Comets

Age-old fascination

Stargazing and discovering comets

Composition, origin, and extinction

Comets and Earth

Bright objects keep humans in the dark

Resources

A comet is an object with a dark, solid core (the nucleus) and a diameter of several kilometers. The core is composed mostly of water-ice and frozen gas. It is surrounded—whenever the comet is close enough to the Sun (or another star) 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 that may be visible from the Earth. Comets spend most of their time far from the Sun, beyond the orbit of the dwarf planet 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 to be observed. A comet that does approach the Sun follows an elliptical or parabolic orbit. An elliptical orbit is oval-shaped, 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. As of September 2006, according to the Minor Planet Center, astronomers who have observed comets on more than one perihelion passage had found and classified 178 periodic comets. However, since then, several of these comets have been lost or destroyed.

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. Most comets are long-period comets.

Age-old fascination

Evidence of a human fascination with the night sky goes back as far as recorded history. Records on Babylonian clay tablets unearthed in the Middle East, dating to at least 3000 BC and rock carvings found in prehistoric sites in Scotland, dating to 2000 BC, record astronomical phenomena that may have been comets.

Until the Arabic astronomers of the eleventh century, the Chinese were by far the world’s most astute sky-watchers. By 400 BC, their intricate cometary classification system included sketches of 29 comet forms, each associated with a past event and predicting a future one. Comet type 9, for example, was named Pu-Hui, meaning calamity in the state, many deaths. Any comet appearing in that form supposedly foretold such a calamity. In fact, from Babylonian civilization right up until the seventeenth century and across cultures, comets have been viewed as omens portending catastrophe.

Of all the Greek and Roman theories on comets— comet is a Greek word meaning long-haired one—the most influential, though entirely incorrect, was that of Greek philosopher Aristotle (384–322 BC). His view of the solar system put Earth at the center circled by the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn (in order of increasing distance from the Earth). Stars were stationary and temporary bodies like comets traveled in straight lines. Aristotle believed that comets were fires in the dry, sublunar “fiery sphere,” which he supposed to be a combustible atmosphere “exhaled” from Earth that accumulated between Earth and the Moon. Comets were therefore considered technically terrestrial—originating from Earth—rather than celestial heavenly bodies.

Aristotle’s model left many unexplained questions about the movement of bodies through the solar system. His theory, which came in the Middle Ages, was so strongly supported by the Christian Church, however, that those who challenged it were often called heretics. His theory remained standard for over 2, 000 years, bringing European investigations into comets virtually to a halt. Fortunately, prolific and accurate cometary records were kept by the Chinese during this period.

Sporadic European scientific investigations into comets also had some influence, however. Although the Church held his theory in check, Polish astronomer Nicolaus Copernicus (1473–1543) suggested that a heliocentric (Sun-centered) solar system would help explain the motions of the planets and other celestial bodies. Through acute observation of the Great Comet of 1577, Danish astronomer Tycho Brahe (1546–1601) calculated that it must be four times further away from Earth than the Moon, refuting Aristotle’s sublunar theory of comets. Also, Brahe found that the comet’s tail pointed away from the Sun and that its orbit might be oval.

Study of the Great Comet by Brahe and his contemporaries was a turning point for astronomical science. Throughout the seventeenth and eighteenth centuries, mathematicians and astronomers proposed conflicting ideas on the origin, formation, orbits, and meaning of comets. In the early 1600s, English physicist and mathematician Sir Isaac Newton (1642–1727) built on theories from the likes of German astronomer Johannes Kepler (1571–1630), who developed the three laws of planetary motion; Polish astronomer Johannes Hevelius (1611–1687), who suggested comets move on parabolas around the Sun; and English physicist Robert Hooke (1635–1703), who introduced the possibility of a universal gravitational influence. Newton developed an mathematical model for the parabolic motion of comets, published in 1687 in his book Principia, one of the most important scientific works ever written.

By this time, comets were viewed as celestial rather than terrestrial, and the focus turned from superstition to science. They were, however, still viewed as singular rather than periodic occurrences. In 1687, English astronomer Edmond Halley (1656– 1742) suggested to Newton that comets may be periodic, following elliptical paths. Newton did not agree. Using Newton’s own mathematical model, Halley argued that the comets of 1531, 1607, and 1682—the latter observed by both he and Newton—were actually one and the same, and predicted that this comet should return late in 1758. It did, and was subsequently named Halley’s comet. Halley’s comet has continued to return on a regular schedule.

By the end of the eighteenth century, comets were believed to be permanent celestial bodies composed of solid material, the movement of which could be calculated using Newton’s laws of planetary motion. The return of two more comets in 1822 and 1832 was accurately predicted. The first, comet Enke, did not follow Newton’s law of planetary motion, as its orbital period of recurrence was decreasing. In 1835, German astronomer Friedreich Bessel (1784–1846) accurately suggested that this was because gases given off by the comet as it passed near the Sun acted like a rocket, thrusting the comet either closer to or further away from the Sun and so affecting its orbital length.

The second predicted periodic comet, comet Biela, with a periodic orbit of 6.75 years upset Newton’s idea of comets when it split in two in 1846. The now-twin comet reappeared in 1852 for the last time.

Earlier in the nineteenth century, scientists had speculated that meteor showers may be flying debris from disintegrating comets. In November 1872, when Biela should have returned, the meteor shower predicted by some astronomers did indeed appear, strengthening the connection between meteors and dying comets.

Stargazing and discovering comets

The first observation of a comet through a telescope was made in 1618. Previously, comets were discovered with the naked eye. Today, most new comet discoveries are made from telescopic photographs and electronic detectors; many comets are discovered by amateur astronomers, and are named after their discoverers.

The long-focal-length, refracting telescope—the primary astronomical observation tool of the 1800s—worked well for direct viewing although, with the relatively insensitive photographic emulsions of the period, it did not collect sufficient light to allow astronomical photography. In 1858, English artist William Usherwood used a short focal-length lens to produce the first photograph of a comet. In 1864, by using a spectroscope, an instrument that separates the wavelengths of light into spectral bands, Italian astronomer Giovanni Donati (1826–1873) first identified a chemical component in a comet’s atmosphere. The first cometary spectrogram (photographed spectrum of light from a comet) was taken by English amateur astronomer William Huggins (1824–1910) of London, England, in 1881.

