| Comets can be spectacular objects seen in the | | | | The action of the Sun's radiation and the magnetic field |
| night-time sky. They have been associated by the | | | | associated with the solar wind remove gas and dust |
| superstitious with disasters and other notable historical | | | | from the coma and it is ‘blown’ away to form |
| events. Until the 1986 opposition of Halley's comet, the | | | | the comet's tail. |
| true nature of a comet's nucleus was the subject of | | | | The Tails of a Comet |
| argument amongst astronomers. The passage of the | | | | The gas which is blown away from the coma is |
| Giotto probe close to the nucleus of Comet Halley and | | | | ionised by solar radiation and becomes electrically |
| the many observations that were carried out | | | | charged. It is then affected strongly by the magnetic |
| worldwide have vastly improved our knowledge of the | | | | fields associated with the solar wind (a stream of |
| nature of comets. | | | | charged particles expelled by the Sun). The gas tail is |
| Because comets can be seen so easily, records of | | | | made visible by line-emission from the excitation of the |
| the observation of comets can be traced back over | | | | gas by the Sun's radiation. This gives the gas tail its |
| many centuries. It was from a study of the historical | | | | characteristic blue colour. The geometric shape of the |
| observations of several comets that Halley, using | | | | tail is governed by the magnetic structures in the solar |
| Newton's new theory of gravitation, showed that the | | | | wind but predominantly the gas tail points directly away |
| orbits of several comets around the Sun were almost | | | | from the direction from the comet to the Sun. |
| identical. He postulated that they were all the same | | | | The dust is blown away from the coma by radiation |
| object and predicted that it would be seen again at a | | | | pressure from the sunlight absorbed by individual dust |
| certain time in the future. As we know, Halley's comet | | | | grains. It moves in a direction which is governed by the |
| did reappear around the predicted date and has been | | | | motion of the comet, by the size of the dust particles |
| seen since then on each of its journeys in towards the | | | | and by the speed of ejection from the coma. The dust |
| Sun. | | | | tail can be complex, multiple and even curved but, in |
| Comets, as seen from the Earth, appear to have | | | | general, will point away from the Sun. Sometimes, due |
| some sort of nucleus which is surrounded by a bright, | | | | to projection effects, part of the dust tail can be seen |
| more or less circular region called the ‘coma’ | | | | pointing in a sunward direction. This is just due to the |
| from which one or more tails may be seen spreading | | | | fact that the comet and the Earth are moving and that |
| out away from the direction to the Sun. These tails | | | | part of the tail has been ‘left behind’ in such a |
| when photographed can be seen to be different | | | | place as to appear to point towards the Sun. The dust |
| colours. There is often a filamentary structured tail | | | | tail is yellow because it reflects the Sun's light to us. |
| which is bluish and a series of more amorphous tails | | | | The gas tail can be about 100 million km long while the |
| which are yellowish. The supposed nucleus of the | | | | dust tail is around 10 million km long. The longest |
| comet is the bright centre of the coma. The coma and | | | | observed tail on record is the Great Comet of 1843, |
| the tails develop markedly as the comet gets closer to | | | | which had a tail that was 250 million km long (greater |
| the Sun with tail lengths sometimes growing as long as | | | | than the distance from the Sun to Mars!). |
| 100 million kilometres. | | | | The Names of Comets |
| The Orbits of Comets | | | | A comet takes the name of its discoverer, or |
| The first computation of cometary orbits was made | | | | discoverers. It also has a serial number consisting of |
