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An 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 not incorporated into planets during the system's formation. Some asteroids have moonss. The vast majority of the asteroids are within the asteroid belt, with elliptical orbits between those of Mars and Jupiter.

Table of contents
1 Definition
2 Earth's Solar System
3 Asteroid classification
4 Asteroid discovery
5 Asteroid deflection
6 Asteroid exploration
7 See also
8 External links


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.

Earth's Solar System

Tens of thousands of asteroids have been discovered within Earth's solar system. As of March 5, 2004, 79,084 "minor planets" had had their orbits calculated well enough to be given official numbers and 11,177 of these had been officially named. Current estimates put the total number of asteroids in the solar system at several million with a half million near-earth asteroids. The largest asteroid in our inner solar system is 1 Ceres, with a diameter of 900-1000 km. Two other large asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km.

See also a List of noteworthy asteroids in our Solar System, or a sequentially-ordered List of asteroids.

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.

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.

Spectral classification

In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison and Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of 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.

Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition:

This has led to great confusion though in that an asteroid's type is NOT indicative of its composition. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

However, scientists have been unable to agree on a new taxonomic system for asteroids and as a result, the spectral classification has stuck.

Asteroid discovery

Asteroid discovery proceeded in several phases.

The first asteroid 1 Ceres was discovered by accident in 1801, and then three others (2 Pallas, 3 Juno, 4 Vesta) over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, Paul Henry and Prosper Henry and Auguste Charlois.

In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This drastically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies".

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.

These first three steps do not constitute asteroid discovery: the observer has only found an apparition. The final step of discovery was to send the locations and time of observations to Brian Marsden of the Minor Planet Center. Dr. Marsden has computer programs that compute whether an apparition tied together previous apparitions into a single orbit. If so, then the observer of the final apparition is declared a discoverer, and the discoverer got the honour of naming the asteroid (subject to the approval of the International Astronomical Union).

When the orbit of an asteroid is confirmed, it is given a number, and later it may also be given a name (e.g. 1 Ceres). The first few are named after figures from Graeco-Roman mythology, but as such names started to run out, others were also used —famous people, the names of the discoverer's wives, even television characters. A few groups have names with a common theme —for instance Centaurs are all named after legendary Centaurs, and Trojans after heroes from the Trojan War. The Centaurs are of special interest; many of them are massive comets, such as 2060 Chiron.

Since 1998, a large majority of the asteroids have been discovered with automated systems that comprise Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. A list of teams using such automated systems include [1]:

The LINEAR system alone has discovered over 40,500 asteroids as of May, 2004 [1]. Between all of the automated systems, 2701 near-Earth asteroids have been discovered [1] including over 600 more than 1 km in diameter.

In 2003, a new class of asteroids, the Apoheles, was discovered when 2003 CP20 was found to have an orbit that always keeps it closer to the Sun than the Earth.

Asteroid deflection

There is increasing interest in identifying asteroids whose orbits cross Earth's orbit, and that could, given enough time, collide with Earth. The three most important groups of near-Earth asteroids are the Apollos, Amors and the Atens. Various asteroid deflection strategies have been proposed.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis and finally 69230 Hermes, which approached within 0.005 AU of the Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the level of alarm: the increasing acceptance of Walter Alvarez's theory of dinosaur extinction being due to an impact event, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 meters across.

All of these considerations helped spur the launch of highly-efficient programs such as LINEAR, NEAT and LONEOS, which have drastically increased the rate of asteroid detection.

Asteroid exploration

The first "nearby" photos of an asteroid were taken by the Galileo spacecraft of 951 Gaspra and 243 Ida (1991), while NEAR Shoemaker landed on 433 Eros (2001). Galileo also discovered the first known asteroid moon Dactyl which orbits 243 Ida.

See also

External links

The Solar System
Sun | Mercury | Venus | Earth | Moon | Mars | Asteroids | Jupiter | Saturn | Uranus | Neptune | Pluto
(For other objects and regions, see: List of solar system objects, Astronomical objects)