The asteroids, or minor planets, are situated almost wholly in the vast
space between Mars and Jupiter. Their orbits are very irregular, both
as to shape and situation; but, so far as is known, only two of them
pass beyond the orbit of Jupiter, and only one has been discovered
which at any point in its journey around the sun comes nearer than the
orbit of Mars.
The minor planets are called by astronomers almost indifferently
asteroids or planetoids. “Asteroids” is probably the name by which they
are most popularly known. But because they are in fact simply little
bodies that revolve about the sun as the planets do, “planetoids” seems
to be more truly descriptive of them, and it is the word I have chosen
to use here.
It was early noted that, except in one instance, the planets seemed
to show in their distance from the sun something like a mathematical
progression. Struck by this appearance, an astronomer named Bode worked
it out into a formula, known ever since as Bode’s law, though the idea
seems to have originated with another astronomer. One almost always
sees it mentioned in any work dealing with this phase of planetary
history, and it is especially interesting because of the part it played
in the discovery of the planetoids. It was as follows: Beginning with
nothing for Mercury, add three for Venus, twice three, or six, for the
earth, twelve for Mars, and continue thus to double the number for each
planet out to and including Saturn. Then to each one of the numbers so
obtained add four, and the numbers resulting will very nearly represent
the relative distances of the planets from the sun. Thus:
0 3 6 12 24 48 96 192 384
4 4 4 4 4 4 4 4 4
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4 7 10 16 28 52 100 196 388
The exception was that at the fifth number, 28, there was no planet to
correspond, and Jupiter was nearly twice as far away from Mars as it
should have been to conform to the law, thus leaving room for another
planet to occupy the allotted position and fill out this very beautiful
progression.
About nine years after this law was set forth Uranus was discovered
circling out in space far beyond Saturn, and was found to conform to
the law in a most satisfactory manner, its distance being approximately
twice that of Saturn. With such close accord between the actual
distances and the prescribed distances of the planets from the sun, and
with the one exception leaving almost exactly the space allotted by
Bode’s law for another planet, astronomers naturally had a very strong
feeling that there must be another planet between Mars and Jupiter.
They accordingly set to work to prove this, if possible, and to find
what had become of this lost member of the planet family, if it ever
existed.
As a result of this work, on January 1, 1801, the first planetoid was
discovered, and in rapid succession many like it were found, until
now many hundreds are known to astronomers. Their discovery seemed at
first almost a certain confirmation of Bode’s law, and the fact that
where one large planet should have been found there proved to be such
a swarm of small ones could be accounted for in no other way than
to suppose that something had happened in the making of the planet.
At any rate, the promulgation of Bode’s law was the direct cause of
the search for the missing planet which led to the discovery of the
planetoids. And this is the only reason why Bode’s law has continued
to be mentioned in the history of the planets. For it was no real
law, it had no scientific foundation, and its conformity to the facts
of the relative distances of the planets was only one of those very
interesting and singular coincidences that startle one for the moment
into thinking that there is some scientific significance in them.
Another example of such a coincidence is in the fact that the mass of
any given planet exceeds the total mass of all the planets of any less
mass than itself.
In less than half a century after the discovery of the first planetoid,
Neptune was discovered at a distance not at all corresponding to that
indicated by Bode’s law. It was not nearly far enough away, and yet,
strangely enough, it was by taking Bode’s law into consideration that
the position was indicated which finally led to the discovery of the
planet. So while Bode’s law has been found to be no law at all, it
is, nevertheless, entitled to some mention because of its having thus
stimulated research that has had such important results.
No really satisfactory and final explanation of the present state of
the planetoids has ever been given. At one time it was suggested that
another planet had originally existed in the space between Mars and
Jupiter, and through some catastrophe had been shattered into the
small bodies that now occupy that space. But this has been shown to be
impossible.