| by Halley, as mentioned above. Since then the orbits of | | | | the year and a letter designation. In this way all comets |
| many hundreds of comets have been determined. | | | | are named uniquely. Halley's comet is one of very few |
| They almost all fall into two types; periodic orbits, which | | | | exceptions to the naming rule. Halley did not discover |
| take the form of very eccentric ellipses, and parabolic | | | | ‘his’ comet but has the honour of having his |
| orbits. | | | | name attached to it because of his pioneering work in |
| The orbits of many comets have periods ranging from | | | | determining the orbits of comets and showing that this |
| hundreds of years to tens of millions of years, | | | | comet was periodic. |
| indicating that they spend much of the time far outside | | | | Prediction of Comets |
| the orbits of Neptune and Pluto. The orbits of the | | | | Apart from the periodic comets, whose orbital periods |
| long-period comets are not confined to a plane, like the | | | | are well known and hence whose returns can be |
| orbits of the planets, and these comets can appear in | | | | predicted with great accuracy, it is impossible to predict |
| any part of the sky. In order to explain the orbits of | | | | when comets may be seen in the sky. Most of the |
| comets, astronomers have postulated the existence | | | | brightest and most spectacular comets have been |
| of two groups of comets on the edges of the solar | | | | ones which have appeared only once and have never |
| system: | | | | been seen again. When a comet is discovered, far |
| The Oort Cloud: | | | | from the Sun, it is very difficult to predict how bright it |
| In 1950, Dutch Astronomer Jan Oort proposed that a | | | | will appear when it comes close to the Earth and the |
| large, spherical cloud of comets surrounds the solar | | | | Sun. Some comets seem to emit a lot of gas and dust |
| system. The Oort Cloud is supposed to be almost 1 | | | | and produce long and spectacular tails whereas others |
| light year in radius and could contain up to a trillion small, | | | | only produce a small amount of gas and dust and |
| icy comets. Small perturbations to the very slow | | | | have almost no tail at all. |
| motions of these bodies will cause one of them to | | | | Name |
| start its long, slow journey towards the inner solar | | | | Orbital Period |
| system under the gravitational pull of the Sun. The orbit | | | | Perihelion Date |
| of such a body will be a parabola with the Sun as its | | | | Perihelion Distance |
| focus. As the comet gets closer to the Sun its velocity | | | | Halley |
| increases reaching a maximum at its closest point | | | | 76.1 yrs. |
| whereupon is starts its journey back out to the outer | | | | 1986-02-09 |
| reaches of the solar system, never to be seen again. | | | | 0.587 AU |
| The Oort Cloud has never been observed, only | | | | Encke |
| theorised, but its existence would explain the orbits of | | | | 3.30 yrs. |
| long period comets, which have orbital periods greater | | | | 2003-12-28 |
| than 200 years. | | | | 0.340 AUd'Arrest |
| Sometimes, during its journey through the solar system, | | | | 6.51 yrs. |
| a comet may pass close to one of the major planets. | | | | 2008-08-01 |
| If this encounter is a close one then the gravitational | | | | 1.346 AU |
| pull of the planet will dramatically change the comet's | | | | Tempel 1 |
| orbit and can alter the parabolic orbit into a closed, | | | | 5.51 yrs. |
| elliptical orbit. The comet the becomes a periodic | | | | 2005-07-07 |
| comet with a definite period for its returns close to the | | | | 1.500 AU |
| Sun. Halley's comet is the best known example of | | | | Borrelly |
| such a comet. The existence of periodic comets, with | | | | 6.86 yrs. |
| orbital periods less than 200 years, led to the proposal | | | | 2001-09-14 |
| of a second source of comets: | | | | 1.358 AU |
| The Kuiper Belt: | | | | Giacobini-Zinner |
