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Royal Greenwich Observatory
Information Leaflet No. 35: 'Comets'
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 the comet's nucleus was the subject of argument amongst astronomers. The passage of the Giotto probe close to the nucleus 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, as it is called, 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.
It is believed that all comets originate from a region far from the Sun,
where there is a reservoir of material left over from the formation of the Solar System.
This region is called the Oort Cloud
and in it there are believed to be many small accumulations of the matter that went into the formation of the planets but which,
because of their great distance from the Sun, never became aggregated into a planet, nor were they dispersed by Solar radiation.
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.
Sometimes during its journey through the inner 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 then 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 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 from the nucleus by the evaporation of the ices by energy from the Sun.
The parent molecules are mainly split up by the energetic radiation from the Sun into simple compounds.
These are not necessarily like the stable chemicals that we know on the Earth, but are simple combinations of atoms.
These are broken down pieces of larger chemicals such as water and organic carbon compounds.
The expelled gas and dust form a roughly spherical ball around the nucleus. 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 ionized by the 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-emissions 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 which is blown away from the coma, is blown by the Sun's radiation and 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 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 and number 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.
Produced by the Information Services Department of the Royal Greenwich Observatory.
PJA Thu Nov 25 10:46:16 GMT, 1993
Updated: March 7 '97, June 26 '14
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For some illustrative images and excellent texts, link to: Comets in Calvin J. Hamilton's Views of the Solar System
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