The modern answer to the question “What are Comets made of?” is this:
Probably the heads are a mixture of solid and gaseous matter. The tails
are gaseous—the result of the volatilisation of the solid matter of
the heads.
The spectroscope shows that gases appear to be a constituent of all
Comets. The spectra of Comets are very similar to those of a Bunsen
flame. Recent spectroscopic photographs have revealed the presence of
hydrocarbons, nitro-carbons, of cyanogen and of the vapours of sodium,
iron and other metals.
The connection between Comets and Meteors implies the presence in
Comets of solid matter. A modern theory, voiced by Schiaparelli, is
that meteor showers are broken up Comets.
The tails of Comets appear to be composed of luminous gases ejected
from the head of the Comet through a solar force held to be “Light
Pressure,” which causes these tails to shoot off and disperse into
space at the rate of 865,000 miles an hour.
The length of some Comets’ tails has been estimated at 125,000,000
miles, while the Comets’ heads themselves are generally much larger in
size than our Earth. Halley’s Comet is more than ten-fold the size of
our Earth.
E. W. Maunder, of the Royal Observatory of Greenwich, a modern
astronomer, has thus summarized the latest theories of the substance of
Comets:
“Though the bulk of comets is huge, they contain extraordinarily
little substance. Their heads must contain some solid matter, but it
is probably in the form of a loose aggregation of stones enveloped in
vaporous material. There is some reason to suppose that Comets are apt
to shed some of these stones as they travel along their paths, for the
orbits of the meteors that cause some of our greatest ‘star showers’
are coincident with the paths of Comets that have been observed. But it
is not only by shedding its loose stones that a Comet diminishes its
bulk; it loses also through its tail. As the Comet gets close to the
Sun its head becomes heated, and throws off concentric envelopes, much
of which consists of matter in an extremely fine station of division.”
The orbits of Comets visible to human eyes are all governed by the Sun.
In the words of C. L. Poor: “The attraction of the Sun is to the Comet
like the flame to the moth. The Comet flutters for a moment about the
Sun, and then swings back into outward space. But not unscathed; like
the moth, the Comet has been singed. The fierce light of the Sun has
beaten upon it, and spread out its particles and scattered them along
its path.”
As a comet swings toward and away from the Sun, it travels at a
tremendous rate of speed—over a million miles an hour. The distance
covered from one end of the orbit to the other is 3,370,000,000 miles.
The great majority of Comets appear to travel in parabolas, open curves
leading from infinite space to and around the Sun, and thence back into
infinite space to some other fixed star invisible to us. As a matter
of fact, though, the parabolic curves of Comets’ orbits through the
gravitational attraction of the planets, whose orbits are crossed by
it, may be changed into hyperbolic curves and ellipses by planetary
perturbations. Hence the differences in time between the returns of
certain Comets, like Halley’s, for instance.
[Illustration: RELATIVE SIZES OF THE EARTH, THE MOON’S ORBIT AND
HALLEY’S COMET.]
[Illustration: ORBIT OF HALLEY’S COMET. THE TAIL ALWAYS POINTS AWAY
FROM THE SUN.]
In a general way, it may be said that every Comet comprises a nucleus,
an envelope (called the “coma”) surrounding the nucleus and measuring
from 20,000 to 1,000,000 miles in diameter, and a long tail which
streams behind the nucleus from sixty to a hundred million miles or
more.
Astronomers have decided that the nucleus is probably a heap of
meteorites varying in size from a grain to masses weighing several
tons each; a heap, moreover, so easily sundered that its elements are
distributed gradually along the orbit. It follows that every Comet must
eventually perish unless it restores its nucleus by collecting stray
meteors. That disintegration does occur has been observed time and time
again.
For example, Biela’s Comet, which was discovered in 1826, burst into
two fragments, which drifted apart a distance of one million miles.
Thus it became a twin Comet. Eventually it disappeared as a Comet, and
in its stead we see a shoal of meteors whenever we cross its track
every six and a half years.
