Asteroids are lumps of rock that orbit the Sun. They are sometimes called the minor planets. Most asteroids - more than half a million - orbit the Sun in the Asteroid Belt, between Mars and Jupiter. Every 50 million years, the Earth is hit by an asteroid measuring over 10 km across. Most asteroids are in the Asteroid Belt, which lies between Mars and Jupiter. 
 Some distant asteroids are made of ice and orbit the Sun beyond Neptune. A few asteroids come near the Earth. These are called Near Earth Objects (NEOs). The first asteroid to be discovered was Ceres in 1801. It was detected by Giuseppi Piazzi, one of the Celestial Police whose mission was to find a ‘missing’ planet. Ceres is the biggest asteroid - 940 km across, and 0.0002 percent of the size of the Earth.
 The Galileo space probe took close-up pictures of the asteroids Ida and Gaspra in 1991 and 1993. There are half a million or so asteroids bigger than 1 km across. More than 200 asteroids are over 100 km across. The Trojan asteroids are groups of asteroids that follow the same orbit as Jupiter. Many are named after warriors in the ancient Greek tales of the Trojan wars.
 Comets are bright objects with long tails, which we sometimes see streaking across the night sky. They may look spectacular, but a comet is just a dirty ball of ice a few kilometres across. Many comets orbit the Sun, but their orbits are very long and they spend most of the time in the far reaches of the Solar System. A comet’s tail is made as it nears the Sun and begins to melt.
 A vast plume of gas millions of kilometres across is blown out behind by the solar wind. The tail is what you see, shining as the sunlight catches it. Comets called periodics appear at regular intervals. Some comets reach speeds of two million km/h as they near the Sun. Far away from the Sun, comets slow down to 1000 km/h or so - that is why they stay away for so long.
 The visit of the comet Hale-Bopp in 1997 gave the brightest view of a comet since 1811, visible even from brightly lit cities. The Shoemaker-Levy 9 comet smashed into Jupiter in July 1994, with the biggest crash .
 The tail of a comet always ever witnessed. The most famous comet of all is Hailey’s comet Comet Kahoutek streaks through the night sky. 

Hailey s comet 

 Hailey’s comet is named after the British scientist Edmund Hailey (1656-1742). Hailey predicted that this particular comet would return in 1758, 16 years after his death. It was the first time a comet’s arrival had been predicted. Hailey’s comet orbits the Sun every 76 years.
 Its orbit loops between Mercury and Venus, and stretches out beyond Neptune.
 Its next visit will be in 2062. The Chinese described a visit of Hailey’s comet as long ago as 240bc.When Hailey’s cometwas seen in 1837, Chinese astronomers wrote that its head was as bright as Venus and its tail stretched right through the sky. Harold, King of England, saw the comet in 1066. 
When he was defeated by William the Conqueror a few months later, people took the comet’s visit as an evil omen. Hailey’s comet was embroidered on the Bayeux tapestry, which shows Harold’s defeat by William.

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