Solar system - Asteroids

Asteroid Facts

About 6000 asteroids have been discovered. Several hundred more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in diameter. Of those in the 10 to 100 km range we have cataloged about half. But we know very few of the smaller ones; perhaps as many as a million 1 km sized asteroids may exist.

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

243 Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. They are the only asteroids which have beens studied closely. Currently is near 433 Eros the NEAR spacecraft, which investigate this asteroid.

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. All other known asteroids are less than 340 km across.

There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos and Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, and perhaps some of the newly discovered moons of Uranus and Neptune are all more similar to asteroids than to the larger moons.

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.

Characteristic
Mass	2 % of Moon mass
Diameter	from 914 km (Ceres) to 10-30 m
Average distance from Sun (AU)	2,2 - 3,3
Revolution period (length of year in Earth years)	3,5 - 6
Orbit inclination (degrees)	10 (majority)
Atmospheric components	without atmosphere
Materials	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

Characteristic