Comets can be spectacularobjects seen in the night-time sky. They have been associated by thesuperstitious with disasters and other notable historical events. Until the1986 opposition of Halley's comet, the true nature of a comet's nucleus was thesubject of argument amongst astronomers. The passage of the Giotto probe closeto the nucleus of Comet Halley and the many observations that were carried outworldwide have vastly improved our knowledge of the nature of comets.
Because comets can be seen soeasily, records of the observation of comets can be traced back over manycenturies. It was from a study of the historical observations of several cometsthat Halley, using Newton's new theory of gravitation, showed that the orbitsof several comets around the Sun were almost identical. He postulated that theywere all the same object and predicted that it would be seen again at a certaintime in the future. As we know, Halley's comet did reappear around thepredicted date and has been seen since then on each of its journeys in towardsthe Sun.
Comets, as seen from the Earth,appear to have some sort of nucleus which is surrounded by a bright, more orless circular region called the ‘coma’ from which one or more tails may be seenspreading out away from the direction to the Sun. These tails when photographedcan be seen to be different colours. There is often a filamentary structuredtail 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 comaand the tails develop markedly as the comet gets closer to the Sun with taillengths sometimes growing as long as 100 million kilometres. />
The Orbits ofComets
The first computation ofcometary orbits was made by Halley, as mentioned above. Since then the orbitsof many hundreds of comets have been determined. They almost all fall into twotypes; periodic orbits, which take the form of very eccentric ellipses, andparabolic orbits.
The orbits of many comets haveperiods ranging from hundreds of years to tens of millions of years, indicatingthat 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 aplane, like the orbits of the planets, and these comets can appear in any partof the sky. In order to explain the orbits of comets, astronomers havepostulated the existence of two groups of comets on the edges of the solarsystem:
The Oort Cloud:
In 1950,Dutch Astronomer Jan Oort proposed that a large, spherical cloud of cometssurrounds the solar system. The Oort Cloud is supposed to be almost 1 lightyear in radius and could contain up to a trillion small, icy comets. Smallperturbations to the very slow motions of these bodies will cause one of them tostart its long, slow journey towards the inner solar system under thegravitational pull of the Sun. The orbit of such a body will be a parabola withthe Sun as its focus. As the comet gets closer to the Sun its velocityincreases reaching a maximum at its closest point whereupon is starts itsjourney back out to the outer reaches of the solar system, never to be seenagain. The Oort Cloud has never been observed, only theorised, but itsexistence would explain the orbits of long period comets, which have orbitalperiods greater than 200 years.
Sometimes, during its journeythrough 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 willdramatically change the comet's orbit and can alter the parabolic orbit into aclosed, elliptical orbit. The comet the becomes a periodic comet with adefinite period for its returns close to the Sun. Halley's comet is the bestknown example of such a comet. The existence of periodic comets, with orbitalperiods less than 200 years, led to the proposal of a second source of comets:
The OortCloud does not explain the existence of comets which have orbital periods of200 years or less. In 1951, astronomer Gerald Kuiper suggested that anotherbelt of comets existed beyond the orbit of Neptune, between 30 and 50 astronomical units (4.5 to 7.5thousand million km) from the Sun. In 1988, a groupof 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 afew hundred km in size and with orbital periods of a few hundred years. TheKuiper Belt may be composed of comets from the Oort Cloud, which have beendeflected into smaller orbits by Jupiter or the other outer planets.
A few comets have very shortperiod orbits. For example, Comet Encke has a period of 3.5 years, the shortestknown, which places its orbit inside the orbit of Jupiter. It is generallythought that these inner solar system comets originated in the Oort Cloud orthe Kuiper Belt but passed close enough to one of the giant planets to bedeflected by its gravitational pull into a much smaller orbit.
/>The Cometary Nucleus
Until the Giotto probe showed us picturesof the nucleus of comet Halley there was considerable discussion of the natureof a comet's nucleus. We now know that the nucleus is small, about 10-20kilometres across, is irregular in shape (rather like a peanut), and is almostblack. From it jets of gas and dust are forced out by the Sun's radiation. Webelieve that under the black skin there is a solid body composed of ices ofvarious kinds, including water-ice, dry-ice (made of carbon dioxide), ammonia,methane and many other organic carbon compound ices all mixed together withdust. The dust contains silicates, carbon and carbon compounds. />
Surrounding the nucleus is thebright coma. This is composed of gas and dust which has been expelled as theSun evaporates the icy nucleus. The parent molecules are mainly split up byenergetic ultraviolet radiation from the Sun into simple compounds. These arenot necessarily like stable chemicals that we know on the Earth but are simplecombinations of atoms. For example, some of the most numerous are CN, C2,OH, C3, H2O+ and NH2. These arebroken down pieces of larger chemicals, such as water (H2O)and organic carbon compounds. The expelled gas and dust form a roughlyspherical 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 thenucleus is only 10km or so across. The coma of the Great Comet of 1811 waslarger than the Sun.
The action of the Sun's radiation and themagnetic field associated with the solar wind remove gas and dust from the comaand it is ‘blown’ away to form the comet's tail. />
The Tails of aComet
The gas which is blown awayfrom the coma is ionised by solar radiation and becomes electrically charged.It is then affected strongly by the magnetic fields associated with the solarwind (a stream of charged particles expelled by the Sun). The gas tail is madevisible 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 ofthe tail is governed by the magnetic structures in the solar wind butpredominantly the gas tail points directly away from the direction from thecomet to the Sun.
The dust is blown away from thecoma by radiation pressure from the sunlight absorbed by individual dustgrains. 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, willpoint away from the Sun. Sometimes, due to projection effects, part of the dusttail can be seen pointing in a sunward direction. This is just due to the factthat 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 dusttail is yellow because it reflects the Sun's light to us.
The gas tail can be about 100 million kmlong while the dust tail is around 10 million km long. The longest observedtail on record is the Great Comet of 1843, which had a tail that was 250million km long (greater than the distance from the Sun to Mars!).
/>The Names of Comets
A comet takes the name of itsdiscoverer, or discoverers. It also has a serial number consisting of the yearand a letter designation. In this way all comets are named uniquely. Halley's cometis 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 hispioneering work in determining the orbits of comets and showing that this cometwas periodic. />
Apart from theperiodic comets, whose orbital periods are well known and hence whose returnscan be predicted with great accuracy, it is impossible to predict when cometsmay be seen in the sky. Most of the brightest and most spectacular comets havebeen ones which have appeared only once and have never been seen again. When acomet is discovered, far from the Sun, it is very difficult to predict howbright it will appear when it comes close to the Earth and the Sun. Some cometsseem to emit a lot of gas and dust and produce long and spectacular tailswhereas others only produce a small amount of gas and dust and have almost notail at all.
Perihelion DistanceHalley 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