Balls of ice and dust

Comets are fossils - frozen relics from the time of the infant Sun. By studying them, astronomers can find out how the Sun and the planets were born. In the early 1950s Fred Whipple and other astronomers provided an insight into the structure of comets. Whipple said that a typical comet has three parts: a frozen central part called a nucleus, a fuzzy cloud surrounding the nucleus called a coma (or head), and a tail consisting of gas and dust. The nucleus, usually only a few kilometres across, is a 'dirty snowball' made of grains of frozen mass consisting of water, methane, ethane, carbon dioxide, ammonia and many other gases. In 1986 the European Space Agency's Giotto spacecraft proved that Whipple's model was fairly accurate when it took close-up photographs (from a distance of 480 kilometres) of the nucleus of Halley's comet: a comet nucleus resembles a fluffy snowball coated with a crust of black material and spouting jets of vaporised ice.

As the comet approaches the Sun, the gases evaporate under the Sun's heat and form its coma, which scatters sunlight. The solar wind, a spray of charged particles from the Sun, blows this material into a tail, reaching typical lengths of 10 million kilometres, and in rare cases, several times that distance. The tail is always away from the Sun. Comets become visible only when they are close to the Sun - between two and three times as far away from the Sun as Earth. That's why astronomers can't be sure when the next new comet will come.

Whipple, often referred to affectionately as 'Dr Comet', not only coined the evocative phrase 'dirty snowball', he also came up with an equally evocative idea: a comet is like a jet engine. Like the heated gases erupting from a jet engine, the evaporating gases from the nucleus exert a force on the nucleus. This force gives the comet its independent thrust. 'When I first realised the jet action of comets', 79-year-old Whipple told Time magazine in

The most famous of all comets is Halley's comet. It's named after the 18th-century British astronomer Edmond Halley, who first calculated its period and successfully predicted its return in 1758. It travels in a giant orbit that takes about 76 years. It was last seen in

1986, and is expected to return in 2061. It's a long wait for Halley's, but we never know - we may be lucky enough to see one night a brilliant comet arcing through the sky trailing a magnificent tail. It will outshine everything in the sky. When Halley's comet was here in 1910, Earth passed through its tail without any effect. It is a different matter if a comet head hits the Earth.

Comets usually pass Earth with speeds greater than 160,000 kilometres per hour. If a comet a couple of kilometres across hits Earth with such a speed, it will gouge out a hole as big as a large city, spewing out so much dust in the atmosphere that the Sun will be blocked out for months. If it hits an ocean, a tidal wave up to 1 kilometre high and travelling at many hundreds of kilometres per hour will submerge most low-lying regions. But the worst threat is fire - fire caused by debris thrown into the atmosphere when the comet head explodes before hitting Earth. Burning forests and cities will throw soot into an already clogged atmosphere. Then there will be an acid rain - a rain of toxic gases and metals. Most plants will die, followed by marine creatures that live near the surface, nearly ending life on the planet.

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