Comets

A comet is an object with a dark, solid core (the nucleus) some miles in diameter. The core is composed mostly of water ice and frozen gas and is surrounded—whenever the comet is close enough to the Sun for part of the core to vaporize—by a cloud of glowing vapor (the coma). Together, the core and coma comprise the comet's head, which appears as a bright, well-defined cloud. As a comet nears the Sun, charged particles streaming outward from the Sun—the "solar wind"—sweep the coma out into a long, luminous tail which may be visible from the earth. Comets spend most of their time far from the Sun, beyond the orbit of Pluto, where hundreds of billions of comets (too dark to observe directly from Earth) orbit the Sun in a mass called the Oort cloud; only a few ever approach the Sun closely enough for to be observed. A comet that does approach the Sun follows either an elliptical or parabolic orbit. An elliptical orbit is oval in shape, with the Sun inside the oval near one end; a parabolic orbit is an open curve like the cross-section of a valley, with the Sun inside the curve near the bottom. A comet following an elliptical orbit will eventually return to the Sun, perhaps after tens, hundreds, or thousands of years; a comet following a parabolic orbit never returns to the Sun.

Perhaps among the most primitive bodies in the solar system, comets are probably debris from the formation of the Sun and its planets some 4.5 billion years ago. Astronomers believe that the Oort cloud is a dense shell of debris surrounding the solar system. Occasionally, disruptive gravitational forces (perturbations) destabilize one or more comets, causing a piece of debris from the cloud to fall into the gravitational pull of one of the large planets, such as Jupiter. Ultimately, such an object may take up an elliptical or parabolic orbit around the Sun. Comets on elliptical (returning) orbits are either short-period, with orbits of less than 200 years, or long-period, with enormous, nearly parabolic orbits with periods of more than 200 years. Of the 710 individual comets recorded from 1680 to mid 1992, 121 were short-period and 589 long-period.

The questions of the birth, composition, and death of comets still defy definitive answers. Increasing knowledge and advancing twentieth-century technology have brought many and, as usual, conflicting theories.

Composition of the nucleus

Two major theories on the composition of the nucleus have developed over time. The "flying sandbank" model, first proposed by Richard Proctor in the mid 1800s and again in the mid 1900s by Raymond Lyttleton, conjectured swarms of tiny solid particles bound together by mutual gravitational attraction. In 1950, U.S. astronomer Fred Whipple (1906–) introduced the "icy-conglomerate" or "dirty-snowball" model, which describes a comet's core as a solid nucleus of meteoric rock particles and dust frozen in ice. Observations of Halley's comet by spacecraft in 1986 strongly support this model.

No one knows the exact composition of the core, but it is believed that rocks and dust are held together by ices of water, methane, ammonia, and carbon monoxide that are contaminated by carbon and sulfur. The 1986 spacecraft encounter showed Halley's nucleus as peanut-shaped or potato-shaped, 9 mi (15 km) long, and 5.5 mi (8 km) wide.

The nuclei of comets are too small for observation through telescopes. As they approach the Sun, however, they produce one of the largest, most spectacular sights in the solar system—a magnificent, glowing tail often visible even to the naked eye. Cometary nuclei have been seen to produce sudden, bright flares and some to split into as many as five pieces.

Development of the coma

As the nucleus of a comet nearing the Sun approaches the distance of the asteroid belt (outside the orbit of Mars), its ices begin to sublimate (turn to gas), releasing hydrogen, carbon, oxygen, nitrogen, and other substances in the form of vapors and particles. Carried away from the nucleus by the solar wind at several hundred meters per second, they create an enormous coma and tail hundreds of thousands of kilometers long, hiding the nucleus. The Sun's ultraviolet light excites the gaseous molecules, causing them to fluoresce (shine). Microscopic mineral particles in the dust reflect and scatter the Sun's light. In 1970, during the first space-based observation of a comet, a gigantic hydrogen cloud was discovered surrounding the coma. Depending on the size of a cometary nucleus and its proximity to the Sun, this cloud can be larger than the Sun itself.

Tail configuration

As the comet swings around the Sun on its elliptical orbit, the gas and dust particles streaming from the coma form two types of tails: a gaseous ion tail (Type I) or a dust tail (Type II). In a Type I tail, ionized gases form a thin, usually straight tail, sometimes millions of miles long. (The tail of the Great Comet of 1843 stretched out more than 136 million mi [220 million km].) The ion tail, glowing brightly, does not trail behind the core along its path of motion but is blown away from the core along a line pointing directly away from the sun. The head collides with the solar wind, which wraps around the nucleus, pulling the ionized particles of the coma with it. Depending on its position relative to the Sun, a comet's tail may even be traveling ahead of the nucleus. A Type II tail is usually shorter and wider, and curves slightly because its heavier particles are carried away from the nucleus at a slower rate.

Comet Hale-Bopp, which streamed across the skies in the spring of 1997, boasted a feature hitherto unseen in comets: a third type of tail composed of electrically neutral sodium atoms. Observers using instruments with spectral filters that eliminated all but the yellow light emitted by fluorescing sodium atoms found that the tail was 373,000 mi (600,000 km) wide and 31,000 million mi (50 million km) long, streaming in a direction slightly different from that of the ion tail. The exact mechanism producing this type of tail is still not understood.

Origins

As the solar system moves slowly through the center of the galaxy, it encounters interstellar molecular gas clouds which, under certain circumstances, strip comets from the Oort cloud. How, then, is the Oort cloud replenished? One theory proposes "capture" of comets from interstellar space. The popularity of the interstellar theory waxes and wanes, and new hypotheses are again being proposed. One suggests the presence of comets in high-density clouds within the galaxy's inner spiral arms. The Sun may capture comets while passing through one of these arms, which happens once every 100 million years. Also, comets may be captured from the very same molecular gas clouds which, under other circumstances, so severely deplete the Oort cloud population. Mathematical calculations and known chemical composition of comets and stars indicate the possibility of interstellar origins.

Death of a comet

Although vaporization during passages by the Sun diminishes the nucleus, it is not believed to be enough to cause a comet's extinction. There are two commonly accepted reasons for a comet's death: splitting, which may result in deterioration and ultimately a meteor shower; and flaring, bright explosions visible in the coma. Another theory postulates that asteroids may be extinct comets.