larger than a grain of sand and becomes vaporized within a fraction of a second, to settle finally as dust or be dispersed in gaseous form. 

At night the passage of some of these objects is marked by a brilliant trail - a "shooting star". The pieces large enough to survive are likely to strike the ocean, which covers more than 3/4 of the planet. Those that reach land usually fall unnoticed. Every tens of millions of years one of these objects proves large enough to obliterate much of life on the planet and thereby change the course of evolution. Chunks of intermediate size pose an increasing threat on a lesser scale as more of the Earth's surface becomes populated and so attract a growing interest amongst astronomers and other investigators. 

Meteors are not only intrinsically interesting, as dynamic apparitions in the night sky, but research into their distribution, orbital characteristics and origins might very well prove instrumental in averting the most colossal of disasters. 

The following links provide more information concerning "near misses" and some other spectacular comet/meteor events in recent history.

	Tunguska Event, Russia, 1908
	Comet Shoemaker/Levy Strikes Jupiter, 1994
	Fireball, 1997
	Yukon, Canada, 2000
	"Empty Quarter" Impact in Arabia ~ 1850(?)

A question sometimes arises concerning the distinction between meteors and asteroids. The term "meteor" is broadly used to denote most naturally occurring objects of any type entering the Earth's atmosphere. Meteors associated with meteor showers are remnants of comets - either known, long since vanished or yet to be discovered. These are by far the most common meteors. Comets and their fragments travel in highly elongated orbits and originate outside the solar system. Asteroids belong to that vast assemblage of rocky debris orbiting the sun in an orderly fashion between Mars and Jupiter. Occasionally some configuration and alignment of planets produces gravitational forces inducing one or more of these into an eccentric orbit which even more rarely intersects that of the Earth. It is well-established in the geological/biological record that collisions with meteors of both types have had a profound effect on the Earth. 

While a good many meteors arrive from random directions (sporadics), large numbers of them are associated with definite patterns and close groupings generally described as "showers" or "storms" (when especially numerous), suggesting a shared origin. Furthermore, these patterns are periodic with a tendency to re-occur at the same time of the year or at longer but, nevertheless, predictable intervals. 

At almost any time there is a meteor shower of some sort in progress and new ones are still discovered from time to time. It is now understood that most meteor showers are remnants of comets, either extinct (in that they have long ago disintegrated) or still very much with us. For example, Halley's comet gives rise to the annual eta Aquarids meteor shower when its orbit intersects that of the Earth.

Owing to their common origin, meteors from within a particular shower appear to come from a single point in the sky called the radiant. The shower is usually named after the constellation containing the radiant, hence, "Leonids", "Perseids" etc.

These links provide a wealth of meteor information including schedules of expected activity:

	Calendar of meteor showers
	International Meteor Organization
	American Meteor Society
	Collected links for this site