It is estimated that between ten and a hundred million meteorites enter
our atmosphere and are cremated, every day. Most of them weigh only an
ounce or two, and are invisible. Some of them weigh a ton or more, but
even against these large masses the air acts as a kind of "torpedo-net."
They generally burst into fragments and fall without doing damage.
It is clear that "empty space" is, at least within the limits of our
solar system, full of these things. They swarm like fishes in the seas.
Like the fishes, moreover, they may be either solitary or gregarious.
The solitary bit of cosmic rubbish is the meteorite, which we have just
examined. A "social" group of meteorites is the essential part of a
comet. The nucleus, or bright central part, of the head of a comet (Fig.
19) consists of a swarm, sometimes thousands of miles wide, of these
pieces of iron or stone. This swarm has come under the sun's
gravitational influence, and is forced to travel round it. From some
dark region of space it has moved slowly into our system. It is not then
a comet, for it has no tail. But as the crowded meteors approach the
sun, the speed increases. They give off fine vapour-like matter and the
fierce flood of light from the sun sweeps this vapour out in an
ever-lengthening tail. Whatever way the comet is travelling, the tail
always points away from the sun.