Information AboutComet |
| CATEGORIES ABOUT COMET | |
| comets | |
| minor planets | |
|
]] 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 primarily from the effects of solar radiation upon the comet's Nucleus , which itself is a minor body composed of rock, dust, and ice. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or ''perturbed'') by relatively close approaches to the major planets. Some are moved into Sun-grazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever. A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO 12 or visiting the all-in-one resource Dave's Virtual Sungrazer Observatory . Most 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 Gas eous) State . Asteroid s originate via a different process, but very old comets which have lost all their Volatile materials may come to resemble asteroids. The word Comet came to the English Language through Latin ''cometes''. From the Greek word ''komē'', meaning "hair of the head," Aristotle first used the derivation ''komētēs'' to depict comets as "stars with hair." The Astronomical Symbol for comets accordingly consists of a disc with a tail of hair. PHYSICAL CHARACTERISTICS ]] Long-period comets are believed to originate in a distant cloud known as the Oort Cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).3 They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely Elliptical Orbits that can bring them very close to the Sun . One theory holds that as a comet approaches the Inner Solar System , Solar Radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven, due to its distance. The streams of Dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the '' Coma '', and the force exerted on the coma by the Sun's Radiation Pressure and Solar Wind cause an enormous ''tail'' to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, pointed in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the '' Nucleus '') is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 Astronomical Unit (150 million km) or more."Yeomans, Donald K. " Comet World Book Online Reference Center. 2005. Also available from World book @ NASA here . Both the comet and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from Ion isation. Most comets are too faint to be visible without the aid of a Telescope , but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad Omen s of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese Oracle Bone s, it is known that their appearances have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry , which records the Norman Conquest of England in AD 1066.4 Surprisingly, cometary nuclei are among the Dark est objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, Asphalt reflects 7% of the light that falls on it. It is thought that complex Organic Compound s are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like Tar or crude Oil . The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing. In 1996, comets were found to emit fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.6 ORBITAL CHARACTERISTICS (red) and Earth (blue), illustrating the high Eccentricity of the orbit and more rapid motion when closer to the Sun .]] Comets are sometimes classified according to the length of their Orbital Period s. ''Short-period comets'', also called ''periodic comets'', have orbital periods of generally 30 years or less (though some take the very arbitrary figures of 50, 100, or even 200 years), while ''long-period comets'' have longer orbital timespans but remain gravitationally bound to the Sun by definition (those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods"), and Main-belt Comets orbit within the Asteroid Belt .IAU bulletin IB74 ''Single-apparition comets'' have Parabolic or Hyperbolic orbits which will cause them to permanently exit the solar system after passing the Sun once. Early observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude. On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter . Short-period comets are thought to originate in the transneptunian region (called by some the " Kuiper Belt "), whereas the source of long-period comets is thought to be the Oort Cloud . Since their elliptical orbits frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a tendency for their Aphelia to coincide with a Giant Planet 's orbital radius, with the Jupiter family of comets being the largest, as the Histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets. | ||||||
|
||||||
|
|