The early twentieth century saw the development of short-focal-length spectrographs which, by the 1950s, allowed identification of several different chemical components in a comet’s tail. Infrared spectrography was introduced in the 1960s and, in 1983, the Infrared Astronomy Satellite (IRAS) began gathering information on cometary dust particles that was unobtainable by ground-based technology. Today, observations are also made by radio astronomy and ultraviolet spectrography.

Composition, origin, and extinction

The questions of the birth, composition, and death of comets still defy definitive answers. Increasing knowledge and 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 British astronomer Richard Anthony Proctor (1837–1888) in the mid-1800s and again in the mid-1900s by British astronomer Raymond Arthur Lyttleton(1911–1995), conjectured swarms of tiny solid particles bound together by mutual gravitational attraction. In 1950, U.S. astronomer Fred Whipple (1906–2004) 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 Giotto, of the European Space Agency, in 1986 strongly support this model. It found that Halley’s comet has a nucleus that reflects about 4% of any light that strikes it. Then, in 1998, NASA launched spacecraft Deep Space 1 from a Delta II rocket. It was able to be unexpectedly sent near Comet Borrelly (officially 19P/Borrelly). It discovered that this comet reflects from 2.5 to 3% of the light fallingon it. Scientists think that dark surface material on comets absorbs most of the heat.

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. Specifically, 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 in front 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 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 that, 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. In addition, comets may be captured from the very same molecular gas clouds that, 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.

Comets and Earth

The paths of comets and asteroids cross the orbital path of the planets and are believed to be the cause of some impact craters on Earth and the Moon. In 1979, United States Air Force satellite P78–1 took the first photograph of a comet colliding with the Sun. Late in 1994, comet Shoemaker-Levy collided with Jupiter. Asteroidal impacts on Earth may have caused the extinction of many species, including the dinosaurs, while making the development of new species— including ourselves—possible.

For millennia, humans have predicted the end of the world from the impact of a giant comet. Now, however, some scientists argue that molecules released by comets’ vaporized gases may have supplied important molecules in Earth’s early atmosphere. When exposed to the Sun’s radiation, these molecules undergo the formation of organic compounds. During the recent passage of Hale-Bopp, for example, scientists discovered a variety of complex organic chemicals in the comet.

The theory gained evidence from data gathered by the Polar spacecraft, launched by NASA in 1996. According to observations by the probe, comet-like objects30to40ft(9.1to12.1m)indiameterarehitting the atmosphere at the rate of 43, 000 per day. These cosmic slushballs are too small to vaporize and provide the spectacular show in the night sky that humans associate with comets; most disintegrate in the upper atmosphere, entering the weather cycle and eventually reaching the terrestrial surface as precipitation. According to estimates by scientists associated with the study, this cosmic rain has added one inch of water to the Earth’s surface each 10, 000 to 20, 000 years, supplying a large quantity of water over geologic time.

Bright objects keep humans in the dark

A pair of space missions launched in 1999 and 2004 are helping scientists reach a better understanding of the physics of comets. NASA’s Stardust mission, launched in 1999, captured dust from the tail of short-period Comet Wild (pronounced vilt) 2 in 2004, and returned the samples to Earth in January 15, 2006.

KEY TERMS

Coma— Glowing cloud of mass surrounding the nucleus of a comet.

Ellipse— An eccentric or elongated circle, or oval.

Ion— An atom or molecule that has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).

Nucleus— Core, or center.

Parabola— Open-ended, elongated ellipse; u-shaped.

Perturbation— Change in the orbit of an astronomical body by the gravitational influence of a body other than the one around which the object orbits.

Solar wind— A stream of charged and neutral particles that emanates from the Sun and moves into the solar system.

Spectrograph— Instrument for dispersing light into its spectrum of wavelengths then photographing that spectrum.

Thousands of samples—most sample grains embedded in the Stardust aerogel were smaller than the width of a human hair—have been distributed to about 150 scientists around the world for analysis.

In February 2003, the European Space Agency’s Rosetta mission—originally scheduled to rendezvous with Comet Wirtanen on its trip around the Sun—was postponed due to launch failures suffered by Europe’s Ariane 5 rocket. In March 2003, ESA scientists retasked the Rosetta mission spacecraft to rendezvous with 67P/Churyumov-Gerasimenko. With a launch on February 2004 (from Kourou in French Guiana), it should rendezvous with the comet in 2014. During its six-month stay near the comet, Rosetta will move closer to the comet’s nucleus until it is only 12 to 15 mi (20 to 25 km) away. It will then map the comet and send a probe to the surface for a landing. The larger size of 67P/Churyumov-Gerasimenko—thus, a stronger gravitational field—poses some problems for the lander that will require recalculation of the landing impact stress on the lander legs. For its remaining mission at the comet, Rosetta will observe the comet as it races toward the Sun. After completing its mission to the comet, the Rosetta will be redirected to a voyage of the outer solar system.

As of 2006, however, despite spaceships probing the outer limits of the solar system; gigantic telescopes in deserts, atop mountains, and floating in space; and satellites designed specifically to capture meteor dust hurtling through Earth’s atmosphere from interstellar space, significant questions about the origin, nature, and fate of comets remains unsolved.

See also Meteors and meteorites; Space probe.

Resources

Books

Brandt, John C. Introduction to Comets. Cambridge, UK, and New York: Cambridge University Press, 2004.

Fernandez, Julio A. Comets: Nature, Dynamics, Origin, and Their Cosmogonical Relevance. Dordrecht, Netherlands: Springer, 2005.

Wickramasinghe, Chandra. Cosmic Dragons: Life and Death on Our Planet. London, UK: Souvenir, 2001.