It is possible that the Comets of 1668, 1843, 1880, 1882 and 1887, all
travelling in approximately the same path, are fragments of a single
large body which was broken up by the gravitational action of other
bodies in the system, or through violent encounter with the Sun’s
surroundings.
The luminous tail which streams behind the nucleus, which Shakespeare
described so beautifully as “crystal tresses,” is startling, to say the
least. Despite a length which may exceed a hundred million miles, it
is so diaphanously light and subtle that it is difficult to compare it
with any earthly fabric. The air that we breathe is a dense blanket in
comparison. Several hundred cubic miles of the matter composing that
wonderful luminous plume would not outweigh a jarful of air. By reason
of its fairy lightness, it is possible for a tail occupying a volume
thousands of times greater than the sun to sweep through our solar
system without causing any perturbations in planetary movements.
No celestial phenomenon has caused more perplexity than the ghostly
sheaf of light we call a Comet’s tail. In a day, in a few hours even,
the form of that wonderful gossamer may change. Hence it is that
periodic Comets are identified when they return, not by the length and
arch of their tails, but by their orbits. These alone are permanent.
When a Comet is first seen in the telescope, it appears as a diminutive
filmy patch, often unadorned by any tail. As it travels on toward the
Sun, at a speed compared with which a modern rifle bullet would seem to
crawl, violent eruptions occur in the nucleus.
The ejected matter is bent back to form the cloak called the “coma.”
With a nearer approach to the sun, the tail begins to sprout,
increasing in size and brightness as it proceeds. Evidently there is
some connection between the Sun and the tail, something akin to cause
and effect.
When the Comet rushes on toward the Sun, invariably the tail drifts
behind the nucleus like the smoke from a locomotive. But when the Comet
swings around the Sun and travels away from it, a startling change
takes place. The tail no longer trails behind, but projects in front as
if some mighty solar wind were blowing it in advance of the head.
This phenomenon has long been an astronomical riddle. Here was a kind
of matter that refused to obey the laws of gravitation and yield to the
enormous pull of the Sun.
[Illustration: OCTOBER 5. OCTOBER 9. DONATI’S COMET OF 1858.]
It was thought for a time that the tail was flung away from the Sun by
stupendous repelling electrical forces. That electricity plays its part
in the formation of the fairy plume is conceivable, and even probable;
but recently the physicist has discovered a new source of repellent
energy which very plausibly explains the mystery of a Comet’s tail.
This new source of energy is nothing less than the pressure or push of
the Sun’s light. Solar gravitation is a force more powerful than we can
realize. If it were possible for us to live on the Sun, we would find
ourselves pulled down so violently that our body would weigh two tons.
Our clothing alone would weigh more than one hundred pounds. Running
would be a very difficult athletic feat. Light-pressure must indeed be
powerful if it can conquer so relentless a force.
Because we have never seen objects torn from our hands by the pressure
of light, it may be inferred that this newly discovered force affects
only bodies that are invisibly small. With the aid of instruments that
feel what our hands can never feel and see what our eyes can never see,
the modern physicist has critically analyzed the radiation that beats
upon the earth from the distant Sun.
Light really does sway infinitely small particles, as was first
experimentally proved by the Russian Lebedev. Two American astronomers,
Nichols and Hull, improved upon his method. They cast the solar
effulgence into mighty mathematical scales and found that the earth
sustains a light-load of no less than 75,000 tons.
Most city-bred people are familiar with the so-called “Sun
Motors”—little mills with black and white wings, enclosed in airtight
vessels, which spin around in “perpetual motion” under the effect of
“Sun Pressure.”
It remained for the broad mind of a Swedish physicist, Svante
Arrhenius, to apply the principle of light-pressure cosmically. He
explained, very simply, that because a Comet’s tail is composed of a
very fine dust it can easily be driven away from the Sun by radiation
pressure.
To understand how it is possible for so immaterial a thing as a sunbeam
to produce so huge an effect, we have only to take a very simple
example.
