Asteroids, Meteors, and Meteorites
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A million asteroids populate the zone between Mars and Jupiter known as the 
Asteroid Belt. Asteroids are essentially chunks of rock but they range in size 
from dust particles to 1,000 km across. Darker asteroids are probably made of 
carbon materials and the brighter ones may contain such metals as iron and 
nickel. Astronomers know of some 2,200 asteroids. The largest of these are Ceres 
(about the width
of France), Pallas, Vesta, and Juno.

See picture of the asteroid 243 Ida and its moon Dactyl (the small dot on it's 
right) 

We find out about the composition of asteroids by analyzing meteorites, which 
are essentially asteroids that have fallen out of orbit and survived the fiery 
passage through Earth's atmosphere. There are about 2,500 meteorites known. The 
largest is in Namibia, Africa and weighs 53.5 metric tons. The second largest, 
which landed in Greenland, weighs 53.5 metric tons and is on display at the 
Hayden Planetarium in New York City. 

When these chunks of rock are still in space they are called meteoroids. They 
strike the Earth's atmosphere at speeds up to 40 km/second. At a height of about 
128km, friction with the atmosphere surrounds the object with hot, glowing air. 
This fireball streaks toward Earth, becoming visible as a bright meteor. If it 
does not break up, the meteoroid will strike the surface and become a meteorite. 

Meteors from much smaller particles, about as large as grains of sand, can be 
seen almost any dark night. The best chance of seeing meteors occurs during 
meteor showers, when Earth passes through streams of dust left behind by comets.
