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Comet is a ball of ice and dust.
A Comet looks like a star with a tail. Some comets do not have tails, looking like hazy, round spots of light. Most Comets have three parts: a nucleus, a head (coma), and a tail.
A Comet is a small body in the Solar System that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail- both due primarily to the effects of solar radiation upon the comet's nucleus, which itself is a minor planet composed of rock, dust, and ices. Due to their origins in the outer Solar System and their propensity to be highly affected by relatively close approaches to the major planets, comets' orbits are constantly evolving. Some are moved into sungrazing orbits that destroy the Comets when they near the sun, while others are thrown out of the Solar System forever.
The "typical" Comet is a small, oblong chunk of ice, about 5-10 miles across, on the average; this chunk of ice is called the comet's "nucleus."
After the Spacecraft Giotto photographed the nucleus of Halley's comet back in 1986, we learned that a comet's nucleus probably has a surface that is best described as a black crust. Although the length of the nucleus of Halley's Comet is about 12km, it is believed that Comet nuclei can range from 1 km to perhaps 50km across. Comet Hale-Bopp of 1997 had a nucleus that was perhaps 40km across.
The black crust of the nucleus helps the Comet absorb heat, which in turn causes some of the ices under the crust to turn to a gas. With pressure now building beneath the crust, the serene, but frozen landscape begins to bulge in places. Eventually the weakest areas of the crust shatter from the pressure beneath, and the gas shoots outward like a geyser and is referred to by Astronomers as a jet. Any dust that had been mixed in with the gas is thrown out as well. As more and more jets appear, a tenuous gas and dust shell forms around the nucleus and this is called the coma.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the Comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
One of the largest Comets in history was the Great Comet of 1811. It was one of the few Comets in history to be discovered with a relatively small telescope at an unusually great distance from the sun, in this case over half-way to the planet Jupiter's orbit. The nucleus has been estimated as between 30 and 40 kilometers in diameter. At one point during September to October 1811 the coma reached a diameter roughly equivalent to the diameter of the Sun and was a very notable naked-eye object seen by people around the world.
Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars. At this distance the particles streaming out from the Sun provide enough force so as to act as a wind and will literally blow the gas and dust particles away from the nucleus and coma. This disruption is the process responsible for a comet's tail, the most spectacular feature of a comet.
When you have a large Comet that moves well inside the orbit of Earth, you have the potential for a long tail. The current record holder for longest tail length is the Great Comet of 1843. Its tail extended more than 250 million kilometers. What this means is that if the comet's nucleus were placed in the center of the Sun the tail would have stretched passed the orbits of Mercury, Venus, Earth, and Mars! As the Comet comes still closer to the sun, the solar wind -- an energetic stream of particles continuously blowing off the sun's surface -- encounters the material in the comet's coma and blows it back behind the nucleus. This creates the comet's "tail," which usually extends behind the Comet in the opposite direction from the sun. (In other words, the tail follows the nucleus and coma as the Comet comes in toward the sun, but leads the coma as the comet recedes.)
You can think of a Comet as a large windsock, with the tail extending in the direction of the solar wind's motion. Quite often, two tails will form: one made up primarily of the sublimated gases (which have been ionized -- i.e., electrically charged -- by the solar wind; one can think of this tail as a large neon sign, in a way) and the other composed of dust grains, which "shine" by reflecting sunlight.
Because a Comet experiences its strongest solar heating at its perihelion point, that is usually when it is brightest. (Of course, a comet's distance from the Earth also plays a role in how bright it appears to us.) Occasionally, Comets have been known to experience "outbursts" when their brightnesses increase dramatically within a short period of time. This can be due to a fresh eruption of new material from a comet's surface, or sometimes this occurs when the nucleus splits into two or more pieces, exposing previously hidden sections of its material to the sun's heat for the first time.
On the average, about a dozen of Jupiter's family of Comets will pass perihelion during any given year. Also, as many as a dozen previously unknown Comets are discovered each year; some of these may turn out to be additional members of Jupiter's family, and others may be in much larger orbits which will not bring them back to the Sun for many centuries or millenia, if ever. On any given clear dark night, two or three dozen Comets may be accessible to professional Astronomers at the world's large observatories; of this number, perhaps two or three are visible to amateur astronomers with suitable telescopes.
About once a year, on the average, there will appear a Comet bright enough to be visible with the naked eye, and the so-called "Great Comets," which are those that are conspicuous to the naked eyes of even non-astronomers, appear about once every decade or so.
Some Comets travel is such long orbit that they are near the sun once in thousands of years. All Comets are part of the suns family, just as the Earth and the other planets.
