An asteroid, also called a minor planet or planetoid, is a member of a group of small, planet-like bodies that are part of our solar system. They are believed to be remnants of the interstellar clouds, nebula, that were not incorporated into planets during the formation of the solar system.
The largest asteroid in the inner solar system is Ceres with a diameter of 1003 km. It also was the first to be discovered, by Giuseppe Piazzi on January 1, 1801. Nowadays, over 9000 asteroids are known, some less than 1 km across. Two other large asteroids are Pallas and Vesta. The first nearby pictures of an asteroid were taken by the Galileo spacecraft of Gaspra and Ida in 1991, while NEAR Shoemaker landed on Eros in 2001.
One large group of asteroids have orbits between the orbits of Mars and Jupiter, roughly 2 to 4 AU, in a region known as the "main belt." These couldn't form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence also results in Kirkwood gaps in the asteroid belt, orbits cleared by orbital resonance.
Another important group is called Trojans; they are in the orbit of Jupiter, on two Lagrangian points. Similar sets of asteriods appear to lie in the trojan points in almost all planetary orbits.
There is increasing interest in identifying asteroids whose orbit crosses the Earth's, and that could, given enough time, collide with the earth. The two most important groups of near-Earth asteroids are the Amors, and the Atens. Various asteroid deflection strategies have been proposed.
Another group of objects that didn't accrete to form planets (but are more icy, and so are not really asteroids) are the Kuiper Belt objects. The Kuiper belt is the source of about half of the comets that come to the inner solar system. Some of these are not much smaller than Pluto and Charon -- the largest found so far is Quaoar, thought to be around 1250 km in diameter, the same size as (or even slightly larger than) Charon - and some astronomers expect that we shall one day find some Trans-Neptunian objects bigger than Pluto.
There are also a few objects that orbit the Sun between the orbits of the giant planets, called Centaurs. The first of these to be discovered was 2060 Chiron in 1977. These are generally supposed to be asteroids or comets that were ejected from their proper orbits.
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 discover'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 Chiron.
Asteroids are classified into spectral types by their optical spectrum, which corresponds to the composition of the asteroid's surface material. 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.
- C-type asteroids - 75% of known asteroids. The C stands for "carbonaceous." They are extremely dark (albedo 0.03), similar to carbonaceous chondrite meteorites. These asteroids have approximately the same chemical composition as the Sun, minus hydrogen, helium and other volatiles. The spectra of these asteroids have relatively blue colors and are fairly flat and featureless.
- S-type asteroids - This type of asteroids represents about 17% of known asteroids. The S stands for silicaceous. They are relatively bright objects (albedo .10-.22). They have a metallic composition (mainly nickel, iron and magnesium-silicates). The spectra of this class are reddish and similar to those of stony-iron meteorites.
- M-type asteroids - This class includes most of the rest of the asteroids. The M stands for metallic; they are bright asteroids (albedo .10-.18), made of pure nickel-iron.
There are also a number of rarer asteroid types, the number of types continuing to grow as more asteroids are studied.
- E-type asteroids - The E stands for enstatite.
- R-type asteroids - The R stands for red.
- V-type asteroids - The V stands for Vesta, a large asteroid these are thought to be fragments of.
The largest asteroids:
|Number||Name||Diameter (km)||Date Discovered||Discoverer|
|1||Ceres||1003||January 1 1801||Piazzi, G.|
|2||Pallas||608||March 28 1802||Olbers, H. W.|
|4||Vesta||538||March 29 1807||Olbers, H. W.|
|10||Hygeia||450||April 12 1849||de Gasparis, A.|
|31||Euphrosyne||370||September 1 1854||Ferguson, J.|
|704||Interamnia||350||October 2 1910||Cerulli, V.|
|511||Davida||323||May 30 1903||Dugan, R. S.|
|65||Cybele||309||March 8 1861||Tempel, E. W.|
|52||Europa||289||February 4 1858||Goldschmidt, H.|
|451||Patienta||276||December 4 1899||Charlois, A.|
|15||Eunomia||272||July 29 1851||de Gasparis, A.|
|16||Psyche||250||March 17 1851||de Gasparis, A.|
|48||Doris||250||September 19 1857||Goldschmidt, H.|
|92||Undina||250||July 7 1867||Peters, C. H. F.|
|324||Bamberga||246||February 25 1892||Palisa, J.|
|24||Themis||234||April 5 1853||de Gasparis, A.|
|95||Arethusa||230||November 23 1867||Luther, R.|
Other noteworthy asteroids:
|Number||Name||Diameter (km)||Year Discovered||Comment|
|243||Ida||56 x 24 x 21||September 29 1884||Visited by Galileo probe|
|Dactyl||1.4||1991||Moon of 243 Ida|
|253||Mathilde||66 x 48 x 46||November 12 1885||Visited by NEAR Shoemaker|
|433||Eros||13 x 13 x 33||August 13 1898||Visited by NEAR Shoemaker|
|624||Hektor||February 10 1907||Largest Jovian Trojan asteroid discovered|
|951||Gaspra||19 x 12 x 11||July 30 1916||Visited by Galileo probe|
|2060||Chiron||170||1977||First Centaur to be discovered|
|3753||Cruithne||5||October 10 1986||Unusual Earth-associated orbit|
|4179||Toutatis||4.5 x 2.4 x 1.9||January 4 1989||Will approach Earth closely in 2004|
|4769||Castalia||1.8 x 0.8||August 9 1989||First asteroid to be imaged|
|5261||Eureka||June 20 1990||First Martian Trojan asteroid (L5 point) discovered|
|2002 AA29||0.1||January 9 2002||Unusual Earth-associated orbit|