Other Worlds / Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries

by Garrett Putman Serviss

We turn to the extraordinary fluctuations in the light of Eros, and the equally extraordinary conclusions drawn from them. While the little asteroid, whose diameter is estimated to be in the neighborhood of twenty or twenty-five miles, was being assiduously watched and photographed during its opposition in the winter of 1900-1901, several observers discovered that its light was variable to the extent of more than a whole magnitude; some said as much as two magnitudes. When it is remembered that an increase of one stellar magnitude means an accession of light in the ratio of 2.5 to 1, and an increase of two magnitudes an accession of 6.25 to 1, the significance of such variations as Eros exhibited becomes immediately apparent. The shortness of the period within which the cycle of changes occurred, about two hours and a half, made the variation more noticeable, and at the same time suggested a ready explanation, viz., that the asteroid was rapidly turning on its axis, a thing, in itself, quite in accordance with the behavior of other celestial bodies and naturally to be expected.

But careful observation showed that there were marked irregularities in the light fluctuations, indicating that Eros either had a very strange distribution of light and dark areas covering its surface, or that instead of being a globular body it was of some extremely irregular shape, so that as it rotated it presented successively larger and smaller reflecting surfaces toward the sun and the earth. One interesting suggestion was that the little planet is in reality double, the two components revolving around their common center of gravity, like a close binary star, and mutually eclipsing one another. But this theory seems hardly competent to explain the very great fluctuation in light, and a better one, probably, is that suggested by Prof. E.C. Pickering, that Eros is shaped something like a dumb-bell.

We can picture such a mass, in imagination, tumbling end over end in its orbit so as to present at one moment the broad sides of both bells, together with their connecting neck, toward the sun, and, at the same time, toward the observer on the earth, and, at another moment, only the end of one of the bells, the other bell and the neck being concealed in shadow. In this way the successive gain and loss of sixfold in the amount of light might be accounted for. Owing to the great distance the real form of the asteroid is imperceptible even with powerful telescopes, but the effect of a change in the amount of reflecting surface presented produces, necessarily, an alternate waxing and waning of the light. As far as the fluctuations are concerned, they might also be explained by supposing that the shape of the asteroid is that of a flat disk, rotating about one of its larger diameters so as to present, alternately, its edge and its broadside to the sun. And, perhaps, in order completely to account for all the observed eccentricities of the light of Eros, the irregularity of form may have to be supplemented by certain assumptions as to the varying reflective capacity of different parts of the misshapen mass.

The invaluable Harvard photographs show that long before Eros was recognized as an asteroid its light variations had been automatically registered on the plates. Some of the plates, Prof. E.C. Pickering says, had had an exposure of an hour or more, and, owing to its motion, Eros had formed a trail on each of these plates, which in some cases showed distinct variations in brightness. Differences in the amount of variation at different times will largely depend upon the position of the earth with respect to the axis of rotation.

Another interesting deduction may be made from the changes that the light of Eros undergoes. We have already remarked that one of the larger asteroids, and the one which appears to the eye as the most brilliant of all, Vesta, has been suspected of variability, but not so extensive as that of Eros. Olbers, at the beginning of the last century, was of the opinion that Vesta's variations were due to its being not a globe but an angular mass. So he was led by a similar phenomenon to precisely the same opinion about Vesta that has lately been put forth concerning Eros. The importance of this coincidence is that it tends to revive a remarkable theory of the origin of the asteroids which has long been in abeyance, and, in the minds of many, perhaps discredited.

This theory, which is due to Olbers, begins with the startling assumption that a planet, perhaps as large as Mars, formerly revolving in an orbit situated between the orbits of Mars and Jupiter, was destroyed by an explosion! Although, at first glance, such a catastrophe may appear too wildly improbable for belief, yet it was not the improbability of a world's blowing up that led to a temporary abandonment of Olbers's bold theory. The great French mathematician Lagrange investigated the explosive force "which would be necessary to detach a fragment of matter from a planet revolving at a given distance from the sun," and published the results in the Connaissance des Temps for 1814.

"Applying his results to the earth, Lagrange found that if the velocity of the detached fragment exceeded that of a cannon ball in the proportion of 121 to 1 the fragment would become a comet with a direct motion; but if the velocity rose in the proportion of 156 to 1 the motion of the comet would be retrograde. If the velocity was less than in either of these cases the fragment would revolve as a planet in an elliptic orbit. For any other planet besides the earth the velocity of explosion corresponding to the different cases would vary in the inverse ratio of the square root of the mean distance. It would therefore manifestly be less as the planet was more distant from the sun. In the case of each of the four smaller planets (only the four asteroids, Ceres, Pallas, Juno, and Vesta, were known at that time), the velocity of explosion indicated by their observed motion would be less than twenty times the velocity of a cannon ball."[6]

Instead, then, of being discredited by its assumption of so strange a catastrophe, Olbers's theory fell into desuetude because of its apparent failure to account for the position of the orbits of many of the asteroids after a large number of those bodies had been discovered. He calculated that the orbits of all the fragments of his exploded planet would have nearly equal mean distances, and a common point of intersection in the heavens, through which every fragment of the original mass would necessarily pass in each revolution. At first the orbits of the asteroids discovered seemed to answer to these conditions, and Olbers was even able to use his theory as a means of predicting the position of yet undetected asteroids. Only Ceres and Pallas had been discovered when he put forth his theory, but when Juno and Vesta were found they fell in with his predictions so well that the theory was generally regarded as being virtually established; while the fluctuations in the light of Vesta, as we have before remarked, led Olbers to assert that that body was of a fragmental shape, thus strongly supporting his explosion hypothesis.

Afterward, when the orbits of many asteroids had been investigated, the soundness of Olbers's theory began to be questioned. The fact that the orbits did not all intersect at a common point could easily be disposed of, as Professor Newcomb has pointed out, by simply placing the date of the explosion sufficiently far back, say millions of years ago, for the secular changes produced by the attraction of the larger planets would effectively mix up the orbits. But when the actual effects of these secular changes were calculated for particular asteroids the result seemed to show that "the orbits could never have intersected unless some of them have in the meantime been altered by the attraction of the small planets on each other. Such an action is not impossible, but it is impossible to determine it, owing to the great number of these bodies and our ignorance of their masses."[7]

Yet the theory has never been entirely thrown out, and now that the discovery of the light fluctuations of Eros lends support to Olbers's assertion of the irregular shape of some of the asteroids, it is very interesting to recall what so high an authority as Professor Young said on the subject before the discovery of Eros: