Comets have excited interest in men’s minds from time immemorial. There are two classes of comets. First, but less important, are the SHORT-PERIOD COMETS moving in eccentric orbits at planetary distances from the Sun, and of these about a hundred are known. Second, are the far more numerous LONG-PERIOD COMETS, of which almost 500 have been observed through the ages, the periods of which are so long, anything from a thousand years to several million, that there must be many million of them gravitationally bound to the Sun and destined to arrive eventually in the inner reaches of the Solar System to become observable from the Earth. Although comets move under the gravitational influence of the Sun and planets, none has ever been found to produce any disturbance itself, though some have even passed within the satellite system of Jupiter.

Comets are usually discovered by systematic searches, using a low-power ‘comet-seeker’ with a large field-of-view, but discovery occurs from time to time on photographic plates, often taken with some other purpose in mind, and this is becoming more frequent with wider and wider sky surveys for different objects. In a few cases, comets have suddenly come into view close to the Sun during total eclipses, suggesting that many comets are present only in the daylight sky and escape observation altogether. Perhaps the most remarkable cometary discovery of all happened in 1896 when Perrine, at the Lick Observatory, received information stating the latest position of a comet he himself had discovered shortly before but the telegram was garbled and gave the wrong position more than 2° away from the true one. Naturally quite unaware of this, Perrine made observation in the stated place and by an amazing coincidence found an entirely new comet right there in the middle of the eye-piece!

The practice for newly-discovered comets is to assign them the names of their discoverer or discoverers, sometimes with an added parenthetical number if the observer already has other comets to his credit, though in a few cases the names given are those of astronomers that have made special theoretical study of a comet, as for instance Halley and Encke. But it has been found more useful to supplement this nomenclature by labelling comets as successively detected in any year by the letters a, b,c, etc. in turn, with the letter P often added for the return of a periodic comet. Thus 1976e P/d’ Arrest was the fifth comet discovered in 1976 and a return of comet d’ Arrest first found in 1851. Often considerably later, when the precise times of perihelion have been established, all comets coming to perihelion in a given year are re-labelled with a Roman numeral affixed to the year concerned: thus I971b P/Holmes became 19721, while 1971c became 1972X1. This notation has obvious advantages, and its uniqueness of designation can fail only for the rare exceptions of comets that undergo large perturbations into entirely different orbits.

The short-period comets are no more than the minor remnants of former long-period comets that have chanced to pass close to Jupiter and be deflected into their present orbits. The Earth itself passes quite close to the paths of some of these comets, and, as we have mentioned, at the appropriate time of year there is an accompanying meteor shower in which tens to even hundreds of thousands per hour can be seen from any station on Earth. The comet itself may be far away at another part of the orbit. These meteors associated with comets are tiny solid particles – none has ever been known to reach ground level – that have gradually moved away from the main mass of the comet, and they establish that comets when quiescent are nothing more than a vast swarm of widely-separated dust particles. Stars are seen undimmed right through the heads of comets.

At the distances from the Sun at which they move, short-period comets cannot hold themselves together by their own gravitation, and it is through collisions and solar radiation acting on their particles that they become spaced out into meteor streams in a few thousand years. By radio techniques, some meteor streams have been found spread all round an orbit, but with no associated comet remaining. Halley’s comet, of which records exist going back over 2,000 years, has an orbital period ten times the average short-period comet, and so may be that much longer lived than most. It is due to return close to the Sun early in 1986, but detailed studies already made suggest that it is unlikely to be at all a brilliant object as seen from the Earth.

On the other hand, the long-period comets can hold themselves together gravitationally when at great distance from the Sun, even though their masses may be only about a millionth that of the Moon and spread through a volume several times that of the Sun. It is at such distances, thousands of times the Sun-Earth distance, that in accordance with the laws of motion they must spend practically all their time, and this may be hundreds of thousands or even millions of years, before coming back in to the immediate neighbourhood of the Sun.

It is this huge irregularly shaped dust-swarm structure of comets that gives them seemingly most curious properties when they arrive at planetary distances and come under the dominating influences of solar gravitation and also solar radiation which can repel small particles of the order of the wavelength of sunlight in size as well as certain gaseous molecules. Leonardo da Vinci was fascinated by comets, and was moved to write: ‘Why, this comet seems variable in shape, so that at one time it is round, at another long, at another divided into two or three parts, at another united, and sometimes invisible and sometimes become visible again.’ All these phenomena are understandable as a result of the development of a vast irregular swarm of tiny particles (some comets are larger than the Sun even in their observable overall bulk and may in fact have far greater extent still) as it pursues its orbit, and the chance relationship of the position of the Earth to the comet at its most active stages when it is nearest the Sun.

