WHERE DO COMETS COME FROM?
Where do comets come from? The answer to this question is not yet fully
made out. Most likely they have not all had a similar origin, and
theories are abundant. Apparently they come into the solar system from
outer space, from any direction whatsoever. The depths of interstellar
space seem to be responsible for most, if not all, of the new ones.
Whether they have come from other stars or stellar systems we cannot
say.
While comets are tremendous in size or volume, their mass or the amount
of real substance in them is relatively very slight. We know this by the
effect they produce on planets that they pass near, or rather by the
effect that they fail to produce. The earth's atmosphere weighs about
one two hundred and fifty thousandth as much as the earth itself, but a
comet's entire mass must be vastly less than this. Even if a comet were
to collide with the earth head on, there is little reason to believe
that dire catastrophe would ensue. At least twice the earth is known to
have passed through the tail of a comet, and the only effect noticed was
upon the comet itself; its orbit had been modified somewhat by the
attraction of the earth. If the comet were a small one, collision with
any of the planets would result in absorption and dissipation of the
comet into vapor.
The whole of a large comet has perhaps as much mass or weight as a
sphere of iron a hundred miles in diameter. Even this could not wreck
the earth, but the effect would depend upon what part of the earth was
hit. A comet is very thin and tenuous, because its relatively small mass
is distributed through a volume so enormous. So it is probable that the
earth's atmosphere could scatter and burn up the invading comet, and we
should have only a shower of meteors on an unprecedented scale.
Diffusion of noxious gases through the atmosphere might vitiate it to
some extent, though probably not enough to cause the extinction of
animal life.
Every comet has an interesting history of its own, almost indeed unique.
One of the smallest comets and the briefest in its period round the sun
is known as Encke's comet. It is a telescopic comet with a very short
tail, its time of revolution is about three and a half years, and it
exhibits a remarkable contraction of volume on approach to the sun.
Biela's comet has a period about twice as long. At one time it passed
within about 15 million miles of the earth, and somewhere about the year
1840 this comet divided into two distinct comets, which traveled for
months side by side, but later separated and both have since completely
disappeared. Perhaps the most beautiful of all comets is that discovered
by Donati of Florence in 1858. Its coma presented the development of
jets and envelopes in remarkable perfection, and its tail was of the
secondary or hydrocarbon type, but accompanied by two faint streamer
tails, nearly tangential to the main tail and of the hydrogen type.
Donati's comet moves in an ellipse of extraordinary length, and it will
not return to the sun for nearly 2,000 years.
The most brilliant comet of the last half century is known as the great
comet of 1882. In a clear sky it could readily be seen at midday. On
September 17 it passed across the disk of the sun and was practically as
bright as the surface of the sun itself. The comet had a multiple
nucleus and a hydrocarbon tail of the second type, nearly a hundred
million miles in length. Doubtless this great comet is a member of what
is known as a cometary group, which consists of comets having the same
orbit and traveling tandem round the sun. The comets of 1668, 1843,
1880, 1882 and 1887 belong to this particular group, and they all pass
within 300,000 miles of the sun's surface, at a maximum velocity
exceeding 300 miles a second. They must therefore invade the regions of
the solar corona, the inference being that the corona as well as the
comet is composed of exceedingly rare matter.
Photography of comets has developed remarkably within recent years,
especially under the deft manipulation of Barnard, whose plates, in
particular during his residence at the Lick Observatory on Mount
Hamilton, California, show the features of cometary heads and tails in
excellent definition. Halley's comet, at the 1910 apparition, was
particularly well photographed at many observatories.
The question is often asked, When will the next comet come? If a large
bright comet is meant, astronomers cannot tell. At almost any time one
may blaze into prominence within only a few days. During the latter half
of the last century, bright comets appeared at perihelion at intervals
of eight years on the average. Several of the lesser and fainter
periodic comets return nearly every year, but they are mostly
telescopic, and are rarely seen except by astronomers who are
particularly interested in observing them.