What are Meteorites

 


 

Meteorites are pieces of other bodies in our solar system that make it to the ground when a meteor or "shooting star" flashes through our atmosphere at speeds of 15 to 70 kilometers per second (roughly 32,000 to 150,000 miles per hour). The majority originate from asteroids shattered by impacts with other asteroids. In a few cases they come from the Moon and, presumably, comets and the planet Mars. Meteorites that are found after a meteoric event has been witnessed are called a "fall," while those found by chance are called a "find." Meteorites are usually named after a town or a large geographic landmark closest to the fall or find, collectively termed localities. The word "meteorite" can refer to an individual specimen, to those collected within a strewnfield, or to a specific locality.

A very large number of meteoroids enter the Earth's atmosphere each day amounting to several hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites. The largest found meteorite (Hoba, in Namibia) weighs 60 tons.

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in 300,000 years on average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences.

 A good example of what happens when a small asteroid hits the Earth is Meteor Crater near Winslow, Arizona. It was formed about 50,000 years ago by an iron meteor about 30-50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on the Earth, so far (see below).

 

 Here are educated guesses about the consequences of impacts of various sizes:

Impactor Diameter (meters) Yield (megatons) Interval (years) Consequences
 
< 50 < 10 < 1 meteors in upper atmosphere most don't reach surface
75 10 - 100 1000 irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city
160 100 - 1000 5000 irons,stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo)
350 1000 - 10,000 15,000 land impacts destroy area size of small state; ocean impact produces mild tsunamis
700 10,000 - 100,000 63,000 land impacts destroy area size of moderate state (Virginia) ocean impact makes big tsunamis
1700 100,000 - 1,000,000 250,000 land impact raises dust with global implication; destroys area size of large state (California, France)

from 'The Impact Hazard', by Morrison, Chapman and Slovic, published in Hazards due to Comets and Asteroids

 

Find Out More About Meteorites

These books will help you learn more about meteorites:

Rocks from Space by O. Richard Norton, Mountain Press, 1994. This book covers just about every aspect of meteorites in a way that the layman can easily understand. Norton devotes most of a chapter to Sikhote-Alin.

Meteorites & Their Parent Planets by Harry McSween, Cambridge U. Pr., 1987. Well written book for a layman with a technical background.

Handbook of Iron Meteorites by Vagn Buchwald, U. of California Press, 1976. A very complete technical description of known iron meteorites including Sikhote-Alin. This book is a major source of the information on this page.

Let's Investigate Magical, Mysterious Meteorites by Madelyn Carlisle, Barron's, 1992. A well-done book for children-but written in a way that even adults will learn from it.