Overview (The Solar System)
Comets are familiar to nearly everyone as majestic, starlike objects with long tails stretching across a wide band of the sky. The most famous comet, Halley’s comet, makes its periodic return to the night skies every seventy-five years. The word “comet” is derived from a Greek word meaning “long-haired.” Comets were greatly feared before the twentieth century as bad omens. Since then, they have been identified and cataloged as objects that come into the inner Solar system from deep space. Most of them occupy orbits that carry them far away from the Sun. Many comets make only a single approach to the Sun and then never return again, while others exist in stable, but highly elliptical, orbits that allow them to return after an extended period of time.
One of the first theories advanced to explain the makeup of comets was proposed by astronomer Fred L. Whipple. Whipple suggested that comets were dirty snowballs, essentially bodies of water ice that incorporate dust and perhaps volatives other than water. This remained the primary theory through the first four decades of the space age. Only when Spacecraft began visiting comets could it be put to the test.
Until the space age, comets were studied only in visible light through optical telescopic images. The first comet to be studied using Earth-orbital instruments, which permitted observations in the ultraviolet as well as the visible, was the much-heralded Comet...
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Applications (The Solar System)
The study of comets requires detailed knowledge of the composition of the outer regions of the solar system and the space between the last planet and 100,000 AU outward from the Sun. Comet studies also seek to understand complex gravitational interactions between bodies separated by wide distances and even gravitational interactions between tiny comets and the entire galaxy. Astronomers who study comets want to learn more about their makeup, their behavior when approaching the Sun, and the makeup and evolution of the early solar system.
New comets approaching the Sun for the first time have been held in deep freeze within the Oort Cloud and are thought to be composed of primordial material of the newly forming solar system. They have been tied up in the Oort Cloud for billions of years at temperatures barely above absolute zero. As they approach the Sun, their internal gases begin to stream away. Detailed study of an approaching comet’s outgassing can inform planetary scientists about the composition of the early solar system. Comets and their approach have also hinted at the existence of the elusive brown dwarfs, thought to be one of the most common bodies of interstellar space. Because they are so dim, they are all but invisible from Earth. On the other hand, because brown dwarfs are thought to be so plentiful, the study of comets and their orbits may give the first real clues to the former’s reality and abundance. The...
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Context (The Solar System)
Humankind has always looked to the heavens in awe and wonder, and sometimes in fear. Perhaps no other astronomical phenomenon except a total solar Eclipse has historically evoked as much fear as comets. When the specter of fear is removed, however, they emerge as strikingly beautiful objects in the sky. It was once believed that if Earth passed through the tail of a comet, its inhabitants would die; this theory has been discredited. Comets are messengers from a time long past. Most are chunks of dirty ice, locked away in the Oort Cloud for billions of years.
Comets have been used to judge vast distances, evaluate the early composition of the solar system, and even test the idea that the gravity of the entire Galaxy can make a difference to the smallest of objects in space. Comets have been used as yardsticks to evaluate what may be the most common type of star in the galaxy, the brown dwarf—which ironically is one that is difficult to observe, even in the infrared. Comets have been called dirty snowballs. Halley’s comet was so black that it was the darkest object ever seen in space. Yet, from these dirty specks of ice, planetary scientists have witnessed some of the most spectacular light shows. Ultimately, comets may also generate clues to some of the most fundamental secrets about the solar system and planets. From these tiny messengers, planetary scientists may unlock and examine pristine elements from the formation of the...
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Further Reading (The Solar System)
Arny, Thomas T. Explorations: An Introduction to Astronomy. 3d ed. New York: McGraw-Hill, 2003. A general astronomy text for the nonscience reader. Includes an interactive CD-ROM and is updated with a Web site.
Benningfield, Damond. “Where Do Comets Come From?” Astronomy 18 (September, 1990): 28-36. A fine summary of comets, superbly illustrated and written for the general public. Addresses the question of the Oort Cloud and Kuiper Belt in detailed, scaled illustrations. Discusses possible linkage to the extinction of the dinosaurs and the latest satellite discoveries.
Brandt, John C., and Robert D. Chapman. Introduction to Comets. New York: Cambridge University Press, 2004. A text suitable for a planetary science course, this comprehensive work covers our knowledge of comets from early observations to telescopic investigations through spacecraft encounters of these mysterious and alluring bodies.
Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2005. An updated version of a classic text that covers all aspects of planetary science. Well-explained material on asteroids and comets. Takes a comparative planetology approach.
Levy, David H. The Quest for Comets: An Explosive Trail of Beauty and Danger. New York: Plenum Press, 1994. Written by one of the codiscoverers of Comet Shoemaker-Levy 9, this book is for the general...
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Comet (Encyclopedia of Science)
A comet Greek word meaning "long-haired"s 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.
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,...
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Comets (World of Earth Science)
Comets are objectselatively small compared to planetshat 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) 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 (1656742) 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 (1642727) 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 (38422 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 terrestrialriginating from Earth, rather than celestialeavenly 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. 8750) 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 (1473543).
In conjunction with the German astronomer and mathematician Johannes Kepler (1571630), Danish astronomer Tycho Brahe's (1546601) observation of the "Great Comet" of 1577 provided evidence that the comet was at least four times further away from Earth than the Moon 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 (1611687), who suggested comets move on a parabola (U-shape) around the Sun; and English scientist Robert Hooke (1635703), 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 (1809882) writings on evolution and German-American physicist Albert Einstein's (1879955) 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 1852ut 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 (1826873) 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 cloudamed for Dutch astronomer Jan Van Oort 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 Earthr 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 300 ft (92 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