# Wikijunior:Solar System/Comets

{{/Coolfacts\|= 1\|\|=
2\|80px\|☄.svg "wikilink") Comet
Facts\|= 3\|

-   Comets are often described as giant \"dirty snowballs\" because they
    are mostly made up of ice and some dirt.
-   Comets have two \"tails\", one made up mostly of rocks and dust, the
    other mostly made of gas.
-   Comet tails always point *away* from the
    Sun.

}}

## What is a Comet?

!The Hale Bopp comet
Think of a comet as a big, dirty, gassy snowball. Comets are formed in
the ring of rocks, dust, and ice that orbits the
Sun beyond Pluto called
the Kuiper Belt. Comets form when rocks,
dust, and ice `<i>`{=html}condense`</i>`{=html} -- that is, join
together . As a comet grows larger, it starts to be pulled towards and
around the Sun. Comets in our Solar System usually take many years to go
around the Sun -- from a few dozen years to many thousands of years.
This is because they start to orbit the sun from very far away. They
make long, egg-shaped orbits around the Sun instead of almost circular
ones like the planets.\

## What does a Comet look like?

{{  }} !The strange surface of
Churyumov-Gerasimenko, a comet. Photograph taken by the Rosetta space
craft.
The comets that can be seen in the sky without telescopes are unusual
because they are the biggest and brightest comets of all. You might have
the chance to see one once or twice during your life. Most comets can
only be seen with a telescope. The few that can be seen with human eyes
are usually just hazy streaks or faint smudges in the night sky.

When comets are very far away from the Sun, they are covered in a
coating of icy, black rocks and dust. As a comet approaches the Sun,
however, the ice starts to melt. This creates large amounts of water and
gas that break through the coating, freeing some of the dust and rocks.
Sometimes this water, gas, rocks, and dust can be seen from the Earth as
one or two tails streaming away from the comet. Even when only one tail
can be seen, there are two, one made from the lighter gas and water, and
the other from the rocks, dust, and chunks of ice.

Comets themselves are usually between a few kilometres and several
hundred kilometres in size, but their tails can be several million
kilometres long.

## Seeing Comets in the Sky

The \"great comets\" that produce particularly spectacular tails are
some of the rarest objects in our solar system. Usually they can only be
seen about once every hundred years, so it is very rare to see one of
these comets. The last great comet appeared in 1910, but it may still be
another hundred years before another one comes near the Earth.
Astronomers can\'t predict exactly how or when they will appear as there
are still things about our Solar System that they don\'t understand. If
you hear about a comet coming into the sky soon, follow the instructions
below to watch it!

1.  Find out if the comet is going to be in your area of the sky.
2.  Get a telescope or binoculars and some chairs to view it. Many of
    the largest comets have never even needed a telescope to see them.
3.  Ask your parents to take you to a park, the woods or another dark
    place away from city lights.
4.  Look up into the sky and enjoy this amazing sight.

Usually the dust making up the comet\'s tail is so faint you can\'t see
it. However, when the Earth\'s orbit takes it through one of these
tails, the dust hits the Earth\'s atmosphere and burns up. These are the
recurring **meteor showers** that happen from time to time, and most
major meteor showers have now been identified either with an existing
comet or the remains of a comet that was observed earlier, usually in
previous centuries. When the Earth travels through this \"swarm\" of
dust left behind, you can see **shooting stars** or **meteors** at
night.

## How many Comets are there?

No one really knows. All comets spend most of their orbit so far away
from the sun that they can\'t be seen \-- even with a telescope.
However, every year amateur
astronomers1
discover over 100 never-seen-before comets that have come close enough
to be discovered2. As of November 2005,
astronomers have discovered 2 857 comets
3. Most of the
comets we see either crash into the Sun, or leave the Solar System
entirely. There might be millions of these comets that sooner or later
will come within range of our telescopes.

Of all the comets that have ever been seen, astronomers only expect 253
comets to ever return 4.

## How is a Comet named?

!Edmond Halley A comet is usually
named after the astronomer who first discovered it. When several people
are involved in its discovery, sometimes you will see multiple names on
a comet, like Comet Hale-Bopp, or Comet Shoemaker-Levy. It is generally
considered to be a great honor to have a comet named after you.\

## What are some famous Comets in history?

right\|thumb\|upright=1.5\|This diagram shows the orbit of Halley\'s
comet around the Sun. There are a few things to note about this orbit.\
\*It is much more elongated than a planet\'s orbit.\
\*It is not in the same plane as the planets.\
\*It goes round its orbit in the opposite direction. This is called
retrograde motion.\


-   Halley\'s Comet - Perhaps the most famous of all comets, and this
    was the first comet to be identified as a recurring comet.
-   Comet Encke - The second comet to be identified as a recurring
    comet.
-   Comet Shoemaker-Levy 9 - This was the first comet to have been
    observed hitting another body in the Solar System. In this case, it
    smashed into the planet Jupiter in what was possibly the most
    studied astronomical event in history.

## Do comets bring bad luck?

In ancient times people didn\'t have a very good understanding of what
comets really were or where they came from. They were seen as very
unusual objects in the sky, and very temporary in nature as well. In
some societies it was often a sign of bad events in the future when a
comet arrived, associated with the death of a king or a significant
military defeat. In other countries comets were considered to bring good
luck, bringing increased fertility and more food. The ancient Chinese
astronomers seem to have done the best job of actually recording when
comets appeared in the sky, and left detailed descriptions of what they
looked like and approximately where in the sky each comet was seen.

Even as recently as the 1910 appearance of Halley\'s Comet there was
widespread panic when it was discovered that the Earth might pass
through the tail of that comet. The panic was over the possibility of
gases from the comet flooding the atmosphere of the Earth with poison.
The reality was that there is so little gas in a comet tail that there
is no measurable effect in the content of the Earth\'s atmosphere when
an event like this occurs. Next Topic: Kuiper
Belt

## References

-   \"What is a comet?\"
    5
-   \"What does a comet look like?\"
    6
    7
    8
    9
-   \"Seeing comets in the sky\"
    10
    11
    12
-   \"How many comets are there?\"
-   \"How is a comet named?\"
    13
    14
-   \"Do comets bring bad luck?\"
    15
