COMETS


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    ABOUT COMETS


    A comet is basically a ball of ice and dust that 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.

    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.

    This is a drawing of the region surrounding the nucleus of comet Hale-Bopp on 1997 March 10. This shows an intense emission or jet coming from the nucleus which is forming features called "hoods" or arcs within the coma. The hoods are features formed as a result of the rotation of the nucleus. In the case of Hale-Bopp, a new hood was formed nearly every 12 hours.

    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.


    THE COMA


    Comets can typically display a coma several thousand kilometers in diameter, with the size being dependent on the comet's distance from the sun and the size of the nucleus. The latter is important because since jets generally spring up on the side of the nucleus facing the sun (that side gets warmest), and since large nuclei have a greater surface area facing the sun, then there is the potential for larger numbers of jets and greater amounts of gas and dust feeding the coma.

    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.


    THE TAIL


    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.


    ELLIPSES ~ PERIHELION

    Comets travel around the sun in paths called 'ellipses'.

    Comets have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

    While most of the planets' orbits are near-circles, the orbits of most comets are extremely elongated ellipses. That point on the comet's orbit that is closest to the sun is called "perihelion."

    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.

    As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus.

    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


    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.

  • Comet's light reveals its birth - BBC News - November 2001



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