Asteroid Gaspra.

Asteroid

    About 6000 asteroids have been discovered.

    The total mass of all the asteroids is less than that of the Moon.

    The largest asteroid by far is 1 Ceres. It is 914 km in diameter and contains about 25% of the mass of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter.

    Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo:
 
        C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles;
        S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;
        M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron.

Asteroids.
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    Asteroids are also categorized by their position in the solar system:

        Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).
        Atens: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;
        Apollos: semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU
        Amors: perihelion distances between 1.017 and 1.3 AU;
        Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). More than 1000 such asteroids are now known; curiously, there are twice as many in the leading point than in the trailing one. (There may also be a few small asteroids in the Lagrange points of Venus and Earth (see Earth's Second Moon) that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)
 
    Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps. These are regions where an object's orbital period would be a simple fraction of that of Jupiter. An object in such an orbit is very likely to be accelerated by Jupiter into a different orbit.

    There also a few "asteroids" known in the outer solar system: 2060 Chiron (aka 95 P/Chiron) orbits between Saturn and Uranus; the orbit of 5335 Damocles ranges from near Mars to beyond Uranus; 5145 Pholus orbits from Saturn to past Neptune.

Comets, Asteroids, relationship?
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Comet Shoemaker Levy 9, on its way to collision with Jupiter.

The Kuiper Belt and The Oort Cloud

    Careful orbital calculations done in 1950 by Jan Oort indicate that a huge spherical "cloud" (now called the Oort Cloud) of perhaps a trillion (1e12) or more comets orbit the Sun far beyond the orbit of Pluto from about 30,000 AU to a light-year or more. This is the source of the long-period comets.
 
    The Oort Cloud may account for a significant fraction of the mass of the solar system, perhaps as much or even more than Jupiter.

    The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets.

Halley's Comet 1986 
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Meteors and Meteorites

Meteorite Facts

    Meteorites are bits of the solar system that have fallen to the Earth. Most come from asteroids, including few are believed to have come specifically from 4 Vesta; a few probably come from comets. A small number have been shown to be of Lunar or Martian origin.

    Meteorites are classified as:

        carbonaceous chondrites: very similar in composition to the Sun less volatiles; similar to type C asteroids .
        ordinary chondrites: by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets .
        irons: primarily iron and nickel; similar to type M asteroids .
        stony irons: mixtures of iron and stoney material like type S asteroids;
        achondrites: similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites .
 
    The largest meteoroids reach the surface to become meteorites.

    The average meteoroid enters the atmosphere at between 10 and 70 km/sec.
 
    This meteoroid burns up to give spectacular meteor showers or shooting stars.

• Information extracted from Britannica 97