Comet

Comet

I

INTRODUCTION

Comet (Latin stella cometa, “hairy star”), relatively small, icy celestial body revolving around the Sun. When a comet nears the Sun, some of the ice in the comet turns into gas. The gas and loose dust freed from the ice create a long, luminous tail that streams behind the comet. See Solar System; Astronomy.

II

HISTORY

Approximately 2,000 comets have been observed and recorded over the past 2,500 years. Several hundred of those were not visible to naked-eye observers on Earth, and were only discovered during the past few decades with the aid of astronomical instruments. Appearances of large comets were regarded as atmospheric phenomena until 1577, when Danish astronomer Tycho Brahe proved that they were celestial bodies. In the 17th century British scientist Isaac Newton demonstrated that the movements of comets are subject to the same laws that control the planets in their orbits.

By comparing the orbital elements of a number of earlier comets, British astronomer Edmond Halley showed the comet of 1682 to be identical with the two that had appeared in 1607 and 1531, and he successfully predicted the comet’s next return, which occurred in 1758. The earlier appearances of what came to be known as Halley’s Comet have now been identified from records dating from as early as 240 bc, and it is probable that the bright comet observed in 466 bc was also an apparition of this famous comet. Halley’s Comet most recently passed around the Sun again early in 1986.

III

COMPOSITION

A comet is generally considered to consist of a small nucleus embedded in a nebulous disk called the coma. American astronomer Fred L. Whipple proposed in 1949 that the nucleus, containing practically all the mass of the comet, is a “dirty snowball” conglomerate of ices and dust.

Major proofs of the snowball theory rest on various data. For one, of the observed gases and meteoric particles that are ejected to provide the coma and tails of comets, most of the gases are fragmentary molecules, or radicals, of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH, and OH may be broken away from the stable molecules CH₄ (methane), NH₃ (ammonia), and H₂O (water), which may exist as ices or more complex, very cold compounds in the nucleus. Another fact in support of the snowball theory is that the best-observed comets move in orbits that deviate significantly from Newtonian gravitational motion. This provides clear evidence that the escaping gases produce a jet action, propelling the nucleus of a comet slightly away from its otherwise predictable path. In addition, short-period comets, observed over many revolutions, tend to fade very slowly with time, as would be expected of the kind of structure proposed by Whipple. Finally, the existence of comet groups shows that cometary nuclei are fairly solid units.

The head of a comet, including the hazy coma, may exceed the planet Jupiter in size. The solid portion of most comets, however, is equivalent to only a few cubic kilometers. The dust-blackened nucleus of Halley’s Comet, for example, is about 15 by 4 km (about 9 by 2.5 mi) in size.

IV

SOLAR EFFECTS

As a comet approaches the Sun, the solar heat evaporates, or sublimates, the ices so that the comet brightens enormously. It may develop a brilliant tail, sometimes extending many millions of kilometers into space. The tail is generally directed away from the Sun, even as the comet recedes again. The great tails of comets are composed of simple ionized molecules, including carbon monoxide and carbon dioxide. The molecules are blown away from the comet by the action of the solar wind, a thin stream of hot gases ejected from the solar corona, the outermost atmosphere of the Sun, at a speed of 400 km (250 mi) per sec. Comets frequently also display a second, curved tail composed of fine dust blown from the coma by the pressure of solar radiation. These dust tails are usually brighter than the ion tails. Astronomers discovered a third type of tail on Comet Hale-Bopp, a comet that was bright in Earth's sky in 1996 and 1997. Hale-Bopp's third tail was very narrow and was not visible to the naked eye. It was composed of neutral (not electrically charged) sodium atoms and glowed faint yellow. The sodium tail was straight, like the ion tail, but pointed in a slightly different direction.

As a comet recedes from the Sun, the loss of gas and accompanying dust decreases in quantity, and the tails disappear. Some of the comets with small orbits have tails so short that they are practically invisible. On the other hand, the tail of at least one comet has exceeded 320 million km (200 million mi) in length. The variation in length of the tail, together with the closeness of approach to the Sun and Earth, accounts for the variation in the visibility of comets. Of all the comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were conspicuous.

