Throughout history, people have
been both awed and alarmed by comets, stars with "long
hair" that appeared in the sky unannounced and unpredictably.
We now know that comets are dirty-ice leftovers from
the formation of our solar system around 4.6 billion
years ago. They are among the least-changed objects
in our solar system and, as such, may yield important
clues about the formation of our solar system. We
can predict the orbits of many of them, but not all.
Around a dozen "new" comets are discovered each year.
Short-period comets are more predictable because they
take less than 200 years to orbit the Sun. Most come
from a region of icy bodies beyond the orbit of Neptune.
These icy bodies are variously called Kuiper Belt
Objects, Edgeworth-Kuiper Belt Objects, or trans-Neptunian
objects. Less predictable are long-period comets,
many of which arrive from a distant region called
the Oort cloud about 100,000 astronomical units (that
is, 100,000 times the mean distance between Earth
and the Sun) from the Sun. These comets can take as
long as 30 million years to complete one trip around
the Sun. (It takes Earth only 1 year to orbit the
Sun.) As many as a trillion comets may reside in the
Oort cloud, orbiting the Sun near the edge of the
Sun's gravitational influence.
Each comet has only a tiny solid part, called a nucleus,
often no bigger than a few kilometers across. The
nucleus contains icy chunks and frozen gases with
bits of embedded rock and dust. At its center, the
nucleus may have a small, rocky core.
As a comet nears the Sun, it begins
to warm up. The comet gets bright enough to see from
Earth while its atmosphere - the coma - grows larger.
The Sun's heat causes ice on the comet's surface to
change to gases, which fluoresce like a neon sign.
"Vents" on the Sun-warmed side may release fountains
of dust and gas for tens of thousands of kilometers.
The escaping material forms a coma that may be hundreds
of thousands of kilometers in diameter.
The pressure of sunlight and the flow
of electrically charged particles, called the solar
wind, blow the coma materials away from the Sun, forming
the comet's long, bright tails, which are often seen
separately as straight tails of electrically charged
ions and an arching tail of dust. The tails of a comet
always point away from the Sun.
Most comets travel a safe distance from
the Sun itself. Comet Halley comes no closer than
89 million kilometers from the Sun, which is closer
to the Sun than Earth is. However, some comets, called
sun-grazers, crash straight into the Sun or get so
close that they break up and vaporize.
Impacts from comets played a major role
in the evolution of the Earth, primarily during its
early history billions of years ago. Some believe
that they brought water and a variety of organic molecules
to Earth.
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