Other

National Aeronautics and Space Administration. “STARDUST Mission.” (2000). <http://stardust.jpl.nasa.gov/mission/> (accessed October 6, 2006).

Marie L. Thompson

Comets

Comets are objects—relatively small compared to planets—that are composed of dust and ices of various compounds. Comets orbit the Sun in elongated elliptical (eccentric, elongated circle) or parabolic orbits. Accordingly, these objects spend the majority of time in the outer regions of the solar system , in some cases well beyond the orbits of Neptune and Pluto. Short-period comets are those with less exaggerated elliptical orbits that carry them out only as far as the region of space between the orbits of Jupiter and Neptune. Comets make periodic, brief, but sometimes-spectacular transits through the inner solar system as they approach the Sun. Comet orbits may be prograde, in the same direction as the planets, or retrograde, in the opposite direction. With the aid of a telescope , a comet is usually visible from Earth.

The term "comet" derives from the Greek aster kmetes (translated literally as "hairy" or long-haired star)—a reference to a sometimes-visible comet tail. If a comet's path takes it close enough to the Sun, the heating causes melting and emission of gases (out gassing) and dust that are then swept behind the comet's orbital path (away from the Sun) by the solar wind to form the characteristic tail.

Fascination with objects in the night sky dates to the dawn of human civilization. Etchings on clay tablets unearthed in the ancient city of Babylon dating to at least 3000 b.c. and rock carvings found in prehistoric sites in Scotland dating to 2000 b.c. depict unexplained astronomical phenomena that may have been comets. Until the Arabic astronomers of the eleventh century, the Chinese were by far the most astute skywatchers in the ancient and medieval world. By 400 b.c., their intricate cometary classification system included sketches of 29 comet forms, each associated with a past event and predicting a future one. Comet type 9 was named Pu-Hui, meaning "Calamity in the state, many deaths." In fact, until the seventeenth century when English Astronomer Edmund Halley (1656–1742) predicted the return of a the comet in 1758 (thereafter known as Halley's comet) based, in part, upon calculations derived from English physicist and mathematician Sir Isaac Newton's (1642–1727) work, comets were widely viewed with superstition, as omens portending human disasters and terrestrial catastrophes.

Recorded observation of comets is evident in the records of the Ancient Chinese culture who termed comets "guest stars," a general term also applied to other apparent temporary solar system transients that were later, of course, identified to be much more distant stellar novae. Chinese records clearly indicate the transit of a guest star in approximately 240 b.c. that we now identify as Halley's comet.

In accord with Anasazi Native American accounts, Chinese astronomers also noted the difference in what is now regarded as comets and the 1054 supernova explosion in the Taurus constellation (i.e., a region of the sky associated with the Taurus constellation) that created the Crab Nebula.

Of all the Classical Greek and Roman theories on comets, the most influential, though entirely incorrect, was that of the Greek philosopher Aristotle (384–322 b.c.). His geocentric view of the solar system put Earth at the center circled by the Sun, Moon , and visible planets. Stars were stationary and the bodies existed on celestial spheres. Aristotle argued that comets were fires in the dry, sublunar "fiery sphere," a combustible atmosphere "exhaled" from Earth, which accumulated between Earth and the Moon. Comets were therefore considered terrestrial—originating from Earth, rather than celestial—heavenly bodies. Moreover, they were seen as a portent of future events controlled by the gods.

Aristotle's writings formed the basis of later Greek-Alexandrian astronomer Ptolemy's (a.d. 87–150) model of the universe that became strongly supported by the Christian church in Western Europe . Because the Ptolemy's model provided accurate results with regard to celestial prediction, it was the most influential astronomical model until the acceptance of the Sun-centered (heliocentric) model put forth by Polish astronomer Nicolaus Copernicus (1473–1543).

In conjunction with the German astronomer and mathematician Johannes Kepler (1571–1630), Danish astronomer Tycho Brahe's (1546–1601) observation of the "Great Comet" of 1577 provided evidence that the comet was at least four times further away from Earth than the Moon—a crushing refutation of Aristotle's sublunar positioning.

The study of the Great Comet by Brahe and his contemporaries was the turning point for astronomical science. Throughout the seventeenth and eighteenth centuries, mathematicians and astronomers refined conflicting ideas on the origin, formation, movement, shape of orbit, and meaning of comets. Polish-born scientist Johannes Hevelius (1611–1687), who suggested comets move on a parabola (U-shape) around the Sun; and English scientist Robert Hooke (1635–1703), independent of Newton, introduced the theory of universal gravitational influence based, in part on the periodic behavior of comets. Newton, however, developed an astounding mathematical model for the parabolic motion of comets, published in his seminal and influential 1687 book, Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy). Until English naturalist Charles Darwin's (1809–1882) writings on evolution and German-American physicist Albert Einstein's (1879–1955) twentieth century writings on relativity theory , Principia remained the single most influential scientific work in the history of science."

By the end of the eighteenth century, comets were understood to be astronomical bodies, the movement of which could be calculated using Newton's laws of planetary motion.

The comet Biela, with a periodic orbit of 6.75 years, split in two during its 1846 appearance. Twin comets reappeared in 1852—but then failed to appear for its next pass. The disappearance fostered scientific speculation regarding comet impacts and their relationships to meteor showers. When Biela's twin offspring should have returned, the meteor shower predicted by some astronomers did indeed appear, strengthening the connection between meteors and dying comets.

The first observation of a comet through a telescope was made in 1618. Until the twentieth century, comets were discovered and observed with the naked eye or through telescopes. Today, most new discoveries are made from photographs of our galaxy and electronic detectors, although many discoveries are still made by amateur astronomers with a passion for careful observation.

The long focal-length refracting telescope, the primary astronomical observation tool of the 1800s, worked well for viewing bright objects but did not collect sufficient light to allow detailed astronomical photography. In 1858, an English artist named Usherwood used a short focal-length lens to produce the first photograph of a comet. In 1864, by using a spectroscope, an instrument that separates the wavelengths of light into spectral bands, Italian astronomer Giovanni Donati (1826–1873) first identified a chemical component in a comet's atmosphere. The first cometary spectrogram (spectral photograph) was taken by amateur astronomer William Huggins of London in 1881.