V

PERIODS AND ORBITS

Comets have elliptical orbits with periods—the time they take to orbit the Sun once—ranging from a few years to tens of thousands of years. The orbits of most comets are so vast that they are indistinguishable from parabolas—open curves that would take the comets out of the solar system—but from technical analyses astronomers assume that they also are ellipses, of great eccentricity, with periods as long as 40,000 years or possibly much longer. The bright Comet Hyakutake, which was visible from Earth in 1996, has an estimated period of 10,000 years.

No comets have been known to approach Earth on a hyperbolic orbit; this would have meant an origin outside the solar system. Some comets, however, may never return to the solar system because of extreme alteration of their original orbits by the gravitational action of the planets. Such action has been observed on a smaller scale. About 60 short-period comets have orbits that have been influenced by the planet Jupiter, and are said to belong to the family of Jupiter. Their periods range from 3.3 to 9 years.

VI

COMET GROUPS

When several comets with different periods travel in nearly the same orbit, they are said to be members of a comet group. The most famous group includes the spectacular sun-grazing comet, Ikeya-Seki, of 1965, and seven other comets having periods of nearly 1,000 years. The American astronomer Brian G. Marsden concluded that Ikeya-Seki and the even brighter comet of 1882 split from a parent comet, possibly the one of 1106. This comet and others of the group probably split away from a truly giant comet thousands of years ago. The European Space Agency’s Solar and Heliospheric Observatory (SOHO) spacecraft, designed to observe the Sun, has detected over 500 comets from another group, the Kreutz sun-grazers.

A close relationship also exists between the orbits of comets and the orbits of meteor showers. The Italian astronomer Giovanni Virginio Schiaparelli proved that the Perseid meteors (see Perseids), which appear in August, move in the same orbit as comet Swift-Tuttle. Similarly, the Leonid meteors (see Leonids), which appear in November, were found to follow the same orbit as comet Tempel-Tuttle. Several other showers have been related with known cometary orbits, and are explained by the earth-type debris scattered by a comet along its orbit.

VII

COMET ORIGINS

Comets were once believed to come from interstellar space. Although no detailed theory of origin is generally accepted, many astronomers now believe that comets originated in the outer, colder part of the solar system from residual planetary matter in the early days of the solar system. Comets are believed to be unchanged from the time the solar system came into existence about 4.6 billion years ago. Consequently, scientists try to study them closely for clues to the chemical makeup of the early solar system. The Dutch astronomer Jan Hendrik Oort proposed that a “storage cloud” of comet material has accumulated far beyond the orbit of Pluto, and that the gravitational effects of passing stars send some of the material sunward, where it becomes visible as comets.

Since the 1990s, it has been realized that long-period comets (those with periods longer than about 200 years) come from the Oort cloud, while short-period comets come from a ring of debris known as the Kuiper Belt. The Kuiper Belt, which starts just beyond the orbit of the planet Neptune, is flattened in the plane of the solar system. Comets that originate there tend to have orbits in the same plane as the planets.

VIII

COMET IMPACTS

Comets have long been regarded by the superstitious as portents of calamity or important events. The appearance of a comet has also given rise to the fear of collision between the comet and Earth. Earth, in fact, has passed through the tails of occasional comets without measurable effect. The collision of the nucleus of a comet with a large city would probably destroy the city but the probability of such an event occurring is exceedingly small. Some scientists suggest, however, that collisions have taken place in the astronomical past. Scientists studying Comet Hale-Bopp in 1997 found chemicals in the comet that are very similar to those that are thought to have led to life on Earth. Comets may have provided Earth with water and important chemicals in its early history, and a collision between Earth and a comet may also have had a climatic role in the extinction of the dinosaurs.