It is now thought probable that in the original nebula the matter
forming the planetoids might have been prevented from condensing
into a planet by the powerful gravitative influence of Jupiter. This
influence, however, was not sufficiently strong to bring them entirely
under his control. Even yet he pulls some of them five or six degrees
out of the path they otherwise would take when they venture within the
limits of his domain; but he does not capture them, so they have been
left to circle around the sun as mere fragments of bodies, with no
force to combine and make a world, no mass to hold an atmosphere, and
with nothing to prevent them from quickly condensing and from radiating
all their heat into space. They are, in the main, just cold, dark,
lifeless rocks and lumps of matter whirling through space in a maze of
interlacing orbits, some of them almost as far from the sun as Jupiter
and some almost as near as Mars--one, indeed, a little nearer than
Mars at certain times--but most of them swarming more thickly about
half-way between Mars and Jupiter, not far from the place that Bode’s
law assigned to a planet.
After the first planetoid was discovered and had been observed for
a few weeks, it was lost and had to be rediscovered by means of
mathematical computation of its orbit. Where this computation showed
that it ought to be, there it was found, on the very last day of the
same year, 1801. Early the next year another body of the same sort was
discovered, two years later another was found, and still three years
later a fourth came into view. These four were the only ones known in
this branch of the solar family for nearly forty years thereafter.
In 1845 another period of discovery commenced, and has ever since
continued, until there are now between six and seven hundred of these
little bodies that have disclosed their right to be known as members
of the sun’s family. It is probable that there may be still many
more of them, since a new one comes to light every now and then on a
photographic plate, and there is no indication of any limit to the
number that may thus appear.
It is likely that about all have been discovered that can be seen even
with a telescope, for a fairly systematic and thorough search has been
made of the heavens for this purpose during the last half-century. This
work has resulted in a continually decreasing number of discoveries,
until this method of search has finally been practically abandoned. But
it not infrequently happens that in photographing the stars a little
trail of light is discovered on the plate, showing that some heavenly
body with sensible motion has been caught on it. And this usually
proves to be a new planetoid. No matter how long a photographic plate
is exposed, the fixed stars imprint themselves on it only as points of
light. When the impression is a little streak of light instead of a
dot, the object is shown to be in motion, and is either a planetoid, a
satellite, or a comet. The fixed stars would make a trail also if the
photographic apparatus were not regulated by clockwork, so as to follow
the star in its apparent daily motion across the skies. The planets
and other bodies in the solar system are sufficiently near to have a
sensible motion in addition to the motion caused by the rotation of the
earth, which is the only motion we have to take into account in dealing
with the aspects of the stars.
The first planetoid discovered was called Ceres, the next one Pallas,
the third Juno, and the fourth Vesta. This pretty custom of naming them
after the gods and goddesses of mythology was continued, with some
variations, until perhaps three hundred had been so christened. But the
number of them became too prodigious; and when so many began to swarm
into view, waiting to be named, the utilitarian method of designating
them simply by numbers in the order of their discovery was adopted. The
only distinguishing feature of so numbering them is that each number
is placed in a little circle. Thus Ceres is ①, Pallas ②, and so
on. Those of them that have any special claim to distinction, however,
are still referred to by their own names, if they have any, in spite of
this most orderly attempt to make them fit for easy reference in a list.
There are so many of the planetoids, and they are so minute, that
even after they have been discovered they are frequently lost again.
Hence it is sometimes uncertain when they register themselves on the
photographic plates whether they are really new to us or have been
known before. In such cases they are named temporarily after the
letters of the alphabet, and, when the alphabet is exhausted, a second
letter is added. Thus A to Z, then AB to AZ, BC to BZ, and so on in a
sort of “round.” Sometimes these combinations of letters become the
fixed designation of a planetoid, as a nickname sometimes clings to a
person. And thus it happens that we sometimes read of one in particular
of these little bodies that is conspicuous for the great eccentricity
of its orbit, called “WD.” The letters are not its initials, but its
nickname. It really has no name other than its number in the list; but
it became famous while it was temporarily designated as “WD,” and thus
it continues to be called.
