Comet - astronomy.

Comet - astronomy. I INTRODUCTION Comet, small icy body in space that sheds gas and dust. Like rocky asteroids, icy comets are ancient objects left over from the formation of the solar system about 4.6 billion years ago. Some comets can be seen from Earth with the unaided eye. Comets typically have highly elliptical (oval-shaped), off-center orbits that swing near the Sun. When a comet is heated by the Sun, some of the ice on the comet's surface turns into gas directly without melting. The gas and dust freed from the ice can create a cloud (coma) around the body (nucleus) of the comet. More gas and dust erupt from cracks in the comet's dark crust. High-energy charged particles emitted by the Sun, called the solar wind, can carry the gas and dust away from the comet as a long tail that streams into space. Gas in the tail becomes ionized and glows as bluish plasma, while dust in the tail is lit by sunlight and looks yellowish. This distinctive visible tail is the origin of the word comet, which comes from Greek words meaning "long-haired star." Humans have observed comets since prehistoric times. Comets were long regarded as supernatural warnings of calamity or signs of important events. Astronomers and planetary scientists now study comets for clues to the chemical makeup and early history of the solar system, since comets have been in the deep-freeze of outer space for billions of years. Materials in comets may have played a major role in the formation of Earth and the origin of life. Catastrophic impacts by comets may also have affected the history of life on Earth, and they still pose a threat to humans. II PERIODS AND ORBITS OF COMETS Comets have elliptical orbits around the Sun that take periods of time ranging from a few years to tens of thousands of years or longer. Astronomers classify comets according to their orbits. A commonly used classification divides comets into short-period comets, which have periods shorter than 200 years, and long-period comets, which have periods 200 years or longer. Under this classification, short-period comets are subdivided into Jupiter-family comets and Halley-type comets. Jupiter-family comets are influenced by the gravitation of the giant planet Jupiter and have periods shorter than 20 years (about 8 years being average), and orbits that mainly lie closer in than the orbit of Jupiter. Halleytype comets (sometimes called intermediate-period comets) have periods from 20 up to 200 years, and have orbits that may reach beyond the planet Neptune. Halley's Comet itself has a period that averages 76 years. Most short-period comets and some long-period comets have orbits that lie close to the plane of the solar system in which the eight major planets orbit the Sun. By contrast, a significant number of long-period comets (more than 30 percent) have orbits that are strongly tilted relative to the plane of the solar system. Short-period comets, particularly in the Jupiter family, usually orbit the Sun in the same counterclockwise direction as planets and other solar system bodies. Some comets such as Halley's Comet, however, may orbit the Sun in a clockwise direction, opposite the planets. Scientists think short-period comets typically originate in the Kuiper Belt, a disk of icy objects that lies beyond the orbit of Neptune at 35 to 55 astronomical units (AU) from the Sun. (An astronomical unit is the average distance from Earth to the Sun, just under 150 million km, or 93 million mi.) In addition to the main disk, the Kuiper Belt also includes a larger region of scattered objects--icy bodies such as the dwarf planet Eris that have more inclined and very out-of-round elliptical orbits. In general, comets from the Kuiper Belt retain the same orbital direction and orientation around the Sun as the rest of the solar system. Some of these comets may spend time as objects called centaurs between the orbits of Neptune and Jupiter, and then move into the inner solar system to become Jupiter-family comets. Comets from the scattered-disk region of the Kuiper Belt can have more variable orbits. Long-period comets and some Halley-type comets likely come from the Oort Cloud, a much more distant sphere of icy objects thought to surround the solar system at a distance of 20,000 to 200,000 astronomical units (AU) from the Sun. Comets from the Oort Cloud can come from random directions. The gravitational pull of distant stars may nudge the orbits of objects in the Oort Cloud, causing the icy bodies to move inward and become comets. The orbits of some long-period 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 the orbits of these comets 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; Comet McNaught, which was visible in late 2006 and early 2007, may have a period of more than 100,000 years. No comets have been known to approach Earth on a hyperbolic orbit--such an orbit would have meant an origin outside the solar system. Another classification system divides comets into two major categories based on how their orbits relate to the main plane of the solar system. Comets with orbits that can bring them into the inner solar system from all directions above or below the plane of the solar system are called nearly isotropic (meaning with properties independent of direction). Comets with orbits that lie close to the orbital planes of the major planets in the solar system are called ecliptic comets. In this scheme, Halley-type comets are classified as nearly isotropic returning comets. 