In this state Comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material lumps of frozen gas and rock.
When a Comet approaches within close to the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.
As the Comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a Hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.
The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the Comet opposite the Sun.
Each time a Comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, Comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, Comets that have lost all of their volatiles.
Comets probably formed at the same time as the Sun and planets, about 4.5 billion years ago. But many of them were kicked far from the Sun by the powerful gravity of the outer planets. The make up Comets is found in sedimentary rocks on Earth.
Astronomers suspect that as many as one trillion of these objects reside in a shell, called the Oort Cloud, that extends as much as a light-year from the Sun. Because Comets are so tiny, though, no one has ever seen a cometary body inside the Oort Cloud.
Billions more of these icy bodies orbit the Sun in the Kuiper Belt, which begins just beyond the orbit of Neptune. Astronomers have discovered several dozen large, icy bodies orbiting beyond Neptune, and perhaps a couple of dozen smaller ones.
English Astronomer Edmund Halley was the first person to suggest that Comets are members of our solar system. Halley thought that several of the bright Comets recorded long before he was born might really be a single Comet approaching the Sun once every 76 years or so. The Comet was recorded in 1531, 1607, and 1682. Halley predicted the Comet would appear again in 1758. When it did, Halley's theory was proved correct. Comet Halley was named in his honor. It last approached the Sun in 1986, and will return again in 2061.
Several countries sent probes to study Comet Halley in 1986. In particular, the European Giotto Spacecraft imaged the Comet from close range. It found that Halley is about 10 miles (16 km) l ong and five miles (8 km) wide, and is coated with organic molecules that make Halley's surface darker than charcoal. Giotto's images showed "jets" of gas spewing off the comet's surface. These jets can be strong enough to change a comet's orbit. Many Comets have probably slammed into Earth during our planet's history, causing global destruction. In 1994, Comet Shoemaker-Levy 9 rammed into Jupiter. Jupiter's powerful gravity pulled the Comet apart long before it reached Jupiter, so the planet was pelted by almost two dozen impacts, which created Earth-sized scars in Jupiter's cloudtops. Some of these dark markings lasted for several months.
Astronomers hope to learn more about Comets through several spacecraft. Perhaps the most exciting mission is Stardust 1, which will pass through the tail of Comet Wild 2 in 2004. It will catch Comet dust on a sticky paddle and bring the material back to Earth for study. Deep space 1, scheduled for launch in 1998, will visit a different comet. And yet another spacecraft, called 'Contour', will fly past three Comets early in the next century.
ORIGINS OF COMETS.
Comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. asteroids originate via a different process, but very old Comets which have lost all their volatile materials may come to resemble asteroids.
Our Solar System began as a vast cloud of gas and dust. Several billion years ago, this cloud slowly rotated around our very young Sun and particles within the cloud collided with one another. During this time some objects were obliterated by these collisions, while others grew in size and were to later become the planets.
Throughout this early period, Comets probably filled the solar system. Their collisions with the early Planets played a major part in the growth and evolution of each planet. The ices that make up Comets appear to have been the very building blocks that formed the early atmospheres of the planets, and scientists now very strongly believe that it was the collisions of Comets that brought water to our world and enabled life to begin.
Over the years, comets actually became rarer within our solar system. They no longer fill the skies as they did 4 billion years ago, and today a prominent naked-eye Comet can be expected only about once a decade. Astronomers with powerful telescopes can see many more comets, but even in this case it is still rare for as many as 15 or 20 Comets to be detectable in the sky at any one time.
Today, most Comets are located outside our Solar System in part of the original cloud of dust and gas that has remained virtually untouched for billions of years. These regions are referred to as the Oort cloud and the Kuiper Belt.
The Oort cloud was first theorized by the Dutch Astronomer Jan Oort in 1950. His investigation of the orbits of Comets with very long orbital periods brought him to conclude that a large "cloud" of Comets existed far outside the solar system, possibly within the range of 5-8 trillion kilometers (or more) from the sun. The total number of Comets within this belt was estimated as a trillion. It is thought that objects within this cloud are occasionally ejected either by collision with one another, or by the gravitational forces of stars. Many of the ejected objects probably never cross the paths of the planets, and still more do not come close enough to be seen with even the largest telescopes. However, a few do manage to travel into the inner Solar System and are subsequently seen from Earth. This cloud remains a theory only, as it has never been directly detected.
The Kuiper belt is a region first theorized by the Dutch-American Astronomer Gerard Kuiper in 1951. Seeing that Oort's cloud of Comets did not adequately account for the population of Comets with short orbital periods (making complete orbits around the Sun in less than 200 years), Kuiper conjectured that a belt of Comets probably existed outside the orbit of Neptune within the range of 30 to 50 astronomical units (2.8 to 4.6 billion miles) from the sun. Collisions and perturbations by the Planets of our Solar System are believed to be the reasons for the ejection of bodies from this belt.
