Asteroids

More than 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.

243Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. They are the only asteroids which have beens studied closely. The NEAR mission will investigate 433 Eros.

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 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.
There are also a dozen or so other rare types.

Because of biases involved in the observations (e.g. the dark C-types are harder to see), the percentages above may not be representative of the true distribution of asteroids. (There are actually several classification schemes in use today.)

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). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth 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" (designated as "Centaurs") 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. There are probably many more, but such planet-crossing orbits are unstable and they are likely to be perturbed in the future. The composition of these objects is probably more like that of comets or the Kuiper Belt objects than that of ordinary asteroids. In particular, Chiron is now classified as a comet.

4 Vesta has been studied recently with HST (left). It is a particularly interesting asteroid in that it seems to have been differentiated into layers like the terrestrial planets. This implies some internal heat source in addition to the heat released by long-lived radio-isotopes which alone would be insufficient to melt such a small object. There is also a gigantic impact basin so deep that it exposes the mantle beneath Vesta's outer crust.

Though they are never visible with the naked eye, many asteroids are visible with binoculars or small telescopes.

Asteroid table

A few asteroids and comets are listed below for comparison. (distance is the mean distance to the Sun in thousands of kilometers; masses in kilograms).

No.  Name      Distance  Radius     Mass  Discoverer   Date
---- ---------  --------  ------  -------  ----------  -----
2062 Aten         144514       0.5   ?      Helin       1976
3554 Amun         145710       ?     ?      Shoemaker   1986
1566 Icarus       161269       0.7   ?      Baade       1949
 951 Gaspra       205000       8     ?      Neujmin     1916
1862 Apollo       220061       0.7   ?      Reinmuth    1932
 243 Ida          270000      35     ?      ?           1880?
2212 Hephaistos   323884       4.4   ?      Chernykh    1978
   4 Vesta        353400     263  2.38e20   Olbers      1807
   3 Juno         399400     123     ?      Harding     1804
  15 Eunomia      395500     136     ?      De Gasparis 1851
   1 Ceres        413900     457  1.17e21   Piazzi      1801
   2 Pallas       414500     261  2.18e20   Olbers      1802
  52 Europa       463300     156     ?      Goldschmidt 1858
  10 Hygiea       470300     215     ?      De Gasparis 1849
 511 Davida       475400     168     ?      Dugan       1903
 911 Agamemnon    778100      88     ?      Reinmuth    1919
2060 Chiron      2051900      85     ?      Kowal       1977

Open Issues

Why are there all those asteroids instead of a planet between Mars and Jupiter?
What mechanism(s) are responsible for the differentiation of the asteroids into metallic and rocky types?
Why are there more Trojan asteroids in Jupiter's L4 point than its L5 point?
Is 4 Vesta really differentiated? What is its geologic history?
How do asteroids get pushed from their 'normal' orbits into Earth-crossing ones? What is the probability that a large comet or asteroid will hit the Earth in any given year?
The original plan for the Cassini included an asteroid encounter on its way to Saturn, but it was dropped for cost reasons.
The NEAR spacecraft will orbit the near-Earth asteroid 433 Eros giving our first in-depth information about an asteroid.