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 AD 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 more than 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 in Earth's atmosphere. The Italian astronomer Giovanni Schiaparelli proved that the Perseid meteors, which appear each year in August, move in the same orbit as Comet Swift-Tuttle. Similarly, the Leonid meteors, 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 dust and other solid debris left behind by a comet along its orbit. Some of the dust collected in Earth's upper atmosphere by scientific instruments on high-altitude aircraft and balloons is thought to come from comets. III COMPOSITION AND ORIGIN OF COMETS Close observations of comets by spacecraft confirm that comets have a rotating solid nucleus made of icy material (mainly water ice) mixed with dust and rock. This "dirty snowball" model of comets was first proposed by American astronomer Fred L. Whipple in a 1950 paper. Earlier theories suggested that comets were made up almost entirely of gas and lacked a large solid core, or were a pile of rubble. The size of the nucleus of a comet may vary, but it is typically a few kilometers across and irregular in shape. For example, the Giotto space probe in 1986 revealed that Halley's Comet has a dust-blackened nucleus about 15 by 7 km (about 9 by 3.6 mi) in size. The hazy coma of gas and dust released around the head of an active comet may exceed the planet Jupiter in size, however. Observations from telescopes on Earth and in space indicate that most of the gases in the coma and tail of a comet 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. All comets were once believed to be made up almost entirely of primitive icy material that existed in the colder outer reaches of the huge cloud of dust and gas that collapsed to form the solar system about 4.6 billion years ago. According to a widely held theory of how the solar system formed, the dust and gas coalesced into tiny clumps that contained differing amounts of ice depending on the distance from the early Sun. A "snow line" in the disk around the Sun meant that objects in the region from Jupiter outward must have contained a much larger proportion of ice than objects closer to the Sun. Over time, these tiny objects clumped together to form planetesimals--the building blocks of planets. Larger and larger objects formed as planetesimals collided and clumped together in a process called accretion, leading to the formation of planets. The gas-giant planets Jupiter and Saturn, and the ice-giant planets Uranus and Neptune, along with their moons, were built from mainly icy material, while the inner planets Mercury, Venus, Earth, and Mars were built from mainly rocky material. Rocky planetesimals that were left over from the planet-building period became asteroids, which now mainly orbit between the planets Mars and Jupiter in a region called the asteroid belt. Icy planetesimals became comets. Gravitational interactions among the giant outer planets as they moved into their modern orbits likely threw most of the icy planetesimals into the distant parts of the solar system where they are now found. The material in comets supposedly remained frozen and unchanged since that time. However, data from space probes and other research indicate that important differences may exist in the composition of objects that become comets. Astronomers have found that some short-period comets contain large amounts of material that is similar to minerals in rocky asteroids--material that has been heated and chemically altered near the Sun. Dynamic processes may have mixed material from the inner solar system with icy debris in the outer regions as the Sun and planets formed. Additionally, short-lived radioactive isotopes of mineral elements may have heated some comets internally, creating liquid water and leading to the formation of clays and other compounds. IV COMETARY ACTIVITY AND TAILS As a comet approaches the Sun, the solar heat begins to evaporate, or sublime, the ices on and under the surface of the nucleus. Dust on the surface and in the ice is mixed with the escaping gas. The cloud of gas and dust (called a coma) expands around the rotating nucleus, held in place by the comet's tiny gravity. Gases may also erupt in jetlike bursts out of cracks on the surface of the nucleus, affecting the rotation and path of the nucleus. This nongravitational force is provided by Newton's third law of motion, which states that every action has an equal and opposite reaction--the same law that propels rockets through space. As the nucleus and coma approach the Sun the comet may develop a brilliant tail that can sometimes extend many millions of kilometers into space. The bluish plasma 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--streams of hot gases ejected from the solar corona, the outermost atmosphere of the Sun--at speeds of 400 km (250 mi) per second or more. Ionization and recombination makes the gas molecules glow. 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, show less detail, and have a yellowish color. Reflected sunlight makes them visible. 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 as a result glowed faint yellow. The sodium tail was straight, like the ion tail, but pointed in a slightly different direction. How much icy material a comet contains and how close it comes to the Sun can determine the size of its tail. Repeated passes near the Sun exhaust the icy volatiles in a comet over time so that a comet that swings by the Sun for the first time is likely to have a bigger, brighter tail than one that has made dozens of orbital journeys. 