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]
[Footnote 6: Grant's History of Physical Astronomy, p. 241.]
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]
[Footnote 7: Popular Astronomy, by Simon Newcomb, p. 335.]
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:
"It is true, as has often been urged, that this theory in its original form, as presented by Olbers, can not be correct. No _single_ explosion of a planet could give rise to the present assemblage of orbits, nor is it possible that even the perturbations of Jupiter could have converted a set of orbits originally all crossing at one point (the point of explosion) into the present tangle. The smaller orbits are so small that, however turned about, they lie wholly inside the larger and can not be made to intersect them. If, however, we admit a _series_ of explosions, this difficulty is removed; and if we grant an explosion at all, there seems to be nothing improbable in the hypothesis that the fragments formed by the bursting of the parent mass would carry away within themselves the same forces and reactions which caused the original bursting, so that they themselves would be likely enough to explode at some time in their later history."[8]
[Footnote 8: General Astronomy, by Charles A. Young. Revised edition, 1898, p. 372.]
The rival theory of the origin of the asteroids is that which assumes that the planetary ring originally left off from the contracting solar nebula between the orbits of Mars and Jupiter was so violently perturbed by the attraction of the latter planet that, instead of being shaped into a single globe, it was broken up into many fragments. Either hypothesis presents an attractive picture; but that which presupposes the bursting asunder of a large planet, which might at least have borne the germs of life, and the subsequent shattering of its parts into smaller fragments, like the secondary explosions of the pieces of a pyrotechnic bomb, certainly is by far the more impressive in its appeal to the imagination, and would seem to offer excellent material for some of the extra-terrestrial romances now so popular. It is a startling thought that a world can possibly carry within itself, like a dynamite cartridge, the means of its own disruption; but the idea does not appear so extremely improbable when we recall the evidence of collisions or explosions, happening on a tremendous scale, in the case of new or temporary stars.[9]
[Footnote 9: "Since the discovery of Eros, the extraordinary position of its orbit has led to the suggestion that possibly Mars itself, instead of being regarded as primarily a major planet, belonging to the terrestrial group, ought rather to be considered as the greatest of the asteroids, and a part of the original body from which the asteroidal system was formed."--J. Bauschinger, Astronomische Nachrichten, No.
3542.]
Coming to the question of life upon the asteroids, it seems clear that they must be excluded from the list of habitable worlds, whatever we may choose to think of the possible habitability of the original planet through whose destruction they may have come into existence. The largest of them possesses a force of gravity far too slight to enable it to retain any of the gases or vapors that are recognized as constituting an atmosphere. But they afford a captivating field for speculation, which need not be altogether avoided, for it offers some graphic illustrations of the law of gravitation. A few years ago I wrote, for the entertainment of an audience which preferred to meet science attired in a garb woven largely from the strands of fancy, an account of some of the peculiarities of such minute globes as the asteroids, which I reproduce here because it gives, perhaps, a livelier picture of those little bodies, from the point of view of ordinary human interest, than could be presented in any other way.
A WAIF OF SPACE
One night as I was waiting, watch in hand, for an occultation, and striving hard to keep awake, for it had been a hot and exhausting summer's day, while my wife--we were then in our honeymoon--sat sympathetically by my side, I suddenly found myself withdrawn from the telescope, and standing in a place that appeared entirely strange. It was a very smooth bit of ground, and, to my surprise, there was no horizon in sight; that is to say, the surface of the ground disappeared on all sides at a short distance off, and beyond nothing but sky was visible. I thought I must be on the top of a stupendous mountain, and yet I was puzzled to understand how the face of the earth could be so far withdrawn. Presently I became aware that there was some one by me whom I could not see.
"You are not on a mountain," my companion said, and as he spoke a cold shiver ran along my back-bone; "you are on an asteroid, one of those miniature planets, as you astronomers call them, and of which you have discovered several hundred revolving between the orbits of Mars and Jupiter. This is the little globe that you have glimpsed occasionally with your telescope, and that you, or some of your fellows, have been kind enough to name Menippe."
Then I perceived that my companion, whose address had hardly been reassuring, was a gigantic inhabitant of the little planet, towering up to a height of three quarters of a mile. For a moment I was highly amused, standing by his foot, which swelled up like a hill, and straining my neck backward to get a look up along the precipice of his leg, which, curiously enough, I observed was clothed in rough homespun, the woolly knots of the cloth appearing of tremendous size, while it bagged at the knee like any terrestrial trousers' leg. His great head and face I could see far above me, as it were, in the clouds. Yet I was not at all astonished.