The early twentieth century saw the development of short focal-length spectrographs that, by the 1950s, allowed identification of several different chemical components in a comet's tail. Infrared spectrography was introduced in the 1960s and, in 1983, the Infrared Astronomy Satellite (IRAS) gathered information on cometary dust particles unattainable from ground-based technology. Observations of comets are now also made by radio wave detection and ultraviolet spectrography.

Spectroscopic evidence indicates that most comets contain a solid nucleus (core) surrounded by a gigantic, glowing mass (coma). Together, the nucleus and coma comprise the comet head. It should be noted that although the tail (when apparent) seems dense with dust and gas, it is still a vacuum that is far less dense than the interplanetary space near the earth (e.g., between the earth and Moon).

Perhaps among the most primitive bodies in the solar system, comets are probably debris from the formation of our Sun and planets some 4.5 billion years ago. One hypothesis

concerning their origin involves the Oort cloud—named for Dutch astronomer Jan Van Oort—a dense shell of debris (dense by interstellar standards) at the frigid, outer edge of the solar system (i.e., the distance at which our Sun's gravitational pull is so weak that beyond this point other stellar bodies exert a greater net attraction). Occasionally, disruptive gravitational forces (perturbations) hurl a piece of debris from the cloud into the gravitational pull of one of the large planets, (e.g. Jupiter or Saturn) that then pull the comet into an elliptical orbit around the Sun. The Kuiper belt, associated with Jupiter's gravitational pull, is more likely the source of the well-known comets, including Halley's comet. Regardless, evidence indicates that comets formed from solar system formation debris.

Short lived comets have orbital durations of less than 200 years. Long-period, having enormous elliptical, nearly parabolic orbital durations of more than 200 years, often traveling far beyond the outer planets. Of the 710 individual comets recorded from 1680 to mid-1992, 121 were short-period comets and 589 were classified as long-period comets.

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, Fred Whipple introduced the "icy-conglomerate" model, which described a comet 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.

Evidence to date indicates that within the comet head or nucleus, rocks and dust are held together with ices from water , methane, ammonia, and carbon monoxide, as well as other ices containing carbon and sulphur. The 1986 studies of Halley's comet revealed the nucleus to be peanut or potato-shaped, 9 mi (15 km) long, and 5.5 mi (8 km) wide. However, visual observation beneath the comet's dark, solid surface proved impossible.

The nuclei of comets are among the smallest bodies in the Solar system, too small, in fact, for observation even through a telescope. As they approach the Sun, however, they produce one of the largest, most spectacular sights in the solar system, a magnificent, glowing coma often visible even to the naked eye. Comet nuclei have been seen to produce sudden, bright flares and some even split into two, three, four, or five refions.

As the nucleus of a comet approaches the Sun, beginning at about the distance of the asteroid belt, its ices begin to vaporize and sublimate (change directly from ice to gas). This off-gassing releases gases of hydrogen, carbon, oxygen , nitrogen and other molecules, as well as dust particles. Streaming away at several hundred meters per second, they create an enormous coma hundreds of thousands of kilometers long, completely hiding the nucleus. The Sun's ultraviolet light electrically charges the gaseous molecules, ionizing and exciting them, causing them to fluoresce (emit light) much like a fluorescent light emits light following electrical stimulation. Microscopic mineral particles in the dust reflect and scatter the Sun's light. Only in 1970, during the first spacecraft observation of a comet, was a gigantic hydrogen cloud discovered surrounding the Coma. Depending on the size of the nucleus and its proximity to the Sun, this cloud can be much larger than the comet itself.

As the comet swings around the Sun on its elliptical orbit, gas and dust particles stream from the coma to create two types of tails: the gaseous ion tail, or Type I; and the dust tail, or Type II. In Type I, ionized gases form a thin, usually straight tail, sometimes millions of kilometers long. (The tail of the Great Comet of 1843 stretched out more than 136 million mi [220 million km].) In fact, the tails of comets are the largest measured entities in the solar system. The ion tail, glowing with incredible brightness, does not trail behind but is blown away from the head in a direction almost opposite the Sun by the "solar wind," a continual flow of magnetized plasma emitted by the Sun. The head collides with this plasma, which wraps around the nucleus, pulling the ionized particles with it. Depending on its position to the Sun, the tail may even be traveling almost ahead of the nucleus. A Type II tail is usually shorter and wider, and curves slightly because the heavier particles are carried away at a slower rate. The Great Comet of 1744 actually displayed six brilliant tails fanning above the horizon like peacock feathers.

Comet Hale-Bopp, which streamed across the skies in 1997, boasted a new feature: a third tail composed of electrically neutral sodium atoms. When completely observed using instruments with spectral filters that eliminated all but the yellow light emitted by fluorescing sodium atoms, the tail was more than 370,000 miles wide (600,000 km) and 31 million miles long (50 million km), streaming in a direction close but slightly different to that of the ion tail. Although the exact mechanism is not understood, the tail is thought to be formed of sodium atoms released by dust particles in the coma.

Comets may strike planets without leaving an impact crater . The atmospheric energy released by comet vaporization in the atmosphere can, however, be more powerful than a nuclear explosion. The Tunguska event in Siberia in 1908 is thought to have been the result of a comet or stony meteoroid explosion above the ground. In 1979, a United States Air Force space-test satellite took the first photograph of a comet colliding with the Sun. Late in 1994, the fragmented comet Shoemaker-Levy made spectacular serial collisions with Jupiter.

Some scientists argue that molecules released by comets' vaporized gases may have supplied important molecules in Earth's early atmosphere. When exposed to the Sun's radiation, these molecules began the formation of biochemical compounds that actually began the process of life on Earth—or gave a huge "jump-start" to the evolution of biomolcules. During the recent passage of Hale-Bopp, for example, scientists discovered a variety of complex organic chemicals in the comet.