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 observed from Earth. Of all the comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were conspicuous. As a comet moves away from the Sun again, the tail disappears as colder temperatures reduce the amount of gas and dust released. Comets are typically most active when they are near the Sun. However, a few comets display unusual outbursts. One of the most spectacular examples was Comet Holmes in 2007. Astronomers have known about this small short-period comet for more than 100 years. The comet suddenly erupted with an enormous coma in late 2007 as its orbit took it into the region of the asteroid belt, long after the comet had passed by the Sun. Comet Holmes became nearly one million times brighter, and the tenuous dust cloud surrounding the nucleus temporarily became the largest object in the solar system--wider than the diameter of the Sun. Only a very small taillike feature was detected. Why Comet Holmes exploded in this way remains under study. When a comet passes through the solar system and around the Sun, the gravitational pulls of the Sun and the planets can alter its orbit. In addition, the jetlike bursts of gas from the nucleus can also shift the comet's path. Such changes mean that the comet's next entry into the inner solar system may come later or earlier than predicted, and from a slightly different angle or direction. Over a period of a few thousand years, a comet is likely either to hit the Sun or a planet, be thrown out of the solar system, or simply die by running out of gas and resemble an asteroid. V COMET COLLISIONS IN EARTH HISTORY From about 4.1 to 3.8 billion years ago, a period of time called the Late Heavy Bombardment, millions of comets and asteroids left over from the formation of the solar system struck the surface of the early Earth and other bodies in the solar system. Because comets contain water and organic compounds, comets that struck Earth may have contributed to the formation of the oceans and provided chemical building blocks for the first life, as well as gases for the atmosphere. For example, 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. Some researchers have even theorized that primitive organisms could arise in the environment inside comets and may have seeded life on Earth and on other planets (see Astrobiology). It is likely that comets have struck Earth a number of times since life arose on our planet. However, comet collisions are rarer than asteroid collisions. Most bodies that strike Earth belong to a population called Near Earth Objects (NEOs) that have orbits in the inner solar system that can cross Earth's orbit. The great majority of these objects are asteroids, but some NEOs appear to be comets or old comets that have lost all their gases. A comet that comes from farther out in the solar system might be much more destructive, particularly if it was relatively large and had a path that came in the opposite direction of Earth's orbit, creating a head-on collision scenario. Scientists have identified nearly 200 craters on Earth that resulted from large impact events. Most of the impacting objects were likely asteroids. The most famous recent collision happened in 1908 over Tunguska, Siberia. An object from space exploded in midair, flattening trees for hundreds of square miles. The general effects of an impact from an asteroid or from a comet would be very similar. A superheated explosion creates a massive cloud of debris and a deep crater. Huge earthquakes shake the planet and giant ocean tsunamis may crash into coastlines. Forest fires may sweep large areas near the blast. Dust and smoke thrown into the atmosphere could block sunlight for months or years. Gases released in the blast may cause acid rain and raise levels of carbon dioxide in the air. The large amount of gases released from a comet impact might have different chemical effects in Earth's atmosphere than those caused by an asteroid impact. A large comet that struck Earth would likely lead to a mass extinction of life, including humans. Scientist think such a major collision with an object from space occurred 65 million years ago, leading to the extinction of the dinosaurs. Scientists are studying possible ways to divert an asteroid from striking Earth. Preventing a comet impact might be more difficult because comets have less predictable orbits and are surrounded with a cloud of dust and debris. A dangerous new comet might be detected too late to plan a space mission to divert or destroy the approaching object. Scientists witnessed the results of a comet collision with Jupiter in 1994. Comet Shoemaker-Levy 9 broke apart into 21 large fragments as it ventured into the strong gravitational field of the planet Jupiter. On a later passage of the comet's orbit in July 1994, the fragments crashed into Jupiter's dense atmosphere in a weeklong bombardment, reaching 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. Dark spots of material dredged up from under Jupiter's cloudtops were visible from telescopes on Earth and in space. Comet impacts on the airless Moon and on the planet Mercury may have left icy deposits at the poles in craters whose interiors are protected from sunlight. Astronauts living in a proposed base near the south pole of the Moon might be able to use cometary ice for water and hydrogen fuel. VI OBSERVATION AND EXPLORATION OF COMETS A Early Observations of Comets Thousands of comets have been observed and recorded over the past 2,500 years. Before comets were understood as natural objects, they were seen as supernatural signs. For example, a bright comet that appeared in 44 BC after the murder of the Roman leader Julius Caesar was hailed as a sign that Caesar had become a god. Although comets appeared to move through the sky like planets, the 4th-century BC Greek philosopher Aristotle described comets as objects in Earth's atmosphere. Appearances of large comets were regarded as atmospheric phenomena until 1577, when Danish astronomer Tycho Brahe, working before the invention of the telescope, proved that they were celestial bodies distant from Earth. In the 17th century British scientist Isaac Newton demonstrated that the movements of comets are subject to the same laws of gravitation and motion 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. Minor variations in the period could be accounted for by gravitational interactions. Halley successfully predicted the comet's