By Helen Briggs BBC News Online science reporter

What are comets?

They are large "dirty snowballs" made up of ice, rock and dust - a sort of cosmic iceberg.

Comets are very precious to scientists because the chemicals and frozen gases from which they are made give a snapshot of the early Solar System.

The appearance of a comet is recorded in the Bayeux tapestry

More than 4.6 billion years ago that system was created from a cloud of interstellar dust and gas.

Since then, matter on Earth and the other planets has been altered by geological, biological and chemical processes.

But matter has retained its primitive form in some asteroids and comets.

What do they look like?

When they are near the Sun and active, comets have several components.

The heart of a comet is its nucleus, typically a few kilometres across. It is made up mainly of ice surrounded by a dark crust of unknown composition.

The nucleus is surrounded by the coma, a dense cloud of water, carbon dioxide and other gases given off by the nucleus.

The most distinctive part of a comet is its dust tail - a trail, up to 10 million km long, of tiny dust particles driven off the nucleus by escaping gases.

How do comets get their names?

They are named after their discoverers. Wirtanen, for example, was named after Carl Wirtanen of the Lick Observatory in California.

He discovered the comet by chance on 15 January, 1948, while examining photographic plates.

Many comets have been discovered by amateur astronomers. Comet Ikeya-Zhang, named after two enthusiasts in Japan and China, was glimpsed in March after returning to the inner Solar System for the first time in 341 years.

Can a comet be seen?

Comets have been viewed from the Earth since ancient times. There are Chinese records of Comet Halley going back to at least 240 BC.

Carl Wirtanen: Discovered Wirtanen in 1948

The Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of the same comet.

Comets are brightest when near the Sun.

They are visible for two reasons. Dust streaming behind a comet's nucleus reflects sunlight as it travels through space. Certain gases, activated by the Sun, also give off light.

Why study comets?

The scientific drive behind comet missions like Rosetta is to determine the composition of the building blocks of the Solar System.

"When we know the composition of comets we also know the composition of the cloud of gas and dust that all the planets formed out of," says Dr Geraint Jones of the space and atmospheric physics department at Imperial College London, UK.

Dr Ian Wright of the Open University in Milton Keynes, UK, says an asteroid or comet could wipe out civilisation and it is important to "know thy enemy".

"These are the solid remnants from the Solar System formation process. If we can look at them now we can look back in time."