Halley's Comet

• The ancient Chinese documented the appearance of Halley's Comet as early as 240 B.C.E. People used to believe that Halley's Comet brought about the fall of Jerusalem in 66 A.C.E. and the Huns' invasion of Rome in 373. Halley's Comet was also believed to have caused the death of King Harold II in the Battle of Hastings in 1066; it is even included in the famous Bayeux Tapestry depicting that event. Even when Halley's Comet returned in 1910, "comet hysteria" caused people to go hide in the mountains, buy gas masks and umbrellas -- even take "anti-comet pills" to protect them from the "noxious fumes" emitted by the comet.
  
  

Comet's Core

• Scientists now believe that comets contain more that just ice, dust and gassesDigital Vision./Digital Vision/Getty Images
  
  The core of a comet -- called the nucleus -- consists of, dust, ice, rock and frozen carbon monoxide, carbon dioxide ammonia and methane. It also contains some organic compounds such as methanol, hydrogen, hydrogen cyanide, formaldehyde, ethane and ethanol. It is now believed that it could also contain complex molecules such as amino acids and long-chain hydrocarbons.
  
  

Color

• The brightness observed is really the ionized dust and gasses of the cometDigital Vision./Digital Vision/Getty Images
  
  A comet's nucleus is one of the darkest known objects in the solar system. It reflects only four percent of the light that hits it compared to asphalt, which reflects seven percent. Its dark surface is what enables the comet to absorb the heat necessary to produce its propulsion, or "outgassing."
  
  

Shape

• While comets may appear round, they actually have irregular shapesDigital Vision./Digital Vision/Getty Images
  
  Comets are not round. They have irregular shapes due to their low mass being insufficient to produce their own gravity, which would cause them to be round.
  
  

The Coma

• When a comet approaches the inner solar system it is bombarded with radiation.Photos.com/Photos.com/Getty Images
  
  There is a large, unstable atmosphere around the comet called a "coma." It is created when the comet approaches the inner solar system and is bombarded with radiation. This causes the gasses within the core to vaporize and stream out of the comet creating its coma.
  
  

Comet Tails

• A comet's long tail is caused by radiation and solar windsJupiterimages/Photos.com/Getty Images
  
  Comets actually have two tails. The enormous tail of a comet is caused by the pressure of the sun's radiation and solar winds on the stream of gasses glowing from ionization. This is why the tail always extends away from the sun. A second is a smaller, curved tail, called the antitail, which is a trail of dust caused by the comet's orbit and magnetic field.
  
  

Orbit

• Some comets can take millions of years between visits to the inner solar system.Jupiterimages/Photos.com/Getty Images
  
  Comets are often classified by their orbital period. Short-period comets have an orbital period of 200 years or less and are classified in either the Jupiter Family, with orbital periods less than 20 years, and the Halley Family, with orbital periods between 20 and 200 years. Long-period comets have orbital periods that can range from 200 years to thousands and even millions of years. Single-apparition comets are those that enter the solar system once and when they leave, they leave permanently.