THE ASTEROIDS

In the year 1772 Bode of Berlin published the statement of a curiously symmetrical relation existing among the planets of our system. The gist of this relation, known as Bode's law, though it was really discovered by Titius of Wittenberg, may be summed up briefly thus: 'The interval between the orbits of any two planets is about twice as great as the inferior interval, and only half the superior one.' Thus the distance between the orbits of the earth and Venus should, according to Bode's law, be half of that between the earth and Mars, which again should be half of that which separates Mars from the planet next beyond him. Since the discovery of Neptune, this so-called law has broken down, for Neptune is very far within the distance which it requires; but at the time of its promulgation it represented with considerable accuracy the actual relative positions of the planets, with one exception. Between Mars and Jupiter there was a blank which should, according to the law, have been filled by a planet, but to all appearance was not. Noticing this blank in the sequence, Bode ventured to predict that a planet would be found to fill it; and his foresight was not long in being vindicated.

Several continental astronomers formed a kind of planet-hunting society to look out for the missing orb; but their operations were anticipated by the discovery on January 1, 1801, of a small planet which occupied a place closely approximating to that indicated for the missing body by Bode's law. The news of this discovery, made by Piazzi of Palermo in the course of observations for his well-known catalogue of stars, did not reach Bode till March 20, and 'the delay just afforded time for the publication, by a young philosopher of Jena named Hegel, of a "Dissertation" showing, by the clearest light of reason, that the number of the planets could not exceed seven, and exposing the folly of certain devotees of induction who sought a new celestial body merely to fill a gap in a numerical series.'

The remarkable agreement of prediction and discovery roused a considerable amount of interest, though the planet actually found, and named Ceres after the patron-goddess of Sicily, seemed disappointingly small. But before very long Olbers, one of the members of the original planet-hunting society, surprised the astronomical world by the discovery of a second planet which also fulfilled the condition of Bode's law; and by the end of March, 1807, two other planets equally obedient to the required numerical standard were found, the first by Harding, the second by Olbers. Thus a system of four small planets, Ceres, Pallas, Juno, and Vesta, was found to fill that gap in the series which had originally suggested the search. To account for their existence Olbers proposed the theory that they were the fragments of a large planet which had been blown to pieces either by the disruptive action of internal forces or by collision with a comet; and this theory remained in favour for a number of years, though accumulating evidence against it has forced its abandonment.

It was not till 1845 that there was any addition to the number of the asteroids, as they had come to be named. In that year, however, Hencke of Driessen in Prussia, discovered a fifth, which has been named AstrÆa, and in 1847 repeated his success by the discovery of a sixth, Hebe. Since that time there has been a steady flow of discoveries, until at the present time the number known to exist is close upon 700, of which 569 have received permanent numbers as undoubtedly distinct members of the solar system; and this total is being steadily added to year by year, the average annual number of discoveries for the years 1902 to 1905 inclusive, being fifty-two. For a time the search for minor planets was a most laborious business. The planet-hunter had to construct careful maps of all the stars visible in a certain small zone of the ecliptic, and to compare these methodically with the actual face of the sky in the same zone, as revealed by his telescope. Any star seen in the telescope, and not found to be marked upon the chart, became forthwith an object of grave suspicion, and was watched until its motion, or lack of motion, relatively to the other stars either proved or disproved its planetary nature. At present this lengthy and wearisome process has been entirely superseded by the photographic method, in which a minor planet is detected by the fact that, being in motion relatively to the fixed stars, its image will appear upon the plate in the shape of a short line or trail, the images of the fixed stars being round dots. Of course the trail may be due to a planet which has already been discovered; but should there be no known minor planet in the position occupied by the trail, then a new member has been added to the system. Minor-planet hunting has always been a highly specialized branch of astronomy, and a few observers, such as Peters, Watson, Charlois and Palisa, and at present Wolf, have accounted for the great majority of the discoveries.

It was, however, becoming more and more a matter of question what advantage was to be gained by the continuance of the hunt, when a fresh fillip was given to interest by the discovery in 1898 of the anomalous asteroid named Eros. Hitherto no minor planet had been known to have the greater portion of its orbit within that of Mars, though several do cross the red planet's borders; but the mean distance of Eros from the sun proves to be about 135,000,000, while that of Mars is 141,000,000 miles. In addition, the orbit of the new planet is such that at intervals of sixty-seven years it comes within 15,000,000 miles of the earth, or in other words nearer to us than any other celestial body except the moon or a chance comet. It may thus come to afford a means of revising estimates of celestial distances. Eros presents another peculiarity. It has been found by E. von Oppolzer to be variable in a period of two hours thirty-eight minutes; and the theory has been put forward that the planet is double, consisting of two bodies which revolve almost in contact and mutually eclipse one another—in short, that Eros as a planet presents the same phenomenon which we shall find as a characteristic of that type of variable stars known as the Algol type. An explanation, in some respects more simple and satisfactory, is that the variation in light is caused by the different reflective power of various parts of its surface; but the question is still open.

The best results for the sizes of the four asteroids first discovered are those of Barnard, from direct measurements with the Lick telescope in 1894. He found the diameter of Ceres to be 485 miles, that of Pallas 304, those of Vesta and Juno 243 and 118 miles respectively. There appears to be as great diversity in the reflective power of these original members of the group as in their diameters. Ceres is large and dull, and, in Miss Clerke's words, 'must be composed of rugged and sombre rock, unclothed probably by any vestige of air,' while Vesta has a surface which reflects light with four times the intensity of that of Ceres, and is, in fact, almost as brilliantly white as newly fallen snow.

In the place of Olbers' discredited hypothesis of an exploded planet, has now been set the theory first suggested by Kirkwood, that instead of having in the asteroids the remnants of a world which has become defunct, we have the materials of one which was never allowed to form, the overwhelming power of Jupiter's attraction having exerted a disruptive influence over them while their formation was still only beginning.

So far as I am aware, they share with Mars the distinction of being the only celestial bodies which have been made the subjects of a testamentary disposition. In the case of Mars, readers may remember that some years ago a French lady left a large sum of money to be given to the individual who should first succeed in establishing communication with the Planet of War; in that of the asteroids, the late Professor Watson, a mighty hunter of minor planets in his day, made provision for the supervision of the twenty-two planets captured by him, lest any of them should get lost, stolen, or strayed.

Small telescopes are, of course, quite impotent to deal with such diminutive bodies as the asteroids; nor, perhaps, is it desirable that the ranks of the minor-planet hunters should be reinforced to any extent.