COMETS

· Your travel guide to the Solar System ·

comets_halebopp (6K)


DEFINITION

Icy chunks of water and dust that originate in the outer Solar System. When they come near the Sun they vapourise, developing a bright tail.

REASONS TO VISIT

  • See the frozen relics of the formation of the Solar System
  • Visit the Kuiper belt or the Oort cloud, where these icy chunks wander through outer space
  • Examine their composition to see if comets brought life to Earth

 


WHAT TO SEE

Comets originate from two regions, the Kuiper Belt and the Oort cloud.

The Kuiper Belt
A belt of icy bodies that extends from Neptune out to past Pluto. Comets from this region are called 'short-period comets', due to the relatively short time they take to orbit the Sun.

Some astronomers think that Pluto is actually a member of the Kuiper belt. They think that rather than being a planet, Pluto should be reclassified as a giant comet.

Explore Pluto further

The Oort cloud
The Oort cloud is a spherical shell that surrounds the Solar System. It contains an estimated 10 trillion comets (1 followed by 13 zeros) with the combined mass of the Earth. This is the main home for comets, nine trillion kilometers from the Sun.

Objects from this area are the source of 'long-period comets'. They are called this due to the long time they take to orbit the Sun.

Comet tails
When a comet approaches the Sun, it starts to vapourise. A 'tail' of gas forms, which is pushed out behind the comet by the solar wind.

Find out more about the Sun and the solar wind

A comet's tail can reach up to ten million kilometres long. It can leave behind trails of gas that can extend several hundred million kilometres further.

Some comets actually have two tails - one of gas and another one inside made up of dust.

SPOTTING COMETS FROM THE EARTH


On average, you can see a comet with the naked eye every five or six years. However, many of these are only just visible, even if you know where to look. More spectacular displays happen about every ten years.

Comets leave other evidence of their existence though. Whenever the Earth passes through the tail of a comet, shooting stars dart across the sky. These are known as 'meteor showers'.

For example, the Perseid showers, which appear over our skies in August, are caused by the tail of the Swift-Tuttle comet.

Find out when the next meteor shower is happening

TRAVEL INFORMATION


Halley's comet
Dimensions · 16 x 8 x 8km
Orbital period · 76 years
Next visible from Earth · 2061

Hale-Bopp
Dimensions · 40km
Orbital period · 4026 years

Before you leave
Make sure you have an accurate timetable. Halley's comet orbits roughly once every 76 years, but its path can be changed by the gravitational fields of the major planets.

Its orbital period has varied many times over the last 2,000 years, sometimes by as much as four years.

When you arrive
Though comets can offer some of the most speculator sights in the Solar System, most are rather small objects. Halley, for example, has a core which is only 16km across, composed mostly of ice with small amounts of dust. This core is then surrounded by a halo of gases.

Comets also can contain amino acids, one of the building blocks of life. So some scientists think that colliding comets may have brought the first ingredients of life to Earth.

Did life arrive on a comet? Examine the evidence and then cast your vote

LOCAL HISTORY


In ancient times, a comet in the night sky signified that disaster was coming. Even the word 'disaster' is derived from the Latin astre meaning 'star'.

The most famous of all the comets is named after the astronomer Edmund Halley, who predicted the comet's visit in 1758. The earliest sightings of this comet were made in China in 240 BC. Halley's comet is also depicted on the Bayeux tapestry, marking its appearance in 1066.

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More from BBC

BBC Space - Did Life Arrive on a Comet?
Did life on our planet arrive on colliding comets?

BBC Space - Comet Gallery
Pictures of comets taken by amateur astronomers

h2g2 - Oort Cloud and the Kuiper Belt
The guide to Life, the Universe and Everything, written by you

Go further

In-depth site on the structure, mythology and composition of the comets

NASA - Small Object Photojournal
Photographs of asteroids and comets

NASA - Comet Factsheets
In-depth data on missions to asteroids and comets




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