Comets: Frozen wanderers

Comets are bodies of ancient ice and dust that orbit the Sun and are believed to be almost pristine material left over from the Solar System's formation some 4.6 billion years ago. One theory is that they hold complex carbon molecules that helped seed life on an infant Earth.
-- As a comet nears the Sun, some of the ice is melted and transformed into gusts of gas, the bright "coma" around its head. The gassy wake, and dust loosened by the melting ice, creates a spectacular tail that is reflected in the Sun's rays and may stretch across millions of kilometres (miles) in space. The word for comet comes from "stella cometa," Latin for "long-haired star".
-- Like solar eclipses, comets have been associated with great events of history, good and bad. The birth of Jesus and Napoleon, the eruption of Vesuvius in 79 AD that destroyed Pompeii, and the Great Plague of 1665 that ravaged London have been linked to comets. "The celestial phenomena called comets (excite) wars, heated and turbulent dispositions in the atmosphere, and in the constitutions of men, with all their evil consequences," warned the first-century Egyptian astronomer and astrologer Ptolemy.
-- Approximately 2,000 comets have been observed and recorded over the past 2,500 years. They follow elliptical orbits, with a return taking anything from a few years to as many as 40,000 years. Some scientists estimate there could be billions of comets, only a tiny fraction of which have ever been seen.
-- The most famous comet is named after British astronomer Edmond Halley, who was the first to prove that comets orbit the Sun and return regularly. He showed that a comet of 1682, now called Halley's Comet, was identical with two that had appeared in 1607 and 1531, and he successfully predicted the comet's next return, which occurred in 1758, 16 years after his death. Halley's Comet last swung by Earth in 1986.
-- Comet 67P/Churyumov-Gerasimenko, the target for Europe's Rosetta space probe, orbits the Sun once every 6.6 years. In July, images from the spacecraft as it neared the comet showed the target to be shaped rather like a duck, with a large body and a head connected by a neck. The comet is named after two Soviet astronomers, Klim Churyumov and Svetlana Gerasimenko, who first identified it, separately, in 1969.
-- The head of a comet can be bigger than a planet, but most are just a few cubic kilometres (miles) in size. For all its celestial splendour, Halley's Comet is only about 15 kilometres long by four kilometers wide (nine by 2.5 miles). Churyumov-Gerasimenko is believed to measure about four kms across.
-- Astronomers once believed that comets were born in interstellar space, but the consensus now is that they are created at two locations on the fringes of the Solar System. So-called long-period comets - ones which take at least 200 years to return - are believed to originate in the Oort Cloud, an accumulation of gas and debris beyond the orbit of Pluto. Short-period comets like Churyumov-Gerasimenko are believed to come from a ring of debris beyond Neptune's orbit called the Kuiper Belt.
-- Comets pose a risk, albeit a very small one, to life on Earth. A collision by a comet or large asteroid 65 million years ago inflicted climate change that probably ended the reign of the dinosaurs. In 1992, the comet Shoemaker-Levy 9 was torn into 21 large fragments as it entered Jupiter's gravitational field. In July 1994, the fragments smashed into Jupiter at speeds of about 210,000 kph (130,000 mph), releasing energy that triggered fireballs larger than the Earth.