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.