Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past.
Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon.
Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.
Bibliography: http://www.solarviews.com/eng/asteroid.htm
DK Ultimate Visual 2005 Edition
Asteroid Classification:
Asteroids are classified into two groups: the characteristics of their orbits, and features of their reflectance of spectrum. (The visible spectrum (or sometimes optical spectrum) is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye.)
Orbit groups and families
Many asteroids have been placed in groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic break-up of a large parent asteroid sometime in the past.
Spectral classification
This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m across can be seen.
In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.[10] These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:
C-type asteroids - carbonaceous, 75% of known asteroids
S-type asteroids - silicaceous, 17% of known asteroids
L-type asteroids - metallic, 8% of known asteroids
This list has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied.
Bibliography: http://www.wikipedia.org/ (Only a little bit taken off)
Question:Will an asteroid hit the Earth?
Answer: A number of recent movies have been about this subject. It is important to remember that they are just movies and often times do not represent good or true science. Sometimes, however, there is a tiny seed of truth behind a movie, although it may be hard to tell what is true and what is fiction. This is the case for the movies about asteroids hitting the Earth.
Let us say right away that at this moment, we know of nothing currently on a collision course with Earth. Last year, it was announced in the news that an asteroid was on a collision course with Earth and would hit in the year 2028. We now know that the calculations were in error and that asteroid will miss the Earth completely.
Bibliography: The Book of Knowledge
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