ANCIENT WORLDS

Comets are very ancient objects that formed from the solar nebula disk that circled around the young Sun.

Comets travel very slowly across the night sky and are usually visible for several weeks. They travel huge distances and their orbits around the Sun take many years. The famous comet Halley visits every 76 years and will next appear in 2061. Comet Hale-Bopp, which appeared in 1997, will not come back for over 2000 years.

Comets were formed in the cold outer reaches of the Solar System. Comets that formed near Uranus and Neptune were thrown out from the disk and into a place called the Oort cloud. This is a vast cloud that surrounds the Solar System, over 7 billion kilometers from the Sun. There may be a trillion comets in the Oort cloud. Comets that formed in the solar nebula disk beyond Neptune are still orbiting the Sun in the plane of the Solar System today; these are known as Kuiper Belt comets.

DIRTY SNOWBALLS

Comets are often called "dirty snowballs" because they are made
of dust and ice.

It is very cold at great distances from the Sun. Along with gas and rocky dust, the solar nebula disk contained water ice and frozen methane. Comets formed by gradual accretion (sticking together) of small pieces of dust and ice. They also contain organic material. Scientists wonder whether organic compounds, brought to Earth by comets, sowed the seeds of life on our planet.

When a comet comes close to the Sun, ices from its surface evaporate, producing a gas cloud known as the coma. Dust particles from the surface of the comet are driven out into space by the escaping gases. The resulting dust tail is like a cloud of smoke and can be millions of kilometers long. The tail of a comet is blown in the solar wind. It always points away from the Sun, like a wind sock. The solid part of a comet, called its nucleus, is typically only a few tens of kilometers across.

[pictured above] The European Space Agency's Giotto spacecraft flew close to Comet Halley in 1992. The solid, "dirty snowball" part of the comet (the nucleus) is 16 km long and is made of dust and ice. Jets of gas and dust are coming off the comet, making the bright rays in the image.

COMET DUST

Tiny pieces of dust collected high in the atmosphere
probably come from comets.

Comet dust rains down onto the Earth every day. This dust is known as "interplanetary dust particles" or IDPs. The best way to find comet dust is to catch it on sticky plates attached to special high-altitude aircraft. IDPs are very fluffy-looking dust balls that are only about one hundredth of a millimeter across. They are made of grains of different minerals like olivine, pyroxene and iron sulfide. This is the dust that was floating at the edges of the Solar System 4.6 billion years ago.

Because IDPs have been heated as they travel close to the Sun, they no longer contain the ices or organic compounds from comets. Our information about these comet materials comes from spacecraft missions such as Giotto, which visited comet Halley in 1986, and Deep Impact, which blasted a crater on the surface of comet Tempel 1 in July 2005. In January 2006, the Stardust mission returned the first comet samples directly from comet Wild 2 for laboratory studies.

[pictured top] This interplanetary dust particle is made of grains of olivine and pyroxene, glass and carbon. The whole piece of dust is 10 micrometers across. Photo: NASA. [pictured above] NASA's Stardust mission brought samples back from comet Wild 2 in 2006. Analysis of the dust shows that comets are made of materials very similar to those in chondritic meteorites. The image shows a grain of olivine returned by Stardust. The grain is 2 micrometers across. Photo: University of Washington.

METEOR SHOWERS

Meteor showers happen when the Earth passes through a comet dust trail.

During a meteor shower, each dust speck that enters the Earth's atmosphere burns up with a bright blaze of light. None of the dust reaches the ground. All the meteors appear to come from one point in the sky, for example the Leonids originate in the constellation of Leo. In a meteor shower you may see more than one hundred shooting stars in an hour.

As the Earth orbits the Sun it passes through dust trails left in space by comets. This means that we can predict when meteor showers will happen. The Leonid meteor shower happens every year in November, when the Earth passes through the dust trail of comet Tempel-Tuttle, which last appeared in 1998.

[above] An old engraving from the 18th Century showing a spectacular meteor shower.