[ROLCAM]

Asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also called minor planets or planetoids. Most of them are located in the asteroid belt between the orbits of Mars and Jupiter. The belt contains more than 1,150 asteroids with diameters greater than 18 miles (30 kilometers). Astronomers estimate that more than 50,000 asteroids exist, but they have accurately determined the orbits of fewer than 10,000 of them. The average temperature of the surface of a typical asteroid is -100 °F (-73 °C).

There are two main theories for the origin of asteroids. One theory is that a planet located between the orbits of Mars and Jupiter broke up, forming the asteroids. According to the other theory, there was enough material for a planet between the two orbits, but no planet formed. Instead, the material simply became a belt of rocky objects.

Size. Asteroids vary greatly in size. The largest and first known asteroid, Ceres, was discovered in 1801. It is about 600 miles (970 kilometers) in diameter. Ceres is believed to contain about 1/3 the total mass of all the asteroids. One of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across.

Astronomers determine the size of an asteroid by comparing its distance from the sun with the amount and the wavelength of light it reflects and the amount of heat it gives off. Astronomers may also measure an asteroid during an occultation, when the asteroid passes in front of a star and is silhouetted against it. In addition, astronomers also use radio telescopes to produce images of asteroids.

In 1991, the United States space probe Galileo took the first detailed photograph of an asteroid. This was Gaspra, an irregularly shaped object measuring about 12 by 71/2 by 7 miles (19 by 12 by 11 kilometers).

Orbits. Most asteroids follow elliptical (oval-shaped) orbits in the asteroid belt. Groups of asteroids that follow the same orbit are called Hirayama families, named after Kiyotsugu Hirayama, the Japanese astronomer who first discovered them.

Many asteroids follow orbits outside the belt. For example, a number of asteroids called Trojans follow the same orbit as does Jupiter. Three groups of asteroids--Atens, Amors, and Apollos--orbit in the inner solar system and are known as near-Earth asteroids. Some near-Earth asteroids cross the path of Mars, while others cross Earth's orbit.

Many scientists believe that a near-Earth asteroid collided with Earth about 65 million years ago, triggering widespread environmental changes that led to the extinction of the dinosaurs. The asteroid created a huge circular depression called the Chicxulub (pronounced CHEEK shoo loob) Basin centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometers). In 1908, an object exploded about 6 miles (10 kilometers) above the Tunguska River area of Siberia. The object may have been a comet's nucleus or a large meteorite--sometimes referred to as a small asteroid. Debris from the explosion flattened forests and burned an area about 50 miles (80 kilometers) across.

The gravitational pull of Jupiter and other large planets causes asteroid orbits to change very slowly. Orbital changes lead to collisions that create smaller asteroids and fragments, increasing the chance of more collisions. Some small fragments reach Earth's surface as meteorites.

Composition. Studies of an asteroid's reflected light as well as analyses of meteorites have provided information about the composition of asteroids. 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 has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials.

Source: http://www.nasa.gov/worldbook/asteroid_worldbook.html

Can you add more information on this very fascinating subject.

[FlyYakker]

On the assumption that this might be homework, but even if not, and in no particular order...

You might consider if any of the other planets' moons are captured asteroids?

Differentiate asteroids from comets, as they also orbit the sun.

You might also note that very recently an asteroid came within about 400,000 miles of earth. I believe it's orbit closely parrales Earth's. Have there been other close passes observed?

Is anyone tracking asteroids for possible collisions with earth?

You might also note that many other asteroid/meteor impacts besides the Chicxulub impact have been found - most very, very, old, but I believe that Meteor Crater in Arizona is relatively (in the great scheme of things) new.

[Capuchin]

You should also differentiate between asteroids and meteoroids.

Also take a look at your second paragraph. You're talking about formation of asteroids in the asteroid belt only, so maybe you could clarify that a little better.

There are major asteroid orbits on the orbit of jupiter, as well, called the trojans:

http://upload.wikimedia.org/wikipedi...rSystem-en.png

[Capuchin]

You could also include a paragraph on one of my favorite asteroids, Ida. It has it's own satellite, Dactyl, which orbits it. Which shows that asteroids can be very planet-like.

243 Ida - Wikipedia, the free encyclopedia

[sarnian]

Can you add more information on this very fascinating subject?

You should at least note that the biggest quantity of asteroids and comets can be found far far out in the solar system, in the Kuiper Belt and the Oort Cloud.
Specially as the influence of other star systems passing the solar system in the galaxy arm is the cause of most of the dangerous intruders that over time threatened the earth.