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    Comets can be spectacular objects seen in the night-time sky. They havebeen associated by the superstitious with disasters and other notablehistorical events. Until the 1986 opposition of Halley's comet, the truenature of a comet's nucleus was the subject of argument amongstastronomers. The passage of the Giotto probe close to the nucleus of Comet
    Halley and the many observations that were carried out worldwide havevastly improved our knowledge of the nature of comets.
    Because comets can be seen so easily, records of the observation of cometscan be traced back over many centuries. It was from a study of thehistorical observations of several comets that Halley, using Newton's newtheory of gravitation, showed that the orbits of several comets around the
    Sun were almost identical. He postulated that they were all the same objectand predicted that it would be seen again at a certain time in the future.
    As we know, Halley's comet did reappear around the predicted date and hasbeen seen since then on each of its journeys in towards the Sun.
    Comets, as seen from the Earth, appear to have some sort of nucleus whichis surrounded by a bright, more or less circular region called the 'coma'from which one or more tails may be seen spreading out away from thedirection to the Sun. These tails when photographed can be seen to bedifferent colours. There is often a filamentary structured tail which isbluish and a series of more amorphous tails which are yellowish. Thesupposed nucleus of the comet is the bright centre of the coma. The comaand the tails develop markedly as the comet gets closer to the Sun withtail lengths sometimes growing as long as 100 million kilometres.
    The Orbits of Comets
    The first computation of cometary orbits was made by Halley, as mentionedabove. Since then the orbits of many hundreds of comets have beendetermined. They almost all fall into two types; periodic orbits, whichtake the form of very eccentric ellipses, and parabolic orbits.
    The orbits of many comets have periods ranging from hundreds of years totens of millions of years, indicating that they spend much of the time faroutside the orbits of Neptune and Pluto. The orbits of the long-periodcomets are not confined to a plane, like the orbits of the planets, andthese comets can appear in any part of the sky. In order to explain theorbits of comets, astronomers have postulated the existence of two groupsof comets on the edges of the solar system:
    The Oort Cloud:

    In 1950, Dutch Astronomer Jan Oort proposed that a large, spherical cloud of comets surrounds the solar system. The Oort Cloud is supposed to be almost 1 light year in radius and could contain up to a trillion small, icy comets. Small perturbations to the very slow motions of these bodies will cause one of them to start its long, slow journey towards the inner solar system under the gravitational pull of the

    Sun. The orbit of such a body will be a parabola with the Sun as its focus. As the comet gets closer to the Sun its velocity increases reaching a maximum at its closest point whereupon is starts its journey back out to the outer reaches of the solar system, never to be seen again. The Oort Cloud has never been observed, only theorised, but its existence would explain the orbits of long period comets, which have orbital periods greater than 200 years.
    Sometimes, during its journey through the solar system, a comet may passclose to one of the major planets. If this encounter is a close one thenthe gravitational pull of the planet will dramatically change the comet'sorbit and can alter the parabolic orbit into a closed, elliptical orbit.
    The comet the becomes a periodic comet with a definite period for itsreturns close to the Sun. Halley's comet is the best known example of sucha comet. The existence of periodic comets, with orbital periods less than
    200 years, led to the proposal of a second source of comets:
    The Kuiper Belt:

    The Oort Cloud does not explain the existence of comets which have orbital periods of 200 years or less. In 1951, astronomer Gerald

    Kuiper suggested that another belt of comets existed beyond the orbit of Neptune, between 30 and 50 astronomical units (4.5 to 7.5 thousand million km) from the Sun. In 1988, a group of astronomers at the

    University of Hawaii and the University of California at Berkeley began searching for Kuiper Belt objects using a 2.2m telescope in

    Hawaii. They discovered the first Kuiper Belt object in 1992.

    Subsequent observations from Hawaii and with the Hubble Space

    Telescope have discovered dozens of icy objects, each a few hundred km in size and with orbital periods of a few hundred years. The Kuiper