Through inevitable dynamical causes, a comet must turn itself inside out, as it were, on the part of its orbit nearest the Sun, t particles at first moving on one side of the median plane crossing through it in one direction and those to the other side crossing through to meet them as two opposing streams. The relative speeds are sufficiently high that a great deal of gas and much finer dust particles are released as a result of collisions, and then solar radition can come into operation to repel certain of the gases and minute dust particles and bring about the impressive tail-effects so well known. When a comet first appears on the way in, it is usually no more than a large hazy patch of light so dim as to be scarcely distinguishable from the background sky, but as it approaches the Sun it contracts in some of its dimensions, as seen from the Earth, and may even extend in others, and it may in¬crease in brightness far more rapidly than could be accounted for by mere approach towards the Sun. The quality of the light may also change, at first being mainly scattered and reflected sunlight but then beginning also to emit light of its own apparent making. The greatest depths in the line of sight through the comet may often give the appearance of higher condensation of light within the general COMA (the overall visible head of the comet), and occasionally within this condensation there may seem to develop a starlike point of light, which is termed the NUCLEUS. Only a small proportion of comets come to exhibit a nucleus, and then not at all times, while on the other hand a few comets have shown more than one nucleus at times. But it has to be remembered that this is merely the impression that the human eye reports to the brain, and there can be no question of any sizeable object being present within a comet capable of emitting an unresolvable point-source of light.

As to where these comets come from in their millionfold numbers, (there must be these huge numbers to keep up the observed average annual supply), study of their orbits reveals remarkable properties that throw considerable light on the question. They are found to be by no means randomly distributed. For instance, ten so-called SUN-GRAZING COMETS are known that all come in from almost exactly the same direction in space, and in almost identical orbits, apart from their different timing. There are over fifty such COMET GROUPS, as they are termed, though with fewer members. There is found to be a very strong association of these directions from which the long-period comets approach the Sun and the direction of motion of the Sun itself relative to the surrounding stars in the Galaxy. Indeed, it has been known for almost a century that more than 60 per cent of comets have their perihelion-points within no more than 45° of the solar apex, which means only 15 per cent of the whole area of the sky. This is exactly the opposite of what would be expected if comets simply came in, as they are, from interstellar space. (It may be added that careful dynamical calculations inescapably show that comets all come in from finite, • though in some cases, very large distances.) Moreover, the directions on the celestial sphere from which they come are found to be strongly concentrated to the galactic plane in a kind of belt-like distribution. These correlations reinforce the theory that cornets represent dense aggregations of interstellar dust formed by the gravitational focusing action of the Sun as it pursues its orbit in the Galaxy and from time to time finds itself passing through one of the vast gas-and-dust clouds that abound in the Galaxy and lie near the galactic plane. Examination of this mechanism explains exactly why comets move in the highly elongated orbits they do, near-parabolas which bring them in almost directly towards the Sun and then back out again on a nearly parallel path.

Unlike the planetary orbits, which are stable for tens of millions of years at the least, these elongated cometary orbits are highly sensitive to the action of the great planets, Jupiter especially because of its great mass. At each return to the Sun, so weakly bound are the long-period comets, there is a high probability, something like an even chance for a comet selected at random, of its being ejected altogether from the Solar System in a hyperbolic orbit, never to return. This makes it almost certain that comets, by astronomical standards, are recent additions to the Solar System, and not all formed at the same time. Much the same conclusion is also indicated by the fact that at each return to the Sun, a comet loses a small but finite proportion of its mass. Halley’s comet, for example, loses material in tail production, which goes on for a matter of months, and decay to meteors (there is a meteor stream associated with the comet), and if at each approach it lost only a millionth of its mass (almost certainly it loses a good deal more) then going backwards in time to the beginning of the Solar System its initial mass would need to be far greater than that of the Sun!

As a consequence of the highly elongated orbits, even a bright comet (most remain faint telescopic objects) may arrive unexpectedly and leave to become undetectable after a few months. This unpredictability, except for the faint short-period comets, leads to their not receiving the degree of attention by means of large telescopes and professional astronomers as do less erratically moving objects. Discovery of comets is made mainly by amateurs using powerful binoculars or small telescopes with large fields of view. But despite this, comets may well be objects of the highest astronomical interest, for they may represent samples of inter¬stellar material accessibly deposited in the Solar System instead of remaining for ever beyond the reach of man.

The material of different comets may well have quite different ages, and differ from the age of the Solar System; this would be a question of the greatest interest to be able to answer. The material of short-period comets is eventually swept into the Sun as a result of the dissipative action of solar radiation on small particles.

There is a great deal still to be learnt about comets, much of it possibly only by means of space missions to the few that are both predictable and accessible. Finally, since every star in the Universe may have a retinue of comets in number of the same order as o the Sun, comets may have special importance if only as the m numerous class of celestial objects in the Universe.

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