Some aspects of this theory gained evidence from data gathered by the Polar spacecraft, launched by NASA in 1996. According to some interpretations of observations by the probe, comet-like objects up to 30–40 ft (9–12 m) in diameter may be hitting the atmosphere at the astounding rate of up to 43,000 per day. These cosmic snowballs usually disintegrate in the upper atmosphere, their content liquids and gases entering the weather cycle and eventually reaching the terrestrial surface as precipitation . Other scientists argue that the evidence of "snowballs" from space is an artifact of instrument background noise or interference.

In a pair of space missions planned for the early part of the twenty-first century, space probes will rendezvous with a pair of short-period comets, hopefully to help scientists reach a better understanding of the physics of comets. In 2004, NASA's Stardust mission plans to capture dust from the tail of Comet Wild 2, returning the samples to Earth for analysis. In 2011, the European Space Agency's Rosetta mission will rendezvous with Comet Wirtanen on its trip around the Sun. The Rosetta spacecraft will orbit the comet and send a probe to the surface.

See also Astronomy; Big Bang theory; Impact crater

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.

Age-old fascination

Evidence of a human fascination with the night sky goes back as far as recorded history. Records on Babylonian clay tablets unearthed in the Middle East, dating to at least 3000 b.c. and rock carvings found in prehistoric sites in Scotland, dating to 2000 b.c., record astronomical phenomena that may have been comets. Until the Arabic astronomers of the eleventh century, the Chinese were by far the world's most astute sky-watchers. By 400 b.c., their intricate cometary classification system included sketches of 29 comet forms, each associated with a past event and predicting a future one. Comet type 9, for example, was named Pu-Hui, meaning "calamity in the state, many deaths." Any comet appearing in that form supposedly foretold such a calamity. In fact, from Babylonian civilization right up until the seventeenth century and across cultures, comets have been viewed as omens portending catastrophe.

Of all the Greek and Roman theories on comets—"comet" is a Greek word meaning "long-haired one"—the most influential, though entirely incorrect, was that of Greek philosopher, Aristotle (384–322 b.c.). His view of the solar system put Earth at the center circled by the Moon , Mercury, Venus , the Sun, Mars , Jupiter, and Saturn (in order of increasing distance from the earth). Stars were stationary, and temporary bodies like comets traveled in straight lines. Aristotle believed that comets were fires in the dry, sublunar "fiery sphere," which he supposed to be a combustible atmosphere "exhaled" from Earth which accumulated between Earth and the Moon. Comets were therefore considered technically terrestrial—originating from Earth—rather than celestial heavenly bodies.

Aristotle's model left many unexplained questions about the movement of bodies through the solar system. His theory came in the Middle Ages to be so strongly supported by the Christian Church, however, that those who challenged it were often called heretics. His theory remained standard for over 2,000 years, bringing European investigations into comets virtually to a halt. Fortunately, prolific and accurate cometary records were kept by the Chinese during this period.

Sporadic European scientific investigations into comets also had some influence, however. Although the Church held his theory in check, Polish astronomer Nicolaus Copernicus (1473–1543) suggested that a heliocentric (Sun-centered) solar system would help explain the motions of the planets and other celestial bodies. Through acute observation of the "Great Comet" of 1577, Danish astronomer Tycho Brahe (1546–1601) calculated that it must be four times further away from Earth than the Moon, refuting Aristotle's sublunar theory of comets. Also, Brahe found that the comet's tail pointed away from the Sun and that its orbit might be oval.

Study of the Great Comet by Brahe and his contemporaries was a turning point for astronomical science. Throughout the seventeenth and eighteenth centuries, mathematicians and astronomers proposed conflicting ideas on the origin, formation, orbits, and meaning of comets. In the early 1600s, English physicist and mathematician Isaac Newton (1642–1727) built on theories from the likes of German astronomer Johannes Kepler (1571–1630), who developed the three laws of planetary motion ; Polish astronomer Johannes Hevelius (1611–1687), who suggested comets move on parabolas around the Sun; and English physicist Robert Hooke (1635–1703), who introduced the possibility of a universal gravitational influence. Newton developed an mathematical model for the parabolic motion of comets, published in 1687 in his book Principia, one of the most important scientific works ever written.

By this time, comets were viewed as celestial rather than terrestrial, and the focus turned from superstition to science. They were, however, still viewed as singular rather than periodic occurrences. In 1687, English astronomer Edmond Halley (1656–1742) suggested to Newton that comets may be periodic, following elliptical paths. Newton did not agree. Using Newton's own mathematical model, Halley argued that the comets of 1531, 1607, and 1682—the latter observed by both he and Newton—were actually one and the same, and predicted that this comet should return late in 1758. It did, and was subsequently named Halley's comet . Halley's comet has continued to return on a regular schedule.

By the end of the eighteenth century, comets were believed to be permanent celestial bodies composed of solid material, the movement of which could be calculated using Newton's laws of planetary motion. The return of two more comets in 1822 and 1832 was accurately predicted. The first, comet Enke, did not follow Newton's law of planetary motion, as its orbital period of recurrence

was decreasing. In 1835, German astronomer Friedreich Bessel (1784–1846) accurately suggested that this was because gases given off by the comet as it passed near the Sun acted like a rocket, thrusting the comet either closer to or further away from the Sun and so affecting its orbital length.

The second predicted periodic comet, comet Biela, with a periodic orbit of 6.75 years, upset Newton's idea of comets'when it split in two in 1846. The now-twinned comet reappeared in 1852 for the last time.

Earlier in the nineteenth century, scientists had speculated that meteor showers may be flying debris from disintegrating comets. In November 1872, when Biela should have returned, the meteor shower predicted by some astronomers did indeed appear, strengthening the connection between meteors and dying comets.

Stargazing and discovering comets

The first observation of a comet through a telescope was made in 1618. Previously, comets were discovered with the naked eye . Today, most new comet discoveries are made from telescopic photographs and electronic detectors; many comets are discovered by amateur astronomers, and are named after their discoverers.