Assume that you have before you a block of wood weighing one pound. The
block exposes a certain amount of surface to the Sun’s light. Saw the
block in half, and you increase the amount of that surface. Divide each
half again into half, and the exposed surface is further augmented. If
this process of subdivision is carried on far enough, the block will be
reduced to sawdust.
The entire mass of sawdust still weighs one pound; but its surface has
been vastly enlarged. Indeed, the particles of sawdust, individually
considered, may be said to consist of much surface and very little
weight. If it were possible to take each granule of visible sawdust
and subdivide it into invisible particles, a point would be reached
where the pressure of light would exactly counterbalance the pull of
gravitation, so that the particles would remain suspended in space,
perfectly balanced in the scale of opposing cosmic forces.
Finally, if the subdivision be continued beyond this critical point,
the particles will be wrenched away from the grip of gravitation and
hurled out into space by the pressure of light.
So much has been discovered about the particles that compose a Comet’s
tail that the more progressive scientists of our day have accepted this
ingenious theory. Thus it has been decided by them that the delicate
tresses of a Comet are to a large extent composed of fine particles of
dust and soot.
[Illustration: June 26.]
[Illustration: June 28.]
[Illustration: June 30.]
[Illustration: July 1.]
[Illustration: July 6.]
[Illustration: July 8. CHANGES IN THE COMET OF 1863.]
Before we can completely accept the view that light-pressure forms
this train of soot we must ascertain whether the pressure of light is
capable of accounting for the flash-like rapidity with which a Comet’s
tail changes.
A Comet may throw out a tail sixty million miles long in two days. Is
it actually possible for light-pressure to accomplish that astonishing
feat? Arrhenius has computed that 865,000 miles an hour is the speed
of a light-flung particle of one-half the critical diameter. Because
they are only one-eighteenth as large as this particle of critical
diameter, the dust grains in a Comet’s tail would be propelled over the
same 865,000 miles in less than four minutes. It follows that the solar
radiation is amply strong enough to toss out a tail of sixty million
miles in two days.
Photography in the hands of Prof. E. E. Barnard, of the Yerkes
Observatory, has revealed some extraordinary changes in Comets’ tails,
changes which are not apparent to the eye and which cannot be explained
by light-pressure or by solar electrical forces. He has collected a
formidable mass of photographic evidence which seems to show that
there are other influences at work besides the Sun’s radiation, and
that these influences manifest themselves in distorting and breaking a
Comet’s tail. In some Comets of recent years, streams of matter have
been shot out in large angles to the main direction of the tail without
being at all bent by the pressure of light. In Morehouse’s Comet of
1908, tails were repeatedly formed and discarded to drift bodily out
into space and melt away. Sometimes the photographic plate has shown
the tail twisted like a corkscrew and sometimes it has revealed masses
of matter at some distance from the head, where apparently no supply
had reached it. At one time the entire tail of Morehouse’s Comet was
thrown violently forward, a peculiarity so utterly opposed to the laws
of gravitation that Professor Barnard suspects some unknown force at
work in planetary space besides a force which undoubtedly resides
in the Comet itself. If Halley’s Comet serves no other purpose than
to throw light upon this mystery, its return will more than repay
astronomers for all their observatory vigils.
From the fact that the matter is ejected from the head to form the
tail, it would follow that, unless it has the means of rejuvenating
itself, a comet must eventually be disintegrated. Instances of this
fragmentation and, eventual disappearance of a Comet are not wanting in
astronomical annals. It has been stated previously that when Biela’s
Comet appeared in 1846 it became distorted and elongated, that it
eventually split up into two separate bodies, that in 1852 it again
appeared in its double form, and that it has since disappeared.
In a way, Comets may be said to bleed to death. At each return of
Halley’s Comet, future astronomers will find it less brilliant than it
was seventy-six or seventy-seven years before. Some time there will
be no Halley’s Comet left, and the most famous Comet of its kind will
be reduced to a shoal of meteors varying in weight from a few ounces
to several tons and faithfully pursuing the orbit which their parent
traced and retraced century after century.
[Illustration: COGGIA’S COMET, 1874: ON JULY 13.]