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Born in the dark outer reaches of our solar system, comets are lonely wanderers.
For thousands of years, comets have drawn the attention of philosophers and astronomers.
Many ancient peoples from cultures all over the world believed that comets were a sign,
either of good fortune or disaster.
Comets were thought to be fireballs or 'hairy stars' traveling through the upper atmosphere.
That is where they got their name: the word 'comet' comes from a Greek word meaning 'wearing
long hair,' referring to their long, bright tails.
It was Isaac Newton who described comets as solid bodies with thin tails of vapor heated
by the sun, a description supported by modern astronomical observations.
Comets are similar to asteroids, but unlike asteroids which are mostly rock and metal
with small amounts of ice, comets are made of ice, dust, and frozen gases.
Because of this comets are often called 'dirty snowballs.'
While out in deep space they are unremarkable lumps of ice and rock, but as they enter the
inner solar system and come closer to the sun they begin to heat up.
The comet's ice begins to sublimate, or turn directly into gas.
This gas creates a cloud around the comet, making it seem fuzzy.
There are three main parts of a comet: the nucleus, or solid central structure made of
rock, dust, and ice; the coma, or dust and gas forming a thin atmosphere around the nucleus;
and the tails.
Comets generally have two different tails: one is made of gases, the other is made of
dust.
These two tails usually point in different directions.
The gas tail is blown directly away from the sun by the solar wind.
No matter which way the comet is traveling, this gas tail will be pointing away from the
sun.
The second tail is made of the dust released by the comet.
The dust tail usually points back along the comet's path.
A comet's tail may stretch as far as 6 million miles or 10 million kilometers long.
The orbit of a comet is usually elliptical, or oval in shape, and may have a length of
only a few years, or as many as millions of years.
Comets may be classified by the length of their orbits.
Periodic or short-period comets are comets that orbit the sun in less than 200 years,
comets with orbits of 200 years to 1000 years in length are called long-term comets, and
comets that take more than 1000 years to orbit the sun are called near-parabolic comets.
Some comets, called hyperbolic comets, pass by only once and then exit the solar system.
Comets that pass extremely close to the sun are called sungrazing comets.
Some large sungrazers survive to go around the sun over and over again.
Other comets travel so close to the sun that they evaporate or break into fragments and
are destroyed.
Comets are believed to come from two places on the outskirts of the solar system: the
Kuiper Belt, a region past Neptune filled with asteroids, dwarf planets, comets and
more, and the Oort Cloud, a sphere of icy leftovers from the creation of the solar system
nearly a light-year away from the sun.
Scientists believe that short-period comets come from the Kuiper Belt, and long-period
comets come from the Oort Cloud.
Because these comets are made of ancient material and come from so far away, astronomers love
to study them as they come near.
In the past comets could only be studied through ground-based telescopes, but over the last
thirty years spacecraft and probes have been used to visit and observe comets.
In 2014 the European Space Agency's Rosetta space probe sucessfully landed on the surface
of a comet, the first time this had ever been accomplished.
Still, even seen from earth, comets can be spectacular to behold.
Probably the most famous comet in history is Halley's comet.
Halley's comet is named after Edmond Halley, who realized that the three bright comets
seen in 1531, 1607, and 1682 were actually the same comet returning over and over and
sucessfuly predicted that it would be seen again in 1759.
Halley's comet comes back every 75 years and last appeared in 1986, meaning it won't return
until 2061.
In the meantime, comets that are visible to the naked eye pass close enough to see from
Earth roughly once a year, although not many of them are bright enough that they are easy
to see.
If you see something that looks like a 'fuzzy star' with a trailing tail that is visible
for a few nights in a row, you're probably looking at a comet!
I hope you enjoyed learning about comets today.
Goodbye till next time!