Asteroid

An asteroid, also called minor planet, 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. One large group 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. Another important group is called Trojans; they are in the orbit of Jupiter, on two Lagrangian points. Jupiter's gravitational influence results in Kirkwood gaps in the asteroid belt, orbits cleared by orbital resonance. There is increasing interest in identifying asteroids whose orbit crosses the Earth's orbit, and that could, given enough time, collide with it. The two most important groups of near-Earth asteroids are the Amors, and the Atens.

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.

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. They are the origin 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 2001 KX76, thought to be around 1200 km around - 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 oribts 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		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

Public domain picture from NASA

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