COLLISION WITH COMET

Evening Star, Issue 22458, 1 October 1936, Page 8

COLLISION WITH COMET

EARTH PROBABLY UNHARMED Bright comets and “ now ” stars arc, from their rare and unheralded appearance, exciting things, both to astronomers and the general public, writes J)r W. H. Steaveuson, F.R.A.S., in the ‘ -Daily Mail.’ This summer we have had one of each, and they have caused much activity in our observatories. It is now fairly well established that a comet is not a single massive body like the earth or moon, but consists of a cloud of small solid particles. The exact size of these particles is not known, but a specimen of ordinarygravel will give a lair idea of the probable range of the dimensions of most of them. This cloud of particles moves, like the planets, under the attraction of the sun, but in a very elongated path. Most planets move in almost circular orbits, but the paths of comets are spindle-shaped, suggesting the outline of the airship Hindeuburg. Comets moving in such paths visit the neighbourhood of the sun again and again; but the paths of others are so much drawn out that we cannot be certain whether they are closed curves or not; the comets may come back in a few thousand years or perhaps not at all. On the average four or five comets are seen every year, but they are mostly small and inconspicuous, being visible only through a telescope and often showing no signs of a tail. But sometimes a large comet passes close to the sun, and when this happens the head glows brightly and throws out an enormous tail like the beam of a searchlight. Comets of this kind appeared in 1843, 1858, 1861, 1882, and 1910, and were really splendid objects, bright enough to be seen in broad daylight. Like nearly all great comets, they came without warning, and if they return at all it will not be for some thousands of years. The only large comet proved to be periodic is that of Halley, which appears regularly every 75 years or so and is generally a conspicuous object when near the sun.

The last return was in 1910, and some who read these lines will doubtless see it at its next visit in 1986, Meanwhile it is always possible that a really bright comet may appear unexpectedly at any time. One often hears queries as to the probable fate of the earth and its inhabitants if an actual collision with a comet should occur.

Briefly, it may be said that, while most comets pass “ above ” or “ below ” the earth’s patli on their way to and from the sun, the paths of some do actually cut into or graze that of our planet. Still, the chances against both bodies arriving at the crossing at the same moment are infinitely great. We have more than once passed through the tail of a- comet without suffering any ill effects. This is hardly surprising when wo remember that the material of the tail is so tenuous that we can see faint stars right through _ its thickness of many thousands of miles. But the effect of a direct encounter with a head of a comet might well be a different matter, and opinions are divided as to the probable result. On the whole it seems most likely that nothing more terrible than a fine shower of meteors or “ shooting stars,” would occur.

On the other hand, if some of the particles of “ gravel ” were of considerable size, some damage to property and life might well be caused over a restricted area of the earth’s surface, as happens during the great hailstorms of the Transvaal.

In any case, the probability of such a collision »is so remote that astronomers are not losing any sleep over it.