Asteroids, sometimes called "minor planets," are composed of rock, metal, or both. Most meteorites probably originate as fragments of asteroids. Asteroids range in diameter from a meter up to 933 kilometers for the largest known asteroid, Ceres. Most asteroids orbit the sun in the main asteroid belt, a flat ring which lies between the orbits of the fourth planet Mars and the fifth planet Jupiter. The main asteroid belt is defined as reaching from 2.2 to 3.3 AU (1 AU is the average distance of the Earth from the Sun). Over 7,000 main belt asteroids are now known. There may be over a million at least one kilometer in diameter. The total mass of all these asteroids is less than 0.09% that of the Earth's Moon.

Asteroid 951 Gaspra
Gaspra (NASA photo)

The main belt asteroid 951 Gaspra was photographed by the Galileo spacecraft on its way to Jupiter. Gaspra's shape is irregular. The asteroid measures about 19 x 12 x 11 kilometers in size. The ratio of small to large craters on Gaspra's surface suggests Gaspra formed relatively recently (e.g., about 200 million years ago) during the collisional breakup of a parent body perhaps ten times the current size of Gaspra.

The main asteroid belt contains several regions in orbital resonance with Jupiter called Kirkwood gaps. The Kirkwood gaps appear at distances from the Sun that are ratios of integer multiples of Jupiter's orbital radius. Hence these Kirkwood gaps have been cleared of asteroids by the gravitional "nudges" applied by Jupiter.

Ten Largest Asteroids
Name Diameter (km)
1 Ceres 933
2 Pallas 538
4 Vesta 576
10 Hygeia 430
704 Interamnia 338
511 Davida 324
65 Cybele 308
52 Europa 292
87 Sylvia 282
451 Patientia 280

Origin Of The Asteroids

The main belt asteroids are believed to be the remnants of a planet which never formed. The gravitational influence of Jupiter prevented the small bodies from coalescing. Over the eons the asteroids have collided with each other, forming smaller fragments. Some of the larger asteroids underwent differentiation, resulting in an iron core and a rocky mantle, like the terrestrial planets. Other asteroids remained in an essentially unchanged and primitive undifferentiated state. Astronomers suggest two principal possible mechanisms by which the asteroids became hot enough to differentiate.

A mystery is why some asteroids differentiated and some did not even though they were of similar size and distance from the Sun. For example, the largest asteroid, Ceres, appears to be undifferentiated, while the smaller asteroid Vesta appears highly differentiated. In fact, as noted below, the majority of asteroids (about 75%) are undifferentiated carbonaceous chondrites.

An alternative hypothesis is that the asteroids are fragments of one or more exploded planets. This was a popular hypothesis during the nineteenth and early twentieth century, but it has fallen out of favor with most astronomers. One reason is that it is difficult to imagine a mechanism whereby a planet could explode. Another is that the distribution of asteroid types (see below) seems to militate against the exploded planet hypothesis. A few astronomers still hold to the exploded planet hypothesis. The late Tom Van Flandern was probably the best known proponent in the United States.

Types Of Asteroids

In terms of chemical composition, asteroids are classified into several main groups:

The inner portion of the main belt consists mostly of S-type asteroids. This graduates smoothly through M-type in the middle of the belt to to predominantly C-type in the outer main belt. This smooth transition is a standard argument against the exploded planet hypothesis for the origin of the asteroids. An explosion would most likely result in a random scatter of asteroid types through the main belt.

Millbillillie meteorite fragment
Fragment of Millbillillie Meteorite -- a piece of Vesta?

There are about a dozen or so other rare classification types. The fourth largest asteroid, Vesta, is the main member of the V-class. Vesta is about 576 km in diameter. It appears to be differentiated into layers like the terrestrial planets. Vesta may be the source of some groups of stony meteorites, in particular, the Howardites, Eucrites, and Diogenites (HEDs). As an example, the Millbillilie meteorite is a Diogenite. Its pyroxene spectral signature closely matches that of Vesta.

Several other small asteroids in Vesta's neighborhood exhibit the same unique pyroxene signature. They too may be fragments blasted from the surface of Vesta by impact events. Collisions between asteroids in the main belt occur with an average velocity of 5 km/sec. This is ample to cause energetic impacts. Astronomers suggest that Millbillillie was originally part of such a fragment that broke up in subsequent collisions, ended up in a Kirkwood gap, and was redirected by gravitational nudges onto a collision course with Earth. The Hubble Space Telescope revealed Vesta possesses a giant impact basin that may be the site of the original collision that produced Millbillillie and other HED meteorites.

Asteroid Families

About half of all the main belt asteroids occur in families. More than one hundred separate families have so far been identified. Families are probably the fragments of disrupted parent bodies. For example, the Eos family with about 60 members is thought to comprise the fragments of a parent object around 100 kilometers in diameter. The Themis family of C-type asteroids contains over 100 members. Its parent may have been 300 km in diameter. The parent of the S-type Koronis family with about 60 members may have been 90 km in diameter.

Near-Earth Asteroids

Some asteroids pursue orbits which cross those of the inner planets, including the Earth. Asteroids which approach the Earth are called Near Earth Asteroids or NEAs for short. Some of these near Earth asteroids were originally main belt asteroids perturbed out of their original orbits by the gravitational influence of the planets. Others appear to be extinct comets. Near-Earth asteroids are divided into four general classes based upon the relative location of their orbits to that of the Earth:

These four groups of asteroids are also referred to collectively as Apollos. Gravitational perturbations can shift a member of one near-Earth asteroid group to another. These near-Earth asteroids pose a significant danger since many will eventually strike the Earth. On the other hand, the Apollo asteroids present an opportunity for mining of raw materials since the Apollos are more easily reached than asteroids in the main belt.

Asteroid 4179 Toutatis
Toutatis (NASA photo)

The chaotic orbit of the Apollo asteroid 4179 Toutatis shown at the right reaches from just inside Earth's orbit out to the main asteroid belt. Toutatis appears to be a "contact binary" consisting of two objects which have fused together. On September 29, 2004 Toutatis passed by the Earth at a distance only about four times that of the Moon. Toutatis tumbles in a complex way as it rotates. This irregular rotation may reflect a history of violent collisions.

Asteroids In The Outer Solar System

There are two other groups of asteroids located outside the main asteroid belt.

Asteroid photos courtesy NASA.

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Last modified by pib on May 12, 2009.