The long-focal-length, refracting telescope—the primary astronomical observation tool of the 1800s—worked well for direct viewing although, with the relatively insensitive photographic emulsions of the period, it did not collect sufficient light to allow astronomical photography . In 1858, an English artist, William Usherwood, used a short focal-length lens to produce the first photograph of a comet. In 1864, by using a spectroscope , an instrument which separates the wave lengths of light into spectral bands, Italian astronomer Giovanni Donati (1826–1873) first identified a chemical component in a comet's atmosphere. The first cometary spectrogram (photographed spectrum of light from a comet) was taken by English amateur astronomer William Huggins (1824–1910) of London in 1881.

The early twentieth century saw the development of short-focal-length spectrographs which, by the 1950s, allowed identification of several different chemical components in a comet's tail. Infrared spectrography was introduced in the 1960s and, in 1983, the Infrared Astronomy Satellite (IRAS) began gathering information on cometary dust particles that was unobtainable by ground-based technology. Today, observations are also made by radio astronomy and ultraviolet spectrography.

Composition, origin, and extinction

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.

Comets and Earth

The paths of comets and asteroids cross the orbital path of the planets and are believed to be the cause of some impact craters on Earth and the Moon. In 1979, United States Air Force satellite P78–1 took the first photograph of a comet colliding with the Sun. Late in 1994, comet Shoemaker-Levy collided with Jupiter. Asteroidal impacts on Earth may have caused the extinction of many species , including the dinosaurs, while making the development of new species—including ourselves—possible.

For millennia, humans have predicted the "end of the world" from the impact of a giant comet. Now, however, some scientists argue that molecules released by comets' vaporized gases may have supplied important molecules in Earth's early atmosphere. When exposed to the Sun's radiation , these molecules undergo the formation of organic compounds. During the recent passage of Hale-Bopp, for example, scientists discovered a variety of complex organic chemicals in the comet.

The theory gained evidence from data gathered by the Polar spacecraft, launched by NASA in 1996. According to observations by the probe, cometlike objects 30–40 ft (9.1–12.1 m) in diameter are hitting the atmosphere at the rate of 43,000 per day. These cosmic slushballs are too small to vaporize and provide the standard show we associate with comets; most disintegrate in the upper atmosphere, entering the weather cycle and eventually reaching the terrestrial surface as precipitation . According to estimates by scientists associated with the study, this cosmic rain has added one inch of water to the earth's surface each 10,000–20,000 years, supplying a large quantity of water over geologic time .

Bright objects keep us in the dark

In a pair of space missions planned for the early part of the twenty-first century, space probes will rendezvous with a pair of short-period comets, hopefully to help scientists reach a better understanding of the physics of comets. NASA's Stardust mission, launched in 1999, is on its way to capture dust from the tail of Comet Wild (pronounced "vilt") 2 in 2004, returning the samples to Earth in 2006 for analysis. In February 2003, the European Space Agency's Rosetta mission—originally scheduled to rendezvous with Comet Wirtanen on its trip around the Sun—was postponed due to launch failures suffered by Europe's Ariane 5 rocket. In March 2003, ESA scientists retasked the Rosetta mission spacecraft to rendezvous with 67P/Churyumov-Gerasimenko. With a launch planned as early as January 2004, Rosetta will orbit the comet and send a probe to the surface. An early 2004 launch date will permit a rendezvous in 2014. The larger size of 67P/Churyumov-Gerasimenko—and thus a stronger gravitational field—poses some problems for the lander that will require recalculation of the landing impact stress on the lander legs.

At present, however, despite spaceships probing the outer limits of our solar system; gigantic telescopes in deserts, atop mountains , and floating in space; and satellites designed specifically to capture meteor dust hurtling through Earth's atmosphere from interstellar space, significant questions about the origin, nature, and fate of comets remains unsolved.

See also Meteors and meteorites; Space probe.

Resources

books

Bailey, M.E., S.V.M. Clube, and W.M. Napier. The Origin of Comets. Oxford: Pergamon Press, 1990.

Gibilisco, Stan. Comets, Meteors & Asteroids: How They Affect Earth. Blue Ridge Summit, PA: Tab Books, 1985.

Levy, David H. The Quest for Comets: An Explosive Trail of Beauty and Danger. New York: Plenum Press, 1994.

Yeomans, Donald K. Comets, A Chronological History of Observation, Science, Myth, and Folklore. New York: John Wiley & Sons, 1991.

other

National Aeronautics and Space Administration. "STARDUST Mission." 2000. (cited October 19, 2002). <http://stardust.jpl.nasa.gov/mission/>.

Marie L. Thompson

KEY TERMS

Coma

—Glowing cloud of mass surrounding the nucleus of a comet.

Ellipse

—An eccentric or elongated circle, or oval.

Ion

—An atom or molecule which has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).

Nucleus

—Core, or center.

Parabola

—Open-ended, elongated ellipse; ushaped.

Perturbation

—Change in the orbit of an astronomical body by the gravitational influence of a body other than the one around which the object orbits.

Solar wind

—A stream of charged and neutral particles that emanates from the Sun and moves into the solar system.

Spectrograph

—Instrument for dispersing light into its spectrum of wavelengths then photographing that spectrum.

Comet

A comet—a Greek word meaning "long-haired"—is best described as a dirty snowball. It is a clump of rocky material, dust, and frozen methane, ammonia, and water that streaks across the sky on a long, elliptical (ovalshaped) orbit around the Sun. A comet consists of a dark, solid nucleus (core) surrounded by a gigantic, glowing mass (coma). Together, the core and coma make up the comet's head, seen as a glowing ball from which streams a long, luminous tail. The tail (which always points away from the Sun) is formed when a comet nears the Sun and melted particles and gases from the comet are swept back by the solar wind (electrically charged particles that flow out from the Sun). A tail can extend as much as 100 million miles (160 million kilometers) in length.

Age-old fascination

Through the ages, comets were commonly viewed as omens, both good and bad, because of their unusual shape and sudden appearance. A comet appearing in 44 b.c. shortly after Roman dictator Julius Caesar was murdered was thought to be his soul returning. A comet that appeared in 684 was blamed for an outbreak of the plague that killed thousands of people.

