Asteroids are sometimes called minor planets because they resemble planets in many ways. But by definition, an asteroid is a rocky or metallic atmosphere-free body that orbits around the Sun, usually less than 1,000 kilometers in diameter. Most asteroids tend to congregate in what has come to be called the main asteroid belt—a ring-shaped area between the orbits of Jupiter and Mars. It's generally thought that asteroids are leftover remnants from the early stages of solar system formation that never got formed into a planet because of Jupiter's intense gravitational field.

The first asteroid known to modern times is named Ceres after the Roman grain goddess. Originally thought to be a new comet by its discoverer, Guiseppe Piazzi, in 1801, this mammoth asteroid measures about 600 miles in diameter. Today there are more than several hundred thousand named asteroids, though most are much smaller than Ceres, and a few thousand more are found each year.

How are asteroids categorized?

Asteroids are commonly separated into types by two major factors: albedo and spectral composition. Albedo is the ratio of reflected light to incident (or source) light; white surfaces have an albedo of one, black surfaces have an albedo of zero. Spectral composition aids in identifying an object based upon the wavelengths of light the material absorbs or reflects.

Types of Asteroids

There are three basic compositional types of asteroids (plus a few other, much rarer classes):

  1. C-TYPE (carbonaceous) are extremely dark due to their hydrocarbon content, and have a reflectivity of around 3 percent. C-type asteroids comprise the basis for some of the oldest bodies in the solar system. Most asteroids, upwards of 75 percent, fall into the C-type category. Ceres and Pallas, for example, are both C-type asteroids.

  2. S-TYPE (silicaceous) asteroids are bright because of their high iron composition. They are constituted entirely of silicate materials, and typically have a reflectivity of around 15 percent to 20 percent. These asteroids represent between 10 percent and 15 percent of the asteroid population.

  3. M-TYPE (metallic) asteroids are comparatively brighter than S-type and C-type asteroids. They are composed of an iron and nickel alloy, and are often thought to be from the nuclei of other bodies. M-type asteroids, such as Psyche, are quite rare.

Asteroid Groupings

Asteroids can also be grouped by their solar system location:

MAIN BELT asteroids are located in the asteroid belt between the orbits of Mars and Jupiter. Most of the asteroids are in the Main Belt, and they can be divided into further orbital and compositional subgroups, each of which is named after the main asteroid in the group. The asteroids do not smoothly span the distance of the belt, however. Orbital resonances with Jupiter cause gaps in locations because any asteroid that happened to end up in that orbit would quickly be moved into a different orbit due to Jupiter's gravity.

The asteroid belt is an unstable place, and objects are often perturbed, or shifted, into orbits that cross paths with other asteroids. Impacts are common, and these impacts can send fragments of material into the inner solar system where they become meteoroids and can eventually impact Earth or other planets. Whole asteroids can also occasionally be perturbed into different orbits, making their way into the inner or outer solar system. The moons of Mars, and many of the small moons in the outer solar system, appear to be captured asteroids. These large objects occasionally impact the planets, producing spectacular impact craters.

NEAR-EARTH ASTEROIDS (NEAs) are asteroids that have been perturbed out of their main belt orbits and get especially close to Earth. Like main belt asteroids, NEAs are also grouped into various classes based on their orbits. Earth-crossing asteroids are those whose orbits cross the orbit of Earth. Scientists are especially interested in finding all the known NEAs because of the possibility of one hitting Earth; such an event could have catastrophic consequences. The Moon bears the scars of such impacts. The Near-Earth Asteroid Tracking (NEAT) program at NASA identifies new NEAs and provides carefully calculated orbits, producing up-to-date information on what we might expect in the near future.

TROJANS are located at stable points in Jupiter's orbit called Lagrange points. Several hundred of these objects have been located and as many as a thousand may exist in total. A few small asteroids may also be located in the stable Lagrange points of the orbits of Venus, Earth, and Mars.

The outer solar system also contains one strange class of objects, called Centaurs, that bear some similarities to asteroids, but probably contain a much higher percentage of frozen volatile gases and are better classified as comets. The most famous Centaur is Chiron, which orbits between Saturn and Uranus. Chiron was originally identified as an asteroid, but was recently reclassified as a comet. These objects are mostly in unstable orbits that are subject to change.

Famous Asteroids

Ceres carries great significance as the first documented asteroid. A mathematical formulation from the mid-1700s known as Bode's law predicted the finding of a planet between Mars and Jupiter, and the discovery of Ceres was originally thought to fulfill this prediction (until its small size was realized). Interestingly enough, Bode's law was also successful in predicting the location of Uranus.

The chances of an asteroid or comet hitting Earth are small but not nonexistent. After all, a giant asteroid did kill off all the dinosaurs 65 million years ago, right? Scientists are always on the lookout.

Ceres is large enough to be seen with the naked eye when its orbit brings it close enough to Earth. Heinrich Wilhelm Olbers, who identified further examples of minor planets, continued Guiseppe Piazzi's work, and asteroid hunting became an accepted part of astronomy.

The asteroid Vesta is one of the only other asteroids that can consistently be seen without binoculars or a telescope. It was observed through the Hubble Space Telescope between November 28 and December 1, 1994, and at that time the asteroid was only 156 million miles from Earth. Vesta is about 326 miles in diameter and has distinguishable light and dark areas, much like the craters on the Moon.

Vesta is thought to be one of the few asteroids to be fully differentiated, meaning that it is sufficiently large and has undergone enough heating to separate into layers based on density, with a metallic core and a mantle made up of lighter, rocky materials.

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