Comets

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Comets

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.