A comet is a ball of mostly ice that moves around in outer space. It is similar to an asteroid. In Earth's solar system, the orbits of comets go farther than Pluto, a dwarf planet that used to be the farthest planet from the Sun. Most are very far away from the Sun, but some come near enough to Earth for us to see at night. They have long "tails", because the Sun melts the ice. Sometimes people call them "dirty snowballs" or "shooting stars".
The hard centre of the comet is the "nucleus". It is one of the blackest things in the solar system. When light shone on Comet Halley's nucleus, only 4% of the light shone back to us.
"Periodic" comets visit again and again. "Non-periodic" or "single-apparition" comets visit only once.
People have seen some comets when they broke into pieces: Comet Biela was one example. Another comet was seen when it hit a planet: Comet Shoemaker-Levy 9 hit Jupiter in 1994. Some comets orbit (go around) together in groups. Astronomers think these comets are broken pieces that used to be one object.
History of comets.
In old times, people used to be scared of comets. They didn't know what they were, or where they came from. Some thought that they were fireballs sent from demons or gods to destroy the earth. They said that each time a comet appeared, it would bring bad luck with it. Whenever a comet appeared, a king would die. One of these examples is shown in the Bayeux Tapestry, when Halley's Comet returned. Comets were also known to end wars and thought to bring famine.
It was not until the Renaissance when scientists started to look at comets with less superstition and base their observations on science. Tycho Brahe reasoned that comets did not come from the earth, and his calculations showed that comets must be six times further than the earth is from the moon.
Edmond Halley (whom Halley's Comet is named after) reasoned that comets are periodic, that is, they appear once every several hundreds of years. This led to the first prediction of a comet's return, Halley's Comet. In honor of this prediction, Halley's Comet was named.
Issac Newton also studied comets, but he thought that comets were do to "vapours rising from the soil". In other words, he thought that gasses came up from the ground to form comets. Newton later said that comets make U-turns around the sun. He asked Edmond Halley to publish the research in his book "Philosophiae Natrualis Principia Mathematica". Before Newton said this, people believed that comets go in to the sun, then another comes out from behind the sun.
All this new information and research gave people confidence, but some still thought that comets were messengers from the gods. One 18th century vision said that comets were the places that hell was, where souls would ride, being burned up by the heat of the sun and frozen by the cold of space.
Although today we know a lot about comets, there will always be people who will be afraid when a comet returns.

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