Our entire solar system, including comets, formed from the collapse of a giant, diffuse cloud of gas and dust about 4.5 billion years ago. When the cloud started its collapse, it was rotating very slowly. But the cloud began to heat up and whirl faster as it shrank, just as twirling ice skaters spin faster by bringing their arms close to their bodies. The fast rotation helped ensure that not all of the material fell into the core. Instead, the material in the fast-spinning cloud spread out into a flattened disk.

Meanwhile, the temperature in the dense, central core was heating up. The core eventually became so hot that it ignited nuclear fusion, creating the Sun. The disk's outer regions, however, were quite cold. The low temperatures allowed water to freeze onto dust grains, which grew in size to make clumps. Some clumps eventually reached a size of several kilometers in diameter. The clumps then began merging, probably by collisions, and formed the planets.

Many theories abound about how these clumps became planets. This topic is at the forefront of scientific research. Whatever the details, large planets were created from the buildup of clumps of matter and gas from the surrounding cloud. But some of this matter did not merge into planets.

Within the last decade, for example, astronomers discovered leftover clumps, called planetesimals, in a region beyond Neptune, although no large planets formed beyond that planet. These bodies form an outer asteroid belt at the edge of the solar system called the Edgeworth-Kuiper Belt, named for the scientists who proposed its existence in the 1950s. Recent calculations show that this asteroid-rich Kuiper Belt (as it is now known) is probably the source of most of the short-period comets, such as Halley's Comet, which orbits the Sun every 76 years.

A comet's tail is its most distinctive feature. As it approaches the Sun it develops an enormous tail of luminous material that extends for millions of kilometers away from the Sun. When far from the Sun, a comet's nucleus is very cold and its material is frozen. Water ice, as well as other compounds such as carbon dioxide and carbon monoxide ice, may be found in the nucleus.

This icy nucleus changes radically when a comet approaches the Sun. The intense solar wind from the Sun transforms the solid nucleus directly into a vapor, bypassing the liquid phase. This process is called sublimation. The vapor helps stir things up in the nucleus, forcing the core to form a cloud-like mixture of gas and dust around it called the coma. There, sunlight and the solar wind interact with the ingredients, creating the tails. The ingredients in the coma determine the types and number of tails.

Some comets may appear to have no tails, but they really do. They are simply very faint. Scientists can identify these tails by using special filters that are sensitive to dust or gas emissions. Other comets like Hale-Bopp, which could be seen from Earth in 1997, have very prominent tails. Although Hale-Bopp's tails could be seen visibly from Earth, scientists using sensitive cameras identified a much more complicated tail structure. One of these images revealed a long, curving dust tail. Other pictures showed dust and gas ion tails. There was even an image of a dust tail and two gas ion tails. The different tails provide scientists with important information about the internal chemistry and structure of a comet's nucleus.

There are two types of comet tails: dust and gas ion. A dust tail contains small, solid particles that are about the same size found in cigarette smoke. This type of tail forms when sunlight pushes on these small particles, gently pushing them away from the comet's nucleus. Because the pressure from sunlight is relatively weak, the dust particles end up forming a diffuse, curved tail.

A gas ion tail forms when ultraviolet sunlight rips one or more electrons from gas atoms in the coma, making them into ions (a process called ionization). A solar wind then carries these ions straight outward away from the Sun. The resulting tail is straighter and narrower. Both types of tails may extend millions of kilometers into space.

As a comet heads away from the Sun, its tail dissipates, its coma disappears, and the matter contained in its nucleus freezes into a rock-like material. Recent observations of the very bright comet Hale-Bopp pinpointed a tail made of sodium (Na), a relative of the gas ion tail. This type of tail forms when sunlight pushes on sodium atoms released from the nucleus.

Most of us probably have seen "shooting stars," or meteors. A meteor is the flash of light that we see in the night sky caused by the friction of a meteoroid passing through our atmosphere. A meteoroid is an interplanetary chunk of matter smaller than a kilometer and frequently millimeters in size. (Note that the term "meteor" refers to the flash of light caused by the meteoroid, not the meteoroid itself.)

Most meteoroids that enter Earth's atmosphere are so small that they vaporize completely and never reach the planet's surface. If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is called a meteorite. Although the vast majority of meteorites are very small, their size can range from about a fraction of a gram (the size of a pebble) to 100 kilograms or more (the size of a huge, Earth-destroying boulder).

