Introduction to Small Bodies Node:
Comet Introduction
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Introduction to Small Bodies Node:Comet Introduction |
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Home | Comet Studies | Asteroid Studies | Current Events | Missions to Small Bodies | Images | Comet and Asteroid Locations
Comets are icy planetesimals formed in the outer solar system. They are studied as an aid to understanding the starting composition of the solar system. Throughout history, comets have been viewed with wonder by many different civilizations. At times they were feared and considered to be omens, usually bad. Scholars tried to explain the existence of comets through both scientific and religious reasoning. Today, science has succeeded in answering many of the questions posed by the ancients, but it has also produced many more of its own.
Comets are thought to be the remnants of the material which gave birth to the solar system. As a large cloud of primordial gas contracted it eventually formed into the Sun and the planets around it. However, some of this material did not get incorporated into these objects and it is this leftover material which is thought to be what formed the comets.
Comets are small, irregularly shaped bodies composed mainly of ice and dust. Most comets have highly elliptