Comet

This article is about the objects in space. For store, see Comet (shop).

Diagram of a comet's orbit

A comet is a ball of mostly ice that moves around in outer space. They are often described as "dirty snowballs". They are very different from asteroids. In the solar system, the orbits of comets go further than Pluto. 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. A comet's tail does not trail behind it, but points directly away from the Sun, because it is blown by the solar wind.

The hard centre of the comet is the nucleus. It is one of the blackest things in the solar system. When light shone on the nucleus of Halley's Comet, the comet reflected only 4% of the light 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.

Famous comets

Main page: list of comets

• Halley's Comet
• Hale-Bopp
• Shoemaker-Levy 9
• Ikeya seki

History of comets

In old times, people used to be scared of comets. They did not 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 farther 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.

Isaac Newton also studied comets. He realised that comets make U-turns around the sun. He asked Edmond Halley to publish this in his book Philosophiae Naturalis 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.

Other pages

• List of comets

Other Websites

• Are Comets Made of Antimatter?