Asteroids are generally larger chunks of rock that come from the asteroid belt located between the orbits of Mars and Jupiter. Comets are asteroid-like objects covered with ice, methane, ammonia, and other compounds that form a coma and sometimes a visible tail whenever they orbit close to the Sun. As a comet rides through the solar system, it leaves little particles in its wake. If Earth's orbit intersects this wake of particles, we see a meteor shower as the particles rain down through Earth's atmosphere.

Comets are found in two main regions of the cosmos: the Kuiper Belt and the Oort Cloud. Short-period comets — comets that frequently return to the solar system — probably originate from an area called the Kuiper Belt. This belt is located within the solar system's ecliptic plane, beyond the orbit of Neptune.

Astronomers found the first object in the Kuiper Belt in 1992. Since that discovery many objects have been discovered within that region. These objects are usually small compared with planets. Their size ranges from 10 to 100 kilometers in diameter. Earth's diameter, for example, is 14,000 kilometers.

The Hubble Space Telescope may have detected a population of small comets dwelling in this region in space. Based upon the Hubble observations, astronomers estimate that this belt contains at least 200 million comets, which are thought to have remained essentially unchanged since the birth of the solar system 4.5 billion years ago.

Long-period comets are thought to emanate from a vast, spherical cloud of frozen bodies called the Oort Cloud, named for the Dutch astronomer Jan Hendrik Oort. This cloud of comets, which also orbits the Sun, resides in the farthest region of the solar system, beyond Neptune and Pluto. The Oort Cloud objects are made up of matter such as frozen ammonia (NH₄), methane (CH₄), cyanogen (HCN), water ice (H₂O), and rock. Occasionally, a gravitational disturbance caused by a passing star or an interstellar cloud causes one of these bodies in the Oort Cloud to begin a journey toward the inner solar system, where it makes a passing rendezvous with our Sun.

Planets have nearly circular orbits, whereas comets have elongated paths around the Sun. A comet is at aphelion when its orbit is farthest from the Sun. It is at perihelion when it is closest to the Sun. Due to gravitational effects, a comet will travel fastest at perihelion and will slow down as it approaches aphelion.

Comets can be classified by their orbital period: that is, the time it takes them to make one complete trip around the Sun. Comets with short and intermediate orbital periods — like comet Halley, whose orbital period is 76 years — spend most of their time between Pluto and the Sun. These comets began as icy, asteroid-sized objects in the Kuiper Belt, but a gravitational "push" from the planets, especially Jupiter, swung them closer to the Sun.

Some of their orbital periods are shorter than 200 years. Comet Shoemaker-Levy 9 is an example of a comet that has been radically perturbed by Jupiter's gravitational effects. A long-period comet will have an orbital period of more than 200 years. Hale-Bopp, for example, completes an orbit every 4000 years. Four thousand years from now, comet Hale-Bopp will make another appearance in the inner solar system. Scientists think that this type of comet spends most of its time way out in the Oort Cloud at the farthest edge of our solar system.

Shoemaker-Levy 9 is a comet discovered by David Levy, Eugene Shoemaker, and Carolyn Shoemaker on the night of March 24, 1993. Instead of seeing a single coma and tail, the threesome discovered a coma in the shape of an elongated bar and several tails extending beyond it. Later, more detailed photographs showed the bar to be many individual fragments of the original comet.

From July 16, 1994 to July 22, 1994, these fragments of Shoemaker-Levy 9 crashed into Jupiter. This was the very first time that scientists knew ahead of time where to view the collision of two bodies in space. The impacts were observed by amateur and professional astronomers, along with other scientists. The impact was recorded by satellites and telescopes, both Earth-based and space-based.

Eugene Shoemaker was a retired geologist whose interest in comets and meteorites led him to search the world for craters that recorded their impacts. Carolyn Shoemaker, Eugene's wife, is a planetary astronomer who collaborated with her husband throughout his career. David Levy, an amateur astronomer, has worked closely with Eugene and Carolyn Shoemaker for years. He has discovered 21 comets; eight of them with his own home telescope.

According to observations by the Hubble Space Telescope, the comet was at most 5 km in diameter before the breakup. As it approached Jupiter, the comet broke apart into at least 21 pieces, but the sizes are uncertain. The diameters of the brightest pieces appear to have been 2 to 3 km.