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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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