Introduction

In addition to the Sun and planets, the Solar System contains many small bodies known as asteroids, comets, and meteoroids. Although these bodies often have distinguishing characteristics, the classification of them can sometimes cause controversy. For example, some moons may actually be asteroids that were trapped by the planet's gravity. The Earth and the Moon can be considered "double planets" and astronomers debate whether Pluto is really a planet. In order to classify these celestial bodies, all their characteristics must be taken into consideration. These characteristics include their composition, location, orbit, and origin.

Asteroids: General Properties of Asteroids

This is a picture of Asteroid 951 Gaspra. Gaspra has an irregular shape with dimensions of about 19 x 12 x 11 kilometers. (Courtesy USGS/NASA/JPL)

Asteroids, which are sometimes referred to as minor planets, are rocky, metallic bodies that revolve around the Sun, usually in a region known as the Asteroid Belt. The Asteroid Belt lies between the orbits of Mars and Jupiter and contains millions of asteroids. Over 6,000 asteroids have been named and 70,000 have been identified.

Although most asteroids revolve within the Asteroid Belt, a few asteroids have eccentric orbits that wander outside the region between Mars and Jupiter. Some asteroids even cross Earth's orbit. The unusually eccentric orbits of these asteroids are a result of collisions in the Asteroid Belt that sent asteroids on a new path.

Asteroids vary in size and composition, as well as orbit. The largest asteroid is Ceres, which has a diameter of 623 miles (1,003 km). Compared with Ceres, the vast majority of asteroids are small. In fact, the mass of Ceres is 25% of the combined mass of all the asteroids.

Asteroids are commonly classified into types according to their spectra. Among these types are type C, type S, and type M. Type C asteroids are extremely dark and comprise more than 75% of all asteroids. Type S asteroids are relatively light asteroids composed of metallic nickel-iron and make up 17% of all asteroids. Most of the other asteroids are type M, which are bright asteroids composed of pure nickel-iron. There are a few other rare types of asteroids.

The Discovery of Asteroids: Bode's Law

In the eighteenth century, a German astronomer, Johann Bode, found a sequence representing the distances of the planets from the Sun. The sequence, known as Bode's Law, works by starting with 0, then 3, and doubling the number each time until you reach 768. Then, by adding four to each number and dividing the result by 10, the numbers give the distance of each planet from the Sun in astronomical units (AU). Scientists noticed that the law worked flawlessly from Mercury to Uranus with one exception between the orbits of Mars and Jupiter where there was no known planet.

Astronomers, thinking that Bode's Law was correct, began to search for a planet between Mars and Jupiter. In 1801, Giuseppe Piazzi discovered an object between Mars and Jupiter and gave it the name Ceres, after the goddess of grain. Although Ceres is the largest known asteroid it is still very small compared to the planets, with a diameter of only 1,003 kilometers.

Scientists were not satisfied with the discovery of Ceres, so they continued to search for a planet. Within the next six years, three more small bodies were found between the orbits of Mars and Jupiter. Scientists soon realized that countless asteroids orbited where they had once expected to find a planet.

Theories on Asteroid Formation

After it became obvious that there was not a planet between Mars and Jupiter, astronomers began to make theories explaining the origins of the many small bodies called asteroids. Wilhelm Olbers, an amateur astronomer who discovered the second known asteroid, suggested that asteroids were the remnants of a planet that had been shattered by an explosion.

The most commonly accepted theory, though, states that asteroids are pieces of matter that never coalesced to form a planet. The fact that the combined mass of all the asteroids is far less than the mass of most planets led scientist to believe asteroids lack the gravity necessary to coalesce into a planet.

Comets: Properties of Comets

Comets are sometimes called "dirty snowballs" because they are mixtures of ices and dust. The nucleus of a comet, which is usually a few kilometers in diameter, is the only active part of a comet when it is further from than Sun than the orbit of Jupiter. Once the comet nears the Sun, the ices contained in the nucleus turn to gas and plasma, creating a long, visible tail that extends from the nucleus.

Comets are divided into two types by the period of their orbit: short period comets complete their orbit in 200 or less years and long period comets take more than 200 years to orbit the Sun. Of the two types, short period comets have less elliptical orbits. Short period comets, like asteroids and meteoroids, are left over bits that were never incorporated into a planet during planet formation.

Comet P/Halley as taken March 8, 1986 by W. Liller on Easter Island. The average period of Comet Halley is 76 years. (Photo LSPN - 1725)

The Kuiper Belt, which is 30-100 AU from the Sun, is a reservoir of these short period comets. During the formation of the Solar System, the Kuiper belt was on the outer part of the pre-planetary disk. Since the outer part of the disk was less dense than the inner part, only small comets could be formed, not large planets.

Long period comets, which have very elliptical orbits, usually originate from the Oort Cloud. The Oort Cloud is located 3,000 - 50,000 AU from the Sun and is thought to contain as many as 100 billion comets. Occasionally, a star passes by the Oort Cloud and disturbs the orbits of the comets within it. Some of these comets change course and enter the Solar System.

The Composition and Parts of a Comet

Comets are composed of dust and ices. The ices contained by comets include water, methane, ammonia, and carbon dioxide. These ices are sublimed off the nucleus when the comet nears the Sun. A dust tail, which is the part of a comet easiest to see, forms from dust particles that are driven off the nucleus by escaping gases and plasma. An ion tail, which can be as long as 2 AU when the comet is near the Sun, forms from plasma that interacts with the solar wind. In addition, a coma forms around the nucleus. A coma is a large, bright cloud of gases and dust ejected from the nucleus.

Meteoroids, Meteors, and Meteorites

This iron meterorite was found at Derrick Peak, Antartica. (Courtesy NASA/JPL)

Meteoroids are small chunks of stone that are capable of intersecting Earth's orbit. If a meteoroid enters the Earth's atmosphere, it is known as a meteor or "shooting star." If the meteor doesn't entirely disintegrate in the atmosphere and strikes the surface of the Earth, it is called a meteorite.

Meteors create a streak of light when they enter the Earth's atmosphere. The meteors travel at high speeds when they enter the atmosphere and the friction they encounter heats the surrounding area. Atoms and molecules in the surrounding area are excited and the atmosphere becomes luminescent.

Meteors enter the Earth's atmosphere on a regular basis and on most nights you can see a few meteors per hour. During meteor showers, however, meteors are visible at a much higher rate. Meteor showers are usually associated with comets. Comets cast off debris when they near the Sun and when Earth passes through the debris, its sky displays astonishing meteor showers.

Meteors also originate from asteroids that collided within the Asteroid Belt. These meteors are usually larger than those left behind by comets and are more likely to become meteorites by striking the Earth.

Meteorites vary in size and, fortunately, most of them are small. The largest meteorite ever discovered weighs 60 metric tons. The meteorite was found on a farm near Grootfontein, Namibia. Since many meteorites originated from asteroids, which are planetary objects that never fully formed a planet, they are studied closely to gain knowledge about planet formation.