For centuries, many people believed Earth was at the center of the solar system, with the Sun and other planets orbiting around it. They also believed that comets were a part of Earth's atmosphere. In the sixteenth century, Polish astronomer Nicolaus Copernicus (1473–1543) proposed a theory that placed the Sun at the center of the solar system, with Earth and the other planets in orbit around it. Once astronomers finally determined that comets existed in space beyond Earth's atmosphere, they tried to determine the origin, formation, movement, shape of orbit, and meaning of comets.

Halley's comet

In 1687, English astronomer Edmond Halley (pronounced HAL-ee; 1656–1742) calculated the paths traveled by 24 comets. Among these, he found three—those of 1531, 1607, and 1682—with nearly identical paths. This discovery led him to conclude that comets follow an orbit around the Sun, and thus reappear periodically. Halley predicted that this same comet would return in 1758. Although he did not live to see it, his prediction was correct, and the comet was named Halley's comet. Usually appearing every 76 years, the comet passed by Earth in 1835, 1910, and 1986.

Words to Know

Astronomical unit (AU): Standard measure of distance to celestial objects, equal to the average distance from Earth to the Sun: 93 million miles (150 million kilometers).

Coma: Glowing cloud of mass surrounding the nucleus of a comet.

Ellipse: An oval or elongated circle.

Interstellar medium: Space between stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.

Nucleus: Core or center of a comet.

During its last pass over the planet, Halley's comet was explored by the European Space Agency probe Giotto. The probe came within 370 miles (596 kilometers) of Halley's center, capturing fascinating images of the 9-mile-long, 5-mile-wide (15-kilometer-long, 8-kilometer-wide) potato-shaped core marked by hills and valleys. Two bright jets of dust and gas, each 9 miles (15 kilometers) long, shot out of the core. Giotto 's instruments detected the presence of water, carbon, nitrogen, and sulfur molecules. It also found that the comet was losing about 30 tons of water and 5 tons of dust each hour. This means that although the comet will survive for hundreds more orbits, it will eventually disintegrate. Halley's comet will next pass by Earth in the year 2061.

Comet Hale-Bopp

On July 22, 1995, American astronomer Alan Hale and American amateur stargazer Thomas Bopp independently discovered a new comet just beyond the orbit of Jupiter. Considered by many astronomers to be one of the greatest comets of all time, Comet Hale-Bopp is immense. Its core is almost 25 miles (40 kilometers) in diameter, more than 10 times that of the average comet and 4 times that of Halley's comet. Hale-Bopp's closest pass to Earth occurred on March 22, 1997, when it was 122 million miles (196 million kilometers) away. Despite its great distance from Earth, the huge comet was visible to the naked eye for months before and after that date. Astronomers believed it was one of the longest times any comet had been visible. They estimate that Hale-Bopp will next visit the vicinity of Earth 3,000 years from now.

Nourishing snowballs

In mid-1997, scientists announced that small comets about 40 feet (12 meters) in diameter are entering Earth's atmosphere at a rate of about 43,000 a day. The discovery was made by the polar satellite launched by the National Aeronautics and Space Administration (NASA) in early 1996. American physicist Louis A. Frank, the principal scientist for the visible imaging system of the satellite, first proposed the existence of the bombarding comets in 1986.

These comets do not strike the surface of Earth because they break up at heights of 600 to 15,000 miles (960 to 24,000 kilometers) above ground. Sunlight then vaporizes the remaining small icy fragments into huge clouds. As winds disperse these clouds and they sink lower in the atmosphere, the water vapor contained within condenses and falls to the surface as rain. Scientists estimate that this cosmic rain adds one inch of water to Earth's surface every 10,000 to 20,000 years. Over the immense span of Earth's history (4.5 billion years), this amount of water could have been enough to fill the oceans.

Scientists also speculate that the simple organic chemicals (carbon-rich molecules) these comets contain might have fallen on Earth as it was first developing. They may have provided the groundwork for the development of the wide range of life on the planet.

The origin of comets

Comets are considered among the most primitive bodies in the solar system. They are probably debris from the formation of our sun and planets some 4.5 billion years ago. The most commonly accepted theory about where comets originate was suggested by Dutch astronomer Jan Oort in 1950. He believed that over 100 billion inactive comets lie at the frigid, outer edge of the solar system, somewhere between 50,000 and 150,000 astronomical units (AU) from the Sun. (One AU equals the distance from Earth to the Sun.) They remain there in an immense band, called the Oort cloud, until the gravity of a passing star jolts a comet into orbit around the Sun.

In 1951, another Dutch astronomer, Gerard Kuiper, suggested that there is a second reservoir of comets located just beyond the edge of our solar system, around 1,000 times closer to the Sun than the Oort cloud. His hypothetical Kuiper Belt was confirmed in 1992 when astronomers discovered the first small, icy object in a ring of icy debris orbiting the Sun. This ring is located between Neptune and Pluto (sometimes beyond Pluto, depending on its orbit), some 3.6 billion miles (5.8 billion kilometers) from Earth. Since 1992, astronomers have discovered more than 150 Kuiper Belt objects. Many of them are upwards of 60 miles (96 kilometers) in diameter. Several are much larger. In 2000, astronomers discovered one, which they call Varuna, that measures 560 miles (900 kilometers) in diameter, about one-third the size of the planet Pluto. Astronomers believe the ring is filled with hundreds of thousands of small, icy objects that are well-preserved remnants of the early solar system. They are interested in studying these objects because they want to know more about how Earth and the other major planets formed.

The death of comets

There are many theories as to what happens at the end of a comet's life. The most common is that the comet's nucleus splits or explodes, which may produce a meteor shower. It has also been proposed that comets eventually become inactive and end up as asteroids. One more theory states that gravity or some other disturbance causes a comet to exit the solar system and travel out into the interstellar medium.