    Belt may be composed of comets from the Oort Cloud, which have been deflected into smaller orbits by Jupiter or the other outer planets.
    A few comets have very short period orbits. For example, Comet Encke has aperiod of 3.5 years, the shortest known, which places its orbit inside theorbit of Jupiter. It is generally thought that these inner solar systemcomets originated in the Oort Cloud or the Kuiper Belt but passed closeenough to one of the giant planets to be deflected by its gravitationalpull into a much smaller orbit.
    The Cometary Nucleus
    Until the Giotto probe showed us pictures of the nucleus of comet Halleythere was considerable discussion of the nature of a comet's nucleus. Wenow know that the nucleus is small, about 10-20 kilometres across, isirregular in shape (rather like a peanut), and is almost black. From itjets of gas and dust are forced out by the Sun's radiation. We believe thatunder the black skin there is a solid body composed of ices of variouskinds, including water-ice, dry-ice (made of carbon dioxide), ammonia,methane and many other organic carbon compound ices all mixed together withdust. The dust contains silicates, carbon and carbon compounds.
    The Cometary Coma
    Surrounding the nucleus is the bright coma. This is composed of gas anddust which has been expelled as the Sun evaporates the icy nucleus. Theparent molecules are mainly split up by energetic ultraviolet radiationfrom the Sun into simple compounds. These are not necessarily like stablechemicals that we know on the Earth but are simple combinations of atoms.
    For example, some of the most numerous are CN, C2, OH, C3, H2O + and NH2.
    These are broken down pieces of larger chemicals, such as water (H2O) andorganic carbon compounds. The expelled gas and dust form a roughlyspherical ball around the nucleus. This is many times larger than thenucleus - the coma of a bright comet can be millions of kilometres in size,whereas the nucleus is only 10km or so across. The coma of the Great Cometof 1811 was larger than the Sun.
    The action of the Sun's radiation and the magnetic field associated withthe solar wind remove gas and dust from the coma and it is 'blown' away toform the comet's tail.
    The Tails of a Comet
    The gas which is blown away from the coma is ionised by solar radiation andbecomes electrically charged. It is then affected strongly by the magneticfields associated with the solar wind (a stream of charged particlesexpelled by the Sun). The gas tail is made visible by line-emission fromthe excitation of the gas by the Sun's radiation. This gives the gas tailits characteristic blue colour. The geometric shape of the tail is governedby the magnetic structures in the solar wind but predominantly the gas tailpoints directly away from the direction from the comet to the Sun.
    The dust is blown away from the coma by radiation pressure from thesunlight absorbed by individual dust grains. It moves in a direction whichis governed by the motion of the comet, by the size of the dust particlesand by the speed of ejection from the coma. The dust tail can be complex,multiple and even curved but, in general, will point away from the Sun.
    Sometimes, due to projection effects, part of the dust tail can be seenpointing in a sunward direction. This is just due to the fact that thecomet and the Earth are moving and that part of the tail has been 'leftbehind 'in such a place as to appear to point towards the Sun. The dusttail is yellow because it reflects the Sun's light to us.
    The gas tail can be about 100 million km long while the dust tail is around
    10 million km long. The longest observed tail on record is the Great Cometof 1843, which had a tail that was 250 million km long (greater than thedistance from the Sun to Mars!).
    The Names of Comets
    A comet takes the name of its discoverer, or discoverers. It also has aserial number consisting of the year and a letter designation. In this wayall comets are named uniquely. Halley's comet is one of very few exceptionsto the naming rule. Halley did not discover 'his' comet but has the honourof having his name attached to it because of his pioneering work indetermining the orbits of comets and showing that this comet was periodic.
    Prediction of Comets
    Apart from the periodic comets, whose orbital periods are well known andhence whose returns can be predicted with great accuracy, it is impossibleto predict when comets may be seen in the sky. Most of the brightest andmost spectacular comets have been ones which have appeared only once andhave never been seen again. When a comet is discovered, far from the Sun,it is very difficult to predict how bright it will appear when it comesclose to the Earth and the Sun. Some comets seem to emit a lot of gas anddust and produce long and spectacular tails whereas others only produce asmall amount of gas and dust and have almost no tail at all.
    | Name | Orbital | Perihelion Date | Perihelion |
    | | Period | | Distance |
    | Halley | 76.1 yrs. | 1986-02-09 | 0.587 AU |
    | Encke | 3.30 yrs. | 2003-12-28 | 0.340 AU |
    | d'Arrest | 6.51 yrs. | 2008-08-01 | 1.346 AU |
    | Tempel 1 | 5.51 yrs. | 2005-07-07 | 1.500 AU |
    | Borrelly | 6.86 yrs. | 2001-09-14 | 1.358 AU |
    | Giacobini-Zinner | 6.52 yrs. | 1998-11-21 | 0.996 AU |
    | Grigg-Skjellerup | 5.09 yrs. | 1992-07-22 | 0.989 AU |
    | Crommelin | 27.89 yrs. | 1984-09-01 | 0.743 AU |
    | Honda-Mrkos-Pajdusakova | 5.29 yrs. | 1995-12-25 | 0.528 AU |
    | Wirtanen | 5.46 yrs. | 2013-10-21 | 1.063 AU |
    | Tempel-Tuttle | 32.92 yrs. | 1998-02-28 | 0.982 AU |
    | Schwassmann-Wachmann 3 | 5.36 yrs. | 2006-06-02 | 0.937 AU |
    | Kohoutek | 6.24 yrs. | 1973-12-28 | 1.571 AU |
    | West-Kohoutek-Ikemura | 6.46 yrs. | 2000-06-01 | 1.596 AU |
    | Wild 2 | 6.39 yrs. | 2003-09-25 | 1.583 AU |
    | Chiron | 50.7 yrs. | 1996-02-14 | 8.460 AU |
    | Wilson-Harrington | 4.29 yrs. | 2001-03-26 | 1.000 AU |
    | Hale-Bopp | 4000 yrs. | 1997-03-31 | 0.914 AU |
    | Hyakutake | ~ 40000 yrs. | 1996-05-01 | 0.230 AU |

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