Around 1988, Astronomers David Jewitt (University of Hawaii) and Jane Luu (University of California at Berkeley) began searching for members of the Kuiper belt using modern electronic cameras attached to a large telescope on Mauna Kea, Hawaii. The equipment was capable of detecting extremely faint objects. After nearly 5 years of systematic searching they found a distinct image on 1992 August 30, which was subsequently designated 1992 QB1. The object was moving very slowly, and calculations eventually revealed the object took 291 years to orbit the sun at an average distance of 43 AU. Since, the discovery of that object over three dozen additional objects had been found as of the end of 1996.
The Oort cloud would have its inner disk at the ecliptic from the Kuiper belt. Although no direct observations have been made of such a cloud, it is believed to be the source of most or all Comets entering the inner Solar System (some short-period Comets may come from the Kuiper belt), based on observations of the orbits of comets.
In 1932 Ernst Opik, an Estonian astronomer, proposed that Comets originate in an orbiting cloud situated at the outermost edge of the solar system.
In 1950 the idea was revived and proposed by Dutch Astronomer Jan Hendrick Oort to explain an apparent contradiction: Comets are destroyed by several passes through the inner solar system, yet if the Comets we observe had existed since the origin of the solar system, all would have been destroyed by now. According to the hypothesis, the Oort cloud contains millions of Comet nuclei, which are stable because the sun's radiation is very weak at their distance.
The cloud provides a continual supply of new comets, replacing those that are destroyed. It is believed that the total mass of Comets in the Oort cloud is many times that of Earth, and estimates range between five and 100 Earth masses.
The Oort cloud is a remnant of the original nebula that collapsed to form the Sun and Planets five billion years ago, and is loosely bound to the solar system.
The most widely-accepted hypothesis of its formation is that the Oort cloud's objects initially formed much closer to the Sun as part of the same process that formed the Planets and asteroids, but that gravitational interaction with young Gas giants such as Jupiter ejected them into extremely long elliptical or parabolic orbits.
This process also served to scatter the objects out of the ecliptic plane, explaining the cloud's spherical distribution. While on the distant outer regions of these orbits, gravitational interaction with nearby stars further modified their orbits to make them more circular.
It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars' Oort clouds may sometimes overlap, causing the occasional intrusion of a Comet into the inner solar system. The star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710.
History of Comet Study.
Historically, Comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh - Revelation and the Book of Enoch as references to comets, or possibly bolides - either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.
In the first book of his Meteorology, Aristotle propounded the view of Comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that Comets were planets, or at least a phenomenon related to the planets, on the grounds that while the Planets confined their motion to the circle of the Zodiac, Comets could appear in any part of the sky.
Instead, he described Comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that Comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other Planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.
However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that Comets must exist outside the earth's atmosphere.
In 1577, a bright Comet was visible for several months. The Danish Astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated observers to determine that the Comet had no measureable parallax. Within the precision of the measurements, this implied the Comet must be at least four times more distant from the Earth than the moon.
Although Comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the Planets moved about the Sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the Planets should also influence the motion of other bodies - he believed that Comets travel among the Planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of Comets moving on straight lines through the upper atmosphere.
The first suggestion that Kepler's laws of planetary motion should also apply to the Comets was made by William Lower in 1610.
In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Jean-Dominique Cassini, all argued for Comets curving about the Sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.
The matter was resolved by the bright Comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months.
In his Principia Mathematica of 1687, Isaac Newton proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the Comet of 1680 as an example.
Newton described Comets as compact, solid, fixed, and durable bodies: in one word, a kind of planets, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry Earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the Comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.
(Earlier, Robert Hooke had identified the Comet of 1664 with that of 1618, [N] while Jean-Dominique Cassini had suspected the identity of the Comets of 1577, 1665, and 1680. Both were incorrect.)
Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.
When the Comet returned as predicted, it became known as Comet Halley or Halley's comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the Comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye.
Since the confirmation of Comet Halley's periodicity, many other periodic Comets have been discovered through the telescope.
The second Comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that Comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 18641866 the Italian Astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between Comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852. A "gravel bank" model of Comet structure arose, according to which Comets consist of loose piles of small rocky objects, coated with an icy layer.
By 1900, seventeen Comets had been observed at more than one perihelion passage and recognized as periodic comets.
As of November 2005, 173 Comets have achieved this distinction, though several have since been destroyed or lost.
The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004.
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