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 |