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Comets can be spectacular objects seen in the night-time sky. They have

been associated by the superstitious with disasters and other notable

historical events. Until the 1986 opposition of Halley's comet, the true

nature of a comet's nucleus was the subject of argument amongst

astronomers. The passage of the Giotto probe close to the nucleus of Comet

Halley and the many observations that were carried out worldwide have

vastly improved our knowledge of the nature of comets.

Because comets can be seen so easily, records of the observation of comets

can be traced back over many centuries. It was from a study of the

historical observations of several comets that Halley, using Newton's new

theory of gravitation, showed that the orbits of several comets around the

Sun were almost identical. He postulated that they were all the same object

and 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 has

been 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 which

is 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 the

direction to the Sun. These tails when photographed can be seen to be

different colours. There is often a filamentary structured tail which is

bluish and a series of more amorphous tails which are yellowish. The

supposed nucleus of the comet is the bright centre of the coma. The coma

and the tails develop markedly as the comet gets closer to the Sun with

tail lengths sometimes growing as long as 100 million kilometres.

The Orbits of Comets

The first computation of cometary orbits was made by Halley, as mentioned

above. Since then the orbits of many hundreds of comets have been

determined. They almost all fall into two types; periodic orbits, which

take the form of very eccentric ellipses, and parabolic orbits.

The orbits of many comets have periods ranging from hundreds of years to

tens of millions of years, indicating that they spend much of the time far

outside the orbits of Neptune and Pluto. The orbits of the long-period

comets are not confined to a plane, like the orbits of the planets, and

these comets can appear in any part of the sky. In order to explain the

orbits of comets, astronomers have postulated the existence of two groups

of 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 pass

close to one of the major planets. If this encounter is a close one then

the gravitational pull of the planet will dramatically change the comet's

orbit and can alter the parabolic orbit into a closed, elliptical orbit.

The comet the becomes a periodic comet with a definite period for its

returns close to the Sun. Halley's comet is the best known example of such

a 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 a

period of 3.5 years, the shortest known, which places its orbit inside the

orbit of Jupiter. It is generally thought that these inner solar system

comets originated in the Oort Cloud or the Kuiper Belt but passed close

enough to one of the giant planets to be deflected by its gravitational

pull into a much smaller orbit.

The Cometary Nucleus

Until the Giotto probe showed us pictures of the nucleus of comet Halley

there was considerable discussion of the nature of a comet's nucleus. We

now know that the nucleus is small, about 10-20 kilometres across, is

irregular in shape (rather like a peanut), and is almost black. From it

jets of gas and dust are forced out by the Sun's radiation. We believe that

under the black skin there is a solid body composed of ices of various

kinds, including water-ice, dry-ice (made of carbon dioxide), ammonia,

methane and many other organic carbon compound ices all mixed together with

dust. The dust contains silicates, carbon and carbon compounds.

The Cometary Coma

Surrounding the nucleus is the bright coma. This is composed of gas and

dust which has been expelled as the Sun evaporates the icy nucleus. The

parent molecules are mainly split up by energetic ultraviolet radiation

from the Sun into simple compounds. These are not necessarily like stable

chemicals 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) and

organic carbon compounds. The expelled gas and dust form a roughly

spherical ball around the nucleus. This is many times larger than the

nucleus - 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 Comet

of 1811 was larger than the Sun.

The action of the Sun's radiation and the magnetic field associated with

the solar wind remove gas and dust from the coma and it is ‘blown’ away to

form the comet's tail.

The Tails of a Comet

The gas which is blown away from the coma is ionised by solar radiation and

becomes electrically charged. It is then affected strongly by the magnetic

fields associated with the solar wind (a stream of charged particles

expelled by the Sun). The gas tail is made visible by line-emission from

the excitation of the gas by the Sun's radiation. This gives the gas tail

its characteristic blue colour. The geometric shape of the tail is governed

by the magnetic structures in the solar wind but predominantly the gas tail

points directly away from the direction from the comet to the Sun.

The dust is blown away from the coma by radiation pressure from the

sunlight absorbed by individual dust grains. It moves in a direction which

is governed by the motion of the comet, by the size of the dust particles

and 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 seen

pointing in a sunward direction. This is just due to the fact that the

comet and the Earth are moving and that part of the tail has been ‘left

behind’ in such a place as to appear to point towards the Sun. The dust

tail 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 Comet

of 1843, which had a tail that was 250 million km long (greater than the

distance from the Sun to Mars!).

The Names of Comets

A comet takes the name of its discoverer, or discoverers. It also has a

serial number consisting of the year and a letter designation. In this way

all comets are named uniquely. Halley's comet is one of very few exceptions

to the naming rule. Halley did not discover ‘his’ comet but has the honour

of having his name attached to it because of his pioneering work in

determining 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 and

hence whose returns can be predicted with great accuracy, it is impossible

to predict when comets may be seen in the sky. Most of the brightest and

most spectacular comets have been ones which have appeared only once and

have 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 comes

close to the Earth and the Sun. Some comets seem to emit a lot of gas and

dust and produce long and spectacular tails whereas others only produce a

small 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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