The aid of a telescope is necessary in order to see the planetoids,
though it is said that Vesta, under very favorable conditions,
sometimes comes within the limit of visibility. She is the brightest of
them all, though not the largest, and her brilliancy is the subject of
much interesting speculation among astronomers, who have not yet been
able to account for it. She seems from her excessive brightness to be
covered with clouds; and yet it is manifestly impossible that so small
a body could have held an atmosphere throughout these long ages, though
clouds presuppose an atmosphere. No doubt, in time this mystery of
Vesta’s brilliancy will be made plain. Bright as she is in proportion
to her size, and even if she sometimes can be seen, one cannot
reasonably expect anything very brilliant to our view from a body not
much more than a hundred miles in diameter, shining by reflected light,
nearly two hundred million miles away.
Ceres, as far as we yet know, is the largest of the planetoids, and
may be something more than four hundred miles in diameter. Juno is
somewhere near the same size. Pallas is about two hundred miles in
diameter, and Vesta about one hundred and eighteen. No doubt, these
four were the first to be discovered, because they are the largest
and so the easiest to be seen. At any rate, no others yet seen exceed
them in size, and some of the more lately discovered are not more
than fifteen or twenty miles in diameter. Many of those discovered by
photography are doubtless even smaller than these, and are, perhaps,
mere meteors in size. The combined mass of all those discovered up
to this time is far smaller than that of any of the large planets,
or even than that of our moon. Their mass cannot, of course, really
be measured, because they are too small to have any perceptible
gravitative effect on other bodies, and mass can only be determined
by the influence of one body on another. But we do know that their
aggregate mass, if it exceeded a certain limit, would show some
disturbing effect on Mars; and, since it does not do this, we know that
all of them taken together would make an extremely insignificant body.
While the planetoids all revolve around the sun in the same manner and
in the same direction as the planets do, yet they are very erratic
in their courses, and do not all keep within the narrow limits of
the zodiac through which--happily for our convenient observation--the
larger bodies travel. The orbits of many of them are extremely
elliptical, while some are almost circles; and their inclination to the
ecliptic varies from almost nothing to nearly fifty degrees. If one
could catch from one side a view of them all together, they would have
much the appearance in space of a flock of swallows, the individuals
darting this way and that, passing above and below one another in such
intricate sweeps and sinuosities that it would be impossible to keep
track of them separately. And yet time has brought these apparently
tangled orbits into such nice adjustment that the little bodies can
continue to cross and recross each other’s paths with no danger of
interference from each other. Such collisions as there may have
been occurred in the very beginning of their careers. Such of them
as came into collision then traveled on together as one body until
accommodation was made for all.
One of the most wide-wandering of these tiny bodies has been named
Eros, after the little god of love, more commonly known as Cupid.
It has a particular interest for us, because of all the heavenly
bodies it at times comes nearer to us than any except the moon and an
occasional comet. At its nearest it is within fourteen million miles of
the earth, which is more than ten million miles nearer than the closest
approach of Venus, the nearest of the large planets.
This little body was thus near us in 1894; but we did not then know
this, for Eros was not discovered until 1898. After its discovery,
however, it was traced back on many photographic plates, and the fact
that it had been in our neighborhood was learned. For untold ages it
has been making these visits to us every thirty-seven years, and we
have known nothing of them. Its next near approach will be in 1931, and
it will continue to come thereafter every thirty-seven years. Now that
we know about them, these visits are not only pleasant to contemplate,
but it is expected that when they occur the planetoid will be of
great scientific value to us in helping to determine more surely and
accurately the exact distance of the sun.
The planetoids, though so minute and of no value as a spectacle, have
been, and still are, very useful little bodies to us in a scientific
way. In addition to furnishing an easy means of measuring the distance
of the sun, they promise to throw some light on various questions of
physics in which the planets, too, are involved. The brilliancy of
Vesta, for instance, which has been mentioned, and the unaccountable
variability in the brightness of some others of them have yet to be
adjusted to known physical laws. Even the extreme eccentricity of some
of their orbits, and the large tilt of some of them to the ecliptic,
may be suggestive in finally solving certain planetary problems, for
these impish little bodies are far from conforming to the regular ways
of the planets, and there is, of course, some mechanical reason for
their apparent waywardness.