[See also Meteors and Meteorites ]

Comets

A bright comet is a spectacular astronomical event. Throughout history, comets have left a strong impression on those who have witnessed their appearances. The name comes from the Greek kometes, meaning "the long-haired one." Ancient Greeks thought comets to be atmospheric phenomena, part of the "imperfect" changeable Earth, not of the "perfect" immutable heavens. Today we know they are "icy conglomerates," as proposed in 1950 by Fred Whipple—that is, chunks of ice and dust left over from the formation of the solar system some 4.6 billion years ago.

Comets are among the most primitive bodies in the solar system. Because of their orbits and small sizes, comets have undergone relatively little processing, unlike larger bodies, such as the Moon and Earth, which have been modified considerably since they formed. The chemical composition of comets contains a wealth of information about their origin and evolution as well as the origin and evolution of the solar system. Hence, comets are often called cosmic fossils.

When a comet is far from the Sun, it is an inert icy body. As it approaches the Sun, heat causes ices in the nucleus to sublimate , creating a cloud of gas and dust known as the coma. Sunlight and solar wind will push the coma gas and dust away from the Sun creating two tails. The dust tail is generally curved and appears yellowish because the dust particles are scattering sunlight. The gas (or ion) tail is generally straight and it appears blue because its light is dominated by emission from carbon monoxide ions. The appearance of comets in photographs can give the erroneous impression that they streak through the night sky like a meteor or a shooting star. In fact, comets move slowly from night to night with respect to the stars and can sometimes be visible for many weeks, as was the case with comet Hale-Bopp in 1997 and with comet Halley during its 1985-1986 appearance.

Comet Halley is not the brightest comet, but it is the most famous, mainly because it is the brightest of the predictable comets. It was named after Edmund Halley, an eighteenth-century British astronomer who was the first to calculate the orbits of comets. Comet Halley's orbit has an average period of seventy-six years. Its closest approach to the Sun (perihelion) is between the orbits of Venus and Mercury (0.59 astronomical units ), and its aphelion (farthest distance from the Sun) is at 35 AU, beyond Neptune's orbit. The orbit has an inclination of 162 degrees with respect to the ecliptic . This means that comet Halley orbits the Sun clockwise when seen from the north, whereas Earth orbits the Sun counterclockwise.

The study of comets is a very active field of science. In 1986 a flotilla of spacecraft were used to study comet Halley. In the first decade of the twenty-first century, several spacecraft are scheduled to be launched to encounter and study a number of comets. In addition to space-based studies, ground-based observations of comets have yielded a wealth of information.

The Comet's Nucleus

All of the activity in a comet originates in its nucleus, which is composed of roughly equal amounts of ices and dust. Water ice is the most abundant of the ices, comprising about 80 percent of the total. So far, only the nuclei of comets Halley and Borrelly have been imaged in detail. Comet Halley turned out to be larger, darker, and less spherical than expected by most astronomers. The images of comet Borrelly's nucleus obtained in September 2001 by NASA's Deep Space 1 spacecraft show considerable similarity with those of comet Halley. Halley's nucleus is peanut-shaped, approximately 18 kilometers (11 miles) long and 8 kilometers (5 miles) wide. The reflectivity (or albedo) is 4 percent, which is as dark as coal. The size, albedo, and approximate shape of several other cometary nuclei have been determined. Comet Halley's nucleus seems to be typical among comets with relatively short orbital periods, and there are much larger nuclei such as that of comet Hale-Bopp. So far, the cometary nuclei studied in detail appear to have most of their surface covered by an inert mantle or crust. The active (exposed ice) fraction of their surface is small; in comet Halley, this fraction is somewhere between 15 and 30 percent.

The development of a crust can suppress the activity of cometary nuclei and give them an asteroidal appearance. The best example to date is comet Wilson-Harrington, which was discovered in 1949 and was lost until it was rediscovered as an inert object and given the asteroid number 4015. The behavior of this object has added credence to the long-held expectation that some Earth-crossing asteroids are extinct or dormant comet nuclei.

The Composition of Comets

The composition of cometary nuclei is primarily inferred from studies of the coma components, namely gas, plasma (ions), and dust. So far, twentyfour different molecules have been identified in comets, ten of which were discovered in comet Hale-Bopp. The molecules observed in comets and their relative abundances are very similar to those observed in dense interstellar molecular cloud cores, which is the environment where star formation occurs. Thus, it appears that comets underwent little processing in the solar nebula and they preserve a good record of its original composition.

Information on the composition of cometary dust particles was scarce before 1986. Studies of the dust in comet Halley and other comets confirmed that some of the grains are silicates, more specifically crystalline olivine (Mg, Fe)₂ SiO₄ and pyroxene (Mg, Fe, Ca) SiO₃. Another major component of the dust in comet Halley was organic dust. These small solid particles were discovered by the visiting spacecraft and were called "CHON" because they were composed almost exclusively of the elements carbon, hydrogen, oxygen, and nitrogen.

The Origins of Comets

Dutch astronomer Jan Hendrik Oort noted in 1950 that the source of new comets was a shell located between 20,000 and 100,000 AU from the Sun. The existence of the Oort cloud is now widely accepted. Astronomers believe that comets in the Oort cloud formed near Uranus and Neptune and were gravitationally scattered by these two planets into their current location. In addition to the Oort cloud, there is another reservoir that was proposed in 1951 by Dutch-born American astronomer Gerard Peter Kuiper as a ring of icy bodies beyond Pluto's orbit. This Kuiper belt is considered to be the main source of Jupiter-family comets, which are those with low-inclination and short-period orbits.

see also Close Encounters (volume 2); Comet Capture (volume 4); Impacts (volume 4); Kuiper Belt (volume 2); Kuiper, Gerard Peter (volume 2); Oort Cloud (volume 2).

Humberto Campins

Bibliography

Oort, Jan H. "The Structure of the Cloud of Comets Surrounding the Solar System and a Hypothesis Concerning Its Origin." Bulletin of the Astronomical Institute of the Netherlands 11 (1950):91-110.

Whipple, Fred L. "A Comet Model I: The Acceleration of Comet Encke." Astrophysical Journal 111 (1950):375-394.