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The asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation. Some asteroids have moons. The vast majority of the asteroids are within the asteroid belt, with elliptical orbits between those of Mars and Jupiter.

1 Definition

The term "asteroid" was coined by William Herschel because of the star-like appearance of the first few asteroids discovered (the other then-known planets all show discs, by comparison). The exact definition of an asteroid is unsettled. The term "minor planet" (or "planetoid") carries no strong suggestion about the composition of the object or its general location in the solar system, and some argue that not every minor planet should be called an "asteroid".

One way to classify asteroids is in terms of size. A working definition is that asteroids are larger than 50 m in diameter, distinguishing them from meteoroids, which are typically boulder-sized or smaller. The distinction is made because asteroids are large enough to survive passage through Earth's atmosphere and strike Earth largely intact while the smaller meteoroids generally break up high in Earth's atmosphere.

Thus, it would be safest to use the term "asteroid" for Solar System objects that are bigger than meteoroids, smaller than planets, and made out of rock, not ice.

See Solar System for a complete taxonomy of objects in our system, and minor planet for a taxonomy of the subplanetary objects that include asteroids.

2 Earth's Solar System

Tens of thousands of asteroids have been discovered within Earth's solar system. As of October 28, 2004, 96,154 "minor planets" had had their orbits calculated well enough to be given official numbers and 11,887 of these had been officially named. Current estimates put the total number of asteroids in the solar system at several million. The largest asteroid in our inner solar system is 1 Ceres1 Ceres ("SEER eez") was the first asteroid to be discovered, with a diameter of 959. It was discovered on January 1, 1801, by Giuseppe Piazzi, and originally named Ceres Ferdinandea after both the mythological figure Ceres ( Roman goddess of plants and m, with a diameter of 900-1000 km. Two other large asteroids are 2 Pallas2 Pallas ("PAL us") was the first asteroid discovered after 1 Ceres. It was found and named by H. Wilhelm Olbers on March 28, 1802. It was named after Pallas of Greek mythology, the daughter of Triton and friend of Athena. There is another Pallas in Greek and 4 Vesta4 Vesta ("VESS tuh") is the third-largest asteroid in the Main belt, between 530 and 468 km in diameter. This and the unusually bright surface make Vesta the brightest asteroid. It is the only asteroid sometimes visible to the naked eye. Vesta was discove; both have diameters of ~500 km.

See also a List of noteworthy asteroidsThe following is a list of noteworthy asteroids in our Solar system. For a more complete list of asteroids in sequential numerical order, see List of asteroids. Note: each asteroid is given a unique sequential identifying number after its orbit is precise in our Solar System, or a sequentially-ordered List of asteroidsThis is a list of numbered asteroids in sequential order. As of September 2004 there were over 90,000 numbered asteroids (and many more not yet numbered). Many asteroids are fairly ordinary and not particularly noteworthy. For a smaller list of "interesti.

3 Asteroid classification

Asteroids are commonly classified into groups based on the characteristics of their orbits and on the details of the spectrum of sunlight they reflect.

3.1 Orbit groups and families

Asteroids are divided into 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.

For a full listing of known asteroid groups and families, see minor planet.

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AsteroidsThis page is about the computer game. For the planet-like space object, see asteroid''. Asteroids is a popular vector-based video arcade game released in 1979 by Atari. The object of the game is for the player to shoot and destroy asteroids without being AsteroidAn asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid , which are much smaller than planets. The asteroids are believed to be remnants of the protoplanetary disc which were no Asteroid beltThe asteroid belt is a region of the solar system falling roughly between the planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found. It is termed the main belt when contrasted with other concentrations of minor planets, Asteroid deflection strategiesSince 1994 when Comet Shoemaker-Levy 9 collided with Jupiter, scientists have been researching if it may be possible to deflect incoming asteroids. With the discovery in 1999 that asteroid (29075) 1950 DA may collide with the Earth in 2880, this problem h Asteroid moonAsteroid satellites An asteroid moon is an asteroid that orbits another asteroid. It is thought that many asteroids may possess moons, in some cases quite substantial in size. Asteroids with moons are commonly referred to as binary asteroids, although tha Asteroid 37452 SpiritAsteroid 37452 Spirit was discovered on September 24, 1960 by Ingrid van Houten-Groeneveld, Cornelis Johannes van Houten and Tom Gehrels. The asteroid was spotted by examining photographic plates of images taken by telescopes at the Palomar Observatory. Asteroid 39382 Opportunity Asteroid light curve