Comets are a mixture of ices including water and frozen gases, along with dust. They don't contain fire. They include materials that weren't incorporated into planets when the solar system was formed, so they contain a rich record of the history. Nearly 900 comets have been cataloged and their orbits calculated. However they rarely come more than a few million miles of Earth.
Parts of a Comet
Comets are normally invisible. However when a comet nears the sun, the heat melts the ices in the nucleus releasing dust particles that can be seen from Earth. Distinct parts of the comet can be identified including the nucleus, coma, hydrogen cloud, dust tail, and ion tail.
Learn more: comet term definitions
Comets in Human History
Throughout human history, people have observed comets. As early as 240BC, the Chinese recorded sightings of Comet Halley. Edmond Halley was the first person to recognize that this comet was periodic. It's next close appearance won't take place until 2060.
The Bayeux Tapestry which depicts the Norman Conquest of England in 1066 includes Comet Halley.
Comets can best be observed using a telescope or binoculars. Every five or six years, a comet can be viewed with the naked eye. Comets with long tails are only seen every 10-12 years. Below is a list of selected comets and their orbital periods:
1P Halley 76.1 yrs. 1986-02-09 2P Encke 3.30 yrs. 2003-12-28 6P d'Arrest 6.51 yrs. 2008-08-01 9P Tempel 1 5.51 yrs. 2005-07-05 19P Borrelly 6.86 yrs. 2001-09-14 21P Giacobini-Zinner 6.52 yrs. 1998-11-21 26P Grigg-Skjellerup 5.09 yrs. 1992-07-22
- Comet Hale-Bopp (C/1995 01)
- Comet Hyakutake (C/1996 B2)
- Comet West (C/1975 V1=1976 V1)
- Comet Kohoutek (1973 E1=1973 XII)
- Comet White-Ortiz-Bolelli (C/1970 K1=1970 VI)
- Comet Bennett (C/1969 Y1=1970 II)
Project Stardust, NASA's recent Comet Sample Return Mission. Launched in 1999, the Stardust spacecraft is the first U.S. space mission dedicated solely to the exploration of a comet. On January 2, 2004, the spacecraft flew within 236 kilometers of the comet Wild 2 and survived an impact.
Aerogel, the technology NASA is using to capture comet dust particles, is extremely interesting. It is the lightest solid in existance, has 39 times the insulative power of fiberglass and is 1,000 times less dense than glass.
Play a cool game called Tails of Wonder