Comets

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.
Contents: Parts of a Comet Comets in Human History Viewing Comets Comet News Definitions Link to the REQUIRED "Page 2"	Image found and used according to copyright rules from NASA-Comet.

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.

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.

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

Viewing Comets

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:

1. Comet Hale-Bopp (C/1995 01)
2. Comet Hyakutake (C/1996 B2)
3. Comet West (C/1975 V₁=1976 V1)
4. Comet Kohoutek (1973 E₁=1973 XII)
5. Comet White-Ortiz-Bolelli (C/1970 K₁=1970 VI)
6. Comet Bennett (C/1969 Y₁=1970 II)

Comet News

Project Stardust, NASA's recent Comet Sample Return Mission
http://stardust.jpl.nasa.gov/ (try this link if you want the link in a SEPARATE window)
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.

Play at cool game called Tails of Wonder at http://spaceplace.jpl.nasa.gov/en/kids/stardust/index.shtml

Definitions

anti-tail

Also known as anomalous tail, this occurs when a comet's tail appears to be pointing toward the Sun. Of course, it's not actually pointing toward the sun because comets orbit the Sun. When a comet produces heavy dust particles, they are left along the comet's orbit instead of being pushed away from the Sun.

apparition

This is the time when a comet is under observation. The term probably comes from the ghostly appearance of comets.

coma

Also known as the comet's head, the coma is the dense, hazy cloud of water, carbon dioxide, another gases around the comet's nucleus. The head and tail are the only things we see from Earth.

dust tail

Composed of dust particles from the nucleus and up to 10 million kilometers long, this is the most prominent element of the comet.

hydrogen cloud

A large, sparse envelope of neutral hydrogen.

ion tail

Composed of plasma laced with rays and streamers from solar wind, this tail can be several hundred million kilometers long.

long-period comets

Orbit periods of more than 200 years.

nucleus

Made up of ice, gas, dust, and small amounts of other materials, this core is solid and stable.

outburst

A comet sometimes experiences an unexpected increase in brightness for a short time due to the release of dust and gas into the coma from the nucleus. This is known as an outburst.

perihelion

The point in a comet's orbit when it is closest to the Sun.

short-period comets

Also known as periodic comets, they have orbit periods of less than 200 years. These coments have a P/ before their name.

I see myself as a huge fiery comet, a shooting star. Everyone stops, points up and gasps "Oh look at that!" Then - whoosh, and I'm gone... and they'll never see anything like it ever again, and they won't be able to forget me - ever.

This page was created as a part of Skill Building Activity 1, for Information Architecture for the Web class
Web Author: Emily Morris
Creation Date: September 2, 2005
Last revision: September 7, 2005
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