In 1992 Comet Shoemaker-Levy 9 broke apart into 21 large fragments as it ventured into the strong gravitational field of the planet Jupiter. During a week-long bombardment in July 1994 the fragments crashed into Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the comets was released in massive explosions, some resulting in fireballs larger than Earth.

IX

EXPLORATION OF COMETS

A number of spacecraft have provided scientists with important data about comets. In 1974 the crew of Skylab, the first U.S. space station, used a solar telescope to observe Comet Kohoutek as it approached the Sun. In 1986 Halley’s Comet was visited by two probes, Vega 1 and 2, which were launched by the Soviet Union, and by another spacecraft called Giotto, which was launched by the European Space Agency (ESA). Giotto made the closest approach to the comet, coming within about 600 km (375 mi) of its nucleus. Two Japanese spacecraft observed Halley’s Comet at a great distance as it passed. Giotto and the Vega spacecraft were equipped with cameras. Their images confirmed that Halley's nucleus was very black, probably due to the presence of hydrocarbons, and showed that the nucleus had an elongated, irregular outline shaped somewhat like a potato. Several bright, localized jets of escaping gas and dust spurted from the potato-shaped body, which was about 16 km (10 mi) long and 8 km (5 mi) wide.

In January 2004 a United States spacecraft called Stardust, which was launched in 1999, became the first spacecraft to gather sample dust grains from a comet as it flew through the coma of Comet Wild (pronounced VILT) 2. The spacecraft encountered the comet as it orbited the Sun about 390 million km (240 million mi) from Earth. Scientists believe that the dust grains represent material unchanged from the time the solar system began about 4.6 billion years ago and so will provide valuable clues about early conditions in the solar system. Stardust’s cameras also took close-up images of the comet’s nucleus from a distance of about 240 km (149 mi). As the spacecraft passed through the coma, it used a special device to gather a tiny amount of microscopic dust grains and sealed them in a canister containing a material known as aerogel, which trapped the particles. Stardust jettisoned a capsule containing the canister when the spacecraft flew by Earth on its return journey in January 2006. The capsule successfully reentered Earth’s atmosphere, its final descent slowed by a parachute, and was recovered on January 15 at a landing site in Utah. Scientists with the National Aeronautics and Space Administration (NASA) then examined the canister. The lead scientist for the mission, astronomer Donald Brownlee, calculated that it contained more than a million microscopic specks of dust, which will be analyzed by more than 150 scientists for clues to the origin of the solar system.

In July 2005 NASA successfully engineered the first collision between a man-made object and a comet in an effort to penetrate a comet’s outer crust and thereby expose chemical compounds located within the comet’s nucleus. NASA’s Deep Impact spacecraft, which was launched from Earth in January 2005, rendezvoused with Comet Tempel 1 about 134 million km (83 million mi) from Earth. As the spacecraft approached the comet, it released a smaller craft known as an impactor that slammed into the comet’s nucleus on July 4 at 1:52 am Eastern Standard Time. The impact sent a plume of debris from the comet billowing into space. Both Earth-based and space-based telescopes, along with cameras and other scientific instruments onboard the Deep Impact spacecraft and the impactor, observed the collision and recorded data for later analysis. Recent study of the data indicates that the comet contains a wide range of chemicals, including carbonates, clays, metal sulfides, crystalline silicates, and aromatic hydrocarbons. Some of the compounds must have formed in the presence of liquid water while others require the extreme high temperatures found near the Sun. The findings suggest that the early solar system mixed materials that came from both its inner and its outer regions as it formed.

The ESA’s Rosetta spacecraft is planned to be the first spacecraft to go into orbit around a comet and to place a lander on its nucleus. The 100-kg (220-lb), box-shaped lander carries a variety of instruments to measure the composition of the nucleus and return both panoramic and microscopic images. Rosetta was launched in March 2004 and is expected to reach Comet 67P/Churyumov-Gerasimenko in 2015.

Reviewed By:
Jay M. Pasachoff

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