Asteroids are ancient massive fragments left from the formation of the solar system and .
may hold clues to that early time, which is why they are such an interest to scientists. Most .
asteroids orbit the sun in a "belt" of debris between Mars and Jupiter, but there are some whose .
orbits cross or come very close to Earth. To seek the asteroids and comets that threaten us, we .
return to 5 billion years ago when a huge star exploded, spreading a shock wave and forming a .
big cloud of hydrogen. This cloud gave off a great amount of heat which formed our sun, this .
was the beginning of our solar system. .
Asteroids are any of numerous small planetary bodies that revolve around the sun. .
Asteroids are called minor planets or planetoids. Most of them are located on the asteroid belt .
between the orbits of Mars and Jupiter. The belt contains more than 1,150 asteroids with .
diameters greater than 18 miles. The average temperature of the surface in a typical asteroid is .
-100 degrees Fahrenheit. Most known asteroids are the shattered remains of a smaller group of .
larger objects. Asteroids vary greatly in size. The largest and first known asteroid, Ceres, was .
discovered in 1801. It is 933 kilometers in diameter. One of the smallest that was discovered in .
1991 and named 1991 BA, is only about 20 feet across. Astronomers classify asteroids into two .
broad groups based on their composition. One group of asteroids dominates the outer part of the .
belt. These asteroids are rich in carbon. Their composition hasn't change much since the solar .
system was formed. Asteroids in the second group, which are located in the inner part of the .
belt, are rich minerals. These asteroids formed from melted materials. The amount of sunlight .
or heat reaching the earth depends on the size of the asteroid and it's distance from the sun. .
Therefore, calculations involving distance and light or heat yield the size of the asteroid.