| The Oort Cloud does not explain the existence of | | | | 6.52 yrs. |
| comets which have orbital periods of 200 years or | | | | 1998-11-21 |
| less. In 1951, astronomer Gerald Kuiper suggested that | | | | 0.996 AU |
| another belt of comets existed beyond the orbit of | | | | Grigg-Skjellerup |
| Neptune, between 30 and 50 astronomical units (4.5 to | | | | 5.09 yrs. |
| 7.5 thousand million km) from the Sun. In 1988, a group | | | | 1992-07-22 |
| of astronomers at the University of Hawaii and the | | | | 0.989 AU |
| University of California at Berkeley began searching | | | | Crommelin |
| for Kuiper Belt objects using a 2.2m telescope in | | | | 27.89 yrs. |
| Hawaii. They discovered the first Kuiper Belt object in | | | | 1984-09-01 |
| 1992. Subsequent observations from Hawaii and with | | | | 0.743 AU |
| the Hubble Space Telescope have discovered dozens | | | | Honda-Mrkos-Pajdusakova |
| of icy objects, each a few hundred km in size and | | | | 5.29 yrs. |
| with orbital periods of a few hundred years. The | | | | 1995-12-25 |
| Kuiper Belt may be composed of comets from the | | | | 0.528 AU |
| Oort Cloud, which have been deflected into smaller | | | | Wirtanen |
| orbits by Jupiter or the other outer planets. | | | | 5.46 yrs. |
| A few comets have very short period orbits. For | | | | 2013-10-21 |
| example, Comet Encke has a period of 3.5 years, the | | | | 1.063 AU |
| shortest known, which places its orbit inside the orbit of | | | | Tempel-Tuttle |
| Jupiter. It is generally thought that these inner solar | | | | 32.92 yrs. |
| system comets originated in the Oort Cloud or the | | | | 1998-02-28 |
| Kuiper Belt but passed close enough to one of the | | | | 0.982 AU |
| giant planets to be deflected by its gravitational pull into | | | | Schwassmann-Wachmann 3 |
| a much smaller orbit. | | | | 5.36 yrs. |
| The Cometary Nucleus | | | | 2006-06-02 |
| Until the Giotto probe showed us pictures of the | | | | 0.937 AU |
| nucleus of comet Halley there was considerable | | | | Kohoutek |
| discussion of the nature of a comet's nucleus. We | | | | 6.24 yrs. |
| now know that the nucleus is small, about 10-20 | | | | 1973-12-28 |
| kilometres across, is irregular in shape (rather like a | | | | 1.571 AU |
| peanut), and is almost black. From it jets of gas and | | | | West-Kohoutek-Ikemura |
| dust are forced out by the Sun's radiation. We believe | | | | 6.46 yrs. |
| that under the black skin there is a solid body | | | | 2000-06-01 |
| composed of ices of various kinds, including water-ice, | | | | 1.596 AU |
| dry-ice (made of carbon dioxide), ammonia, methane | | | | Wild 2 |
| and many other organic carbon compound ices all | | | | 6.39 yrs. |
| mixed together with dust. The dust contains silicates, | | | | 2003-09-25 |
| carbon and carbon compounds. | | | | 1.583 AU |
| The Cometary Coma | | | | Chiron |
| Surrounding the nucleus is the bright coma. This is | | | | 50.7 yrs. |
| composed of gas and dust which has been expelled | | | | 1996-02-14 |
| as the Sun evaporates the icy nucleus. The parent | | | | 8.460 AU |
| molecules are mainly split up by energetic ultraviolet | | | | Wilson-Harrington |
| radiation from the Sun into simple compounds. These | | | | 4.29 yrs. |
| are not necessarily like stable chemicals that we know | | | | 2001-03-26 |
| on the Earth but are simple combinations of atoms. For | | | | 1.000 AU |
| example, some of the most numerous are CN, C2, OH, | | | | Hale-Bopp |
| C3, H2O+ and NH2. These are broken down pieces of | | | | 4000 yrs. |
| larger chemicals, such as water (H2O) and organic | | | | 1997-03-31 |
| carbon compounds. The expelled gas and dust form a | | | | 0.914 AU |
| roughly spherical ball around the nucleus. This is many | | | | Hyakutake |
| times larger than the nucleus - the coma of a bright | | | | ~40000 yrs. |
| comet can be millions of kilometres in size, whereas | | | | 1996-05-01 |
| the nucleus is only 10km or so across. The coma of | | | | 0. |
| the Great Comet of 1811 was larger than the Sun. | | | | |