"This is all right," I said to myself. "Of course on Menippe the people must be as large as this, for the little planet is only a dozen miles in diameter, and the force of gravity is consequently so small that a man without loss of activity, or inconvenience, can grow three quarters of a mile tall."
Suddenly an idea occurred to me. "Just to think what a jump I can make!
Why, only the other day I was figuring it out that a man could easily jump a thousand feet high from the surface of Menippe, and now here I actually am on Menippe. I'll jump."
The sensation of that glorious rise skyward was delightful beyond expression. My legs seemed to have become as powerful as the engines of a transatlantic liner, and with one spring I rose smoothly and swiftly, and as straight as an arrow, surmounting the giant's foot, passing his knee and attaining nearly to the level of his hip. Then I felt that the momentum of my leap was exhausted, and despite my efforts I slowly turned head downward, glancing in affright at the ground a quarter of a mile below me, on which I expected to be dashed to pieces. But a moment's thought convinced me that I should get no hurt, for with so slight a force of gravity it would be more like floating than falling.
Just then the Menippean caught me with his monstrous hand and lifted me to the level of his face.
"I should like to know," I said, "how you manage to live up here; you are so large and your planet is so little."
"Now, you are altogether too inquisitive," replied the giant. "You go!"
He stooped down, placed me on the toe of his boot, and drew back his foot to kick me off.
It flashed into my mind that my situation had now become very serious. I knew well what the effects of the small attractive force of these diminutive planets must be, for I had often amused myself with calculations about them. In this moment of peril I did not forget my mathematics. It was clear that if the giant propelled me with sufficient velocity I should be shot into space, never to return. How great would that velocity have to be? My mind worked like lightning on this problem.
The diameter of Menippe I knew did not exceed twelve miles. Its mean density, as near as I could judge, was about the same as that of the earth. Its attraction must therefore be as its radius, or nearly 660 times less than that of the earth. A well-known formula enables us to compute the velocity a body would acquire in falling from an infinite distance to the earth or any other planet whose size and force of gravity are known. The same formula, taken in the opposite sense, of course, shows how fast a body must start from a planet in order that it may be freed from its control. The formula is V = square root of (2 gr.), in which "g" is the acceleration of gravity, equal for the earth to 32 feet in a second, and "r" is the radius of the attracting body. On Menippe I knew "g" must equal about one twentieth of a foot, and "r"
31,680 feet. Like a flash I applied the formula while the giant's muscles were yet tightening for the kick: 31,680 1/20 2 = 3,168, the square root of which is a fraction more than 56. Fifty-six feet in a second, then, was the critical velocity with which I must be kicked off in order that I might never return. I perceived at once that the giant would be able to accomplish it. I turned and shouted up at him:
"Hold on, I have something to say to you!"
I dimly saw his mountainous face puckered into mighty wrinkles, out of which his eyes glared fiercely, and the next moment I was sailing into space. I could no more have kept a balance than the earth can stand still upon its axis. I had become a small planet myself, and, like all planets, I rotated. Yet the motion did not dizzy me, and soon I became intensely interested in the panorama of creation that was spread around me. For some time, whenever my face was turned toward the little globe of Menippe, I saw the giant, partly in profile against the sky, with his back bent and his hands upon his knees, watching me with an occasional approving nod of his big head. He looked so funny standing there on his little seven-by-nine world, like a clown on a performing ball, that, despite my terrible situation, I shook my sides with laughter. There was no echo in the profundity of empty space.
Soon Menippe dwindled to a point, and I saw her inhospitable inhabitant no more. Then I watched the sun and the blazing firmament around, for there was at the same time broad day and midnight for me. The sunlight, being no longer diffused by an atmosphere, did not conceal the face of the sky, and I could see the stars shining close to the orb of day. I recognized the various planets much more easily than I had been accustomed to do, and, with a twinge at my heart, saw the earth traveling along in its distant orbit, splendid in the sunshine. I thought of my wife sitting alone by the telescope in the darkness and silence, wondering what had become of me. I asked myself, "How in the world can I ever get back there again?" Then I smiled to think of the ridiculous figure I cut, out here in space, exposed to the eyes of the universe, a rotating, gyrating, circumambulating astronomer, an animated teetotum lost in the sky. I saw no reason to hope that I should not go on thus forever, revolving around the sun until my bones, whitening among the stars, might be revealed to the superlative powers of some future telescope, and become a subject of absorbing interest, the topic of many a learned paper for the astronomers of a future age.
Afterward I was comforted by the reflection that in airless space, although I might die and my body become desiccated, yet there could be no real decay; even my garments would probably last forever. The _savants_, after all, should never speculate on my bones.
I saw the ruddy disk of Mars, and the glinting of his icy poles, as the beautiful planet rolled far below me. "If I could only get there," I thought, "I should know what those canals of Schiaparelli are, and even if I could never return to the earth, I should doubtless meet with a warm welcome among the Martians. What a lion I should be!" I looked longingly at the distant planet, the outlines of whose continents and seas appeared most enticing, but when I tried to propel myself in that direction I only kicked against nothingness. I groaned in desperation.
Suddenly something darted by me flying sunward; then another and another. In a minute I was surrounded by strange projectiles. Every instant I expected to be dashed in pieces by them. They sped with the velocity of lightning. Hundreds, thousands of them were all about me. My chance of not being hit was not one in a million, and yet I escaped. The sweat of terror was upon me, but I did not lose my head. "A comet has met me," I said. "These missiles are the meteoric stones of which it is composed." And now I noticed that as they rushed along collisions took place, and flashes of electricity darted from one to another. A pale luminosity dimmed the stars. I did not doubt that, as seen from the earth, the comet was already flinging the splendors of its train upon the bosom of the night.
While I was wondering at my immunity amid such a rain of death-threatening bolts, I became aware that their velocity was sensibly diminishing. This fact I explained by supposing that I was drawn along with them. Notwithstanding the absence of any collision with my body, the overpowering attraction of the whole mass of meteors was overcoming my tangential force and bearing me in their direction. At first I rejoiced at this circumstance, for at any rate the comet would save me from the dreadful fate of becoming an asteroid. A little further reflection, however, showed me that I had gone from the frying-pan into the fire. The direction of my expulsion from Menippe had been such that I had fallen into an orbit that would have carried me around the sun without passing very close to the solar body. Now, being swept along by the comet, whose perihelion probably lay in the immediate neighborhood of the sun, I saw no way of escape from the frightful fate of being broiled alive. Even where I was, the untempered rays of the sun scorched me, and I knew that within two or three hundred thousand miles of the solar surface the heat must be sufficient to melt the hardest rocks. I was aware that experiments with burning-glasses had sufficiently demonstrated that fact.
But perforce I resigned myself to my fate. At any rate it would the sooner be all over. In fact, I almost forgot my awful situation in the interest awakened by the phenomena of the comet. I was in the midst of its very head. I was one of its component particles. I was a meteor among a million millions of others. If I could only get back to the earth, what news could I not carry to Signor Schiaparelli and Mr.
Lockyer and Dr. Bredichin about the composition of comets! But, alas!
the world could never know what I now saw. Nobody on yonder gleaming earth, watching the magnificent advance of this "specter of the skies,"
would ever dream that there was a lost astronomer in its blazing head. I should be burned and rent to pieces amid the terrors of its perihelion passage, and my fragments would be strewn along the comet's orbit, to become, in course of time, particles in a swarm of aerolites. Perchance, through the effects of some unforeseen perturbation, the earth might encounter that swarm. Thus only could I ever return to the bosom of my mother planet. I took a positive pleasure in imagining that one of my calcined bones might eventually flash for a moment, a falling star, in the atmosphere of the earth, leaving its atoms to slowly settle through the air, until finally they rested in the soil from which they had sprung.
From such reflections I was aroused by the approach of the crisis. The head of the comet had become an exceedingly uncomfortable place. The collisions among the meteors were constantly increasing in number and violence. How I escaped destruction I could not comprehend, but in fact I was unconscious of danger from that source. I had become in spirit an actual component of the clashing, roaring mass. Tremendous sparks of electricity, veritable lightning strokes, darted about me in every direction, but I bore a charmed life. As the comet drew in nearer to the sun, under the terrible stress of the solar attraction, the meteors seemed to crowd closer, crashing and grinding together, while the whole mass swayed and shrieked with the uproar of a million tormented devils.
The heat had become terrific. I saw stone and iron melted like snow and dissipated in steam. Stupendous jets of white-hot vapor shot upward, and, driven off by the electrical repulsion of the sun, streamed backward into the tail.
Suddenly I myself became sensible of the awful heat. It seemed without warning to have penetrated my vitals. With a yell I jerked my feet from a boiling rock and flung my arms despairingly over my head.
"You had better be careful," said my wife, "or you'll knock over the telescope."
I rubbed my eyes, shook myself, and rose.
"I must have been dreaming," I said.
"I should think it was a very lively dream," she replied.
I responded after the manner of a young man newly wed.
At this moment the occultation began.
CHAPTER VI
JUPITER, THE GREATEST OF KNOWN WORLDS