THE ASTEROIDS, A FAMILY OF DWARF WORLDS
Beyond Mars, in the broad gap separating the terrestrial from the Jovian
planets, are the asteroids, of which nearly five hundred have been
discovered and designated by individual names or numbers. But any
statement concerning the known number of asteroids can remain valid for
but a short time, because new ones are continually found, especially by
the aid of photography. Very few of the asteroids are of measurable
size. Among these are the four that were the first to be
discovered--Ceres, Pallas, Juno, and Vesta. Their diameters, according
to the measurements of Prof. E.E. Barnard, of the Yerkes Observatory,
are as follows: Ceres, 477 miles; Pallas, 304 miles; Juno, 120 miles;
Vesta, 239 miles.
It is only necessary to mention these diameters in order to indicate how
wide is the difference between the asteroids and such planets as the
earth, Venus, or Mars. The entire surface of the largest asteroid,
Ceres, does not equal the republic of Mexico in area. But Ceres itself
is gigantic in comparison with the vast majority of the asteroids, many
of which, it is believed, do not exceed twenty miles in diameter, while
there may be hundreds or thousands of others still smaller--ten miles,
five miles, or perhaps only a few rods, in diameter!
Curiously enough, the asteroid which appears brightest, and which it
would naturally be inferred is the largest, really stands third in the
order of measured size. This is Vesta, whose diameter, according to
Barnard, is only 239 miles. It is estimated that the surface of Vesta
possesses about four times greater light-reflecting power than the
surface of Ceres. Some observations have also shown a variation in the
intensity of the light from Vesta, a most interesting fact, which
becomes still more significant when considered in connection with the
great variability of another most extraordinary member of the asteroidal
family, Eros, which is to be described presently.
The orbits of the asteroids are scattered over a zone about 200,000,000
miles broad. The mean distance from the sun of the nearest asteroid,
Eros, is 135,000,000 miles, and that of the most distant, Thule,
400,000,000 miles. Wide gaps exist in the asteroidal zone where few or
no members of the group are to be found, and Prof. Daniel Kirkwood long
ago demonstrated the influence of Jupiter in producing these gaps.
Almost no asteroids, as he showed, revolve at such a distance from the
sun that their periods of revolution are exactly commensurable with that
of Jupiter. Originally there may have been many thus situated, but the
attraction of the great planet has, in the course of time, swept those
zones clean.
Many of the asteroids have very eccentric orbits, and their orbits are
curiously intermixed, varying widely among themselves, both in
ellipticity and in inclination to the common plane of the solar system.
Considered with reference to the shape and position of its orbit, the
most unique of these little worlds is Eros, which was discovered in 1898
by De Witt, at Berlin, and which, on account of its occasional near
approach to the earth, has lately been utilized in a fresh attempt to
obtain a closer approximation to the true distance of the sun from the
earth. The mean distance of Eros from the sun is 135,000,000 miles, its
greatest distance is 166,000,000 miles, and its least distance
105,000,000 miles. It will thus be seen that, although all the other
asteroids are situated beyond Mars, Eros, at its mean distance, is
nearer to the sun than Mars is. When in aphelion, or at its greatest
distance, Eros is outside of the orbit of Mars, but when in perihelion
it is so much inside of Mars's orbit that it comes surprisingly near the
earth.
Indeed, there are times when Eros is nearer to the earth than any other
celestial body ever gets except the moon--and, it might be added,
except meteors and, by chance, a comet, or a comet's tail. Its least
possible distance from the earth is less than 14,000,000 miles, and it
was nearly as close as that, without anybody knowing or suspecting the
fact, in 1894, four years in advance of its discovery. Yet the fact,
strange as the statement may seem, had been recorded without being
recognized. After De Witt's discovery of Eros in 1898, at a time when it
was by no means as near the earth as it had been some years before,
Prof. E.C. Pickering ascertained that it had several times imprinted its
image on the photographic plates of the Harvard Observatory, with which
pictures of the sky are systematically taken, but had remained
unnoticed, or had been taken for an ordinary star among the thousands of
star images surrounding it. From these telltale plates it was
ascertained that in 1894 it had been in perihelion very near the earth,
and had shone with the brilliance of a seventh-magnitude star.
It will, unfortunately, be a long time before Eros comes quite as near
us as it did on that occasion, when we failed to see it, for its close
approaches to the earth are not frequent. Prof. Solon I. Bailey selects
the oppositions of Eros in 1931 and 1938 as probably the most favorable
that will occur during the first half of the twentieth century.
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]
[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.