THE SPEEDING COMETS

Taranaki Daily News, 23 November 1929, Page 17 (Supplement)

THE SPEEDING COMETS

ARE THEY CAPTURED?

(By

Rev. B. Dudley, F.R.A.S.)

Most comets are inconspicuous bodies requiring a telescope in order to be seen at all, and almost every year at least one of these telescopic objects is visible. Many of them in approaching us are but paying return visits, being periodic comets that enter our neighbourhood at scheduled times; others, following a greatly elongated path, return only after long absences of hundreds, or even thousands of years. Some perhaps never do re-visit the solar regions, their orbits being hyperbolas. The greater number, however, are recognised to be permanent members of the solar system. Perhaps the characteristics and general behaviour of comets may be best understood by taking a typical case. As it approaches from infinity, let us say, the wanderer, a more or less globular body in the distance, looking like an ordinary star, and only distinguished therefrom by its motion among the known stars, puts forth strange-looking projections into space. These are at first directed towards the sun. They then en?fold the head or nucleus, trail behind the head, and, becoming longer, form a tail increasing in width and splendour, and always turned away from the sun. Perhaps there is more than one tail; there may be even several. The magnetic power of the sun’s attraction on the approaching comet is so great that its ever-increasing velocity becomes enormous, attaining, in some cases, the rate of three hundred miles a second as it dashes through the very atmosphere of the sun. As. the appendage always points away from the sun, it must, of course, in rounding him, sweep through space at a much greater velocity than the head, which is nearer, the farthermost limits travelling at a speed that is simply appalling. The sun being passed, the comet retires, still with its tail directed away from the sun (thus appearing to go backwards), into the distance again, losing speed meanwhile as it loses in magnitude and splendour. The tail becomes more and more abbreviated until eventually it disappears altogether. What has become of it? It may be captured by a large planet, very likely, Jupiter, whose pull it is unable to resist. In this case its path may be completely altered so that thereafter its aphelion (or that point in its grbit furthest removed from the sun) will, lie close to that planet. Jupiter, that is to say, has changed the trespasser’s hitherto long orbit into a much shorter one. It is known that ..this huge planet lays claim to some thirty or forty comets. The reason why his captures are so many is because his mass is greater than that of any other member of the solar family, greater, in fact, than that of all the other planets taken together. All the aphelia of these comets are near to Jupiter's orbit. The other major planets, Saturn, Uranus and Neptune, have their own comets, captives to their gravitational influence.

It is further known that three groups of comets return to us. from maximum | distances that are situated far beyond (Neptune’s orbit. Although at present (Neptune is the remotest known among | the planetary bodies, there is no reason j to think that he is'the outermost. And |it is possible that future study of the ■ orbits of these vagrants may lead to the : discovery of . trans-Neptunian worlds. : The sun's gravitational field reaches out I enormously beyond the orbit of NepItune, and he could hold several planets at vastly greater distances. Comets, therefore, that may not have been taken possession of by the known planets may have been captured by these supposed bodies, their aphelia lying along the orbits of such distant spheres. These will, of course, be comets of long period. On the other hand, there are numerous comets of short period, the shortest of which (Encke’s) never gets further away than Jupiter’s orbit, add returns to us every 3 1-3 years. Halley’s comet, whose last adventure into our regions took place in 1910, returns every 76 years. An interesting theory in explanation of the prevalence of comets having greatly elongated orbits, and periods measurable in tens of thousand? of years, over short-period’ comets, comes into view here. It is suggested that those of short period, having made so many visits to the sun, and- thus run frequent risks of disintegration, have already exhausted their gas supplies, and are now mere meteor-swarms without tails or envelopes. The long-period comets, having less often run the gauntlet, last for a greater time. Two comets (Bielas and Brorsen’s) are known to have run out their career. One of these (Bielas) exists no longer as a.comet, having lost its supply of gas. The meteors, however, which once formed its nucleus, as well as those dispersed along its track, are well known to observers as affording the mid-Noveni-ber meteoric display (the “Andromedids,” so called because they appear to come fmm the constellation Andromeda).

The capture theory, it should be stated, is not universally accepted. Dr. Crommcliti, a competent authority, for example, rather inclines to the opinion, put forward many years ago by Richard Proctor, that these rioting bodies have been ejected- from the major planets during tremendous explosions. There are certain features in the planet Jupiter th.’.t lend themselves to the suggestion that huge outbursts have taken place on his surface. This view, it is considered, explains the famous marking known as the “red spot,” as well as the rows of white spots on the Jovian surface.

The head or “coma” of a comet is sometimes of prodigious size, as in the case of Donati’s comet of 1853, the diameter of whose coma was 250,000 miles, or more than thirty times the diameter of the earth. That of the comet of 1811 was at one time no less than a million miles in diameter. The tail of a comet which, as we have seen, does not develop until it has come into the vicinity of the sun, sometimes stretches out nu millions of miles in length. That of the year 1843 was more than 200,000,000 miles long,