next return, which occurred in 1758 after his death. Many 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 this famous comet. Halley's Comet most recently passed around the Sun again early in 1986, but was not especially bright compared to past appearances. Its future reappearance in 2061 is not predicted to be especially bright either. The Hubble Space Telescope is now able to study Halley's Comet as a faint spot throughout its orbit, though it does not currently have a tail B Modern Study of Comets The scientific study of comets was greatly aided by the invention of the telescope in the early 17th century. Detecting and cataloging comets became an important task for professional astronomers. However, amateur astronomers with telescopes have been the first to detect a number of notable comets. A comet is usually detected after it starts to form a coma that reflects sunlight, making the object brighten. Most comets found since the late 20th century were discovered with the aid of astronomical instruments such as the Large Angle Spectroscopic Coronagraph (LASCO) on the SOHO satellite that observes the Sun, or with Earth-based computercontrolled telescope systems such as the Lincoln Near Earth Asteroid Research (LINEAR) and Near Earth Asteroid Tracking (NEAT) projects that scan the skies and detect small objects that move. 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, reflecting only a small percent of the sunlight that strikes it. Its dark color probably comes from the presence of hydrocarbons. Images also 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 nucleus, which was about 15 km (9 mi) long and 7 km (3.6 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. Stardust's cameras also took closeup 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 an extremely low-density 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. Later analysis of the dust revealed that the composition of Comet Wild 2 was surprisingly similar to material found in asteroids. The dust in the comet had been heated and chemically altered from the primitive material that first coalesced into the early solar system. Much of the material apparently formed close to the early Sun and not in the cold, icy outer regions of the solar system. Some process may have mixed material in the early solar system. In July 2005 NASA successfully engineered the first collision between a human-launched 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 Daylight Time. The impactor was destroyed and 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 itself, observed the approach and 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, findings in line with the Stardust results. 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 lander is named Champollion in honor of the famous 19th-century French scholar who decoded the Egyptian hieroglyphs on the Rosetta stone. 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. VII NAMING COMETS The International Astronomical Union (IAU) is the official body that names comets and assures that reported new comets are valid. Comets with their full scientific designations are recorded in a special catalog published for the IAU by the Smithsonian Astrophysical Observatory and maintained online. This catalog is separate from the official catalog of minor planets. (Minor planets include asteroids, dwarf planets, KBOs, and centaurs.) A few objects such as the centaur Chiron are listed in both the comet and the minor planet catalogs. Unlike asteroids, comets are not given specially created names with sequential numbers such as 433 Eros or 100,000 Astronautica. Instead, comets are named for their discoverers, using the last name of the person credited with the first sighting of the comet. If more than one person detected the comet at almost the same time, multiple last names are used with hyphens, such as Comet Hale-Bopp. Comets discovered using computer-controlled telescopes are named with the acronyms of the devices, such as LINEAR or NEAT. The full scientific designation of the comet also includes the year of discovery, and an alphabetical code that indicates the half of the month when the comet was found, beginning with A for the first half of January, B for the second half, and so on (the I is not used). The designation also indicates its ordinal rank among all the comets discovered in that half-month. Periodic comets have numerical designations such as 1P, 2P, and 3P. Although not mandatory, the name of the discoverer or discoverers often follows. The first comet listed in the official catalog is 1P/Halley, named in honor of Edmond Halley, the scientist who first understood its orbit and predicted its return. The second comet listed is 2P/Encke, named for the German astronomer Johann Encke. It has a period of only 3.3 years. Encke's Comet is never bright but has been seen on more passages than any other comet. For comets discovered after 1995, the letter C is used for comets with periods 200 years or longer and P for short-period comets (less than 200 years). For example, Comet McNaught was named for Australian astronomer Robert McNaught, who first spied the comet in early August of 2006. The comet's official designation is C/2006 P1 (McNaught) (first long-period comet discovered in the first half of August, 2006). Comet McNaught became so bright and developed such a large tail when it passed near the Sun in January 2007 that it also earned the name "great comet of 2007." Astronomers give the designation "great comet" to comets that are particularly spectacular when observed from Earth. Other recent "great comets" include Comet Bennett in 1970, Comet West in 1976, Comet Hyakutake in 1996, and Comet Hale-Bopp in 1997. Great comets seem to arrive without notice once or twice each decade. Reviewed By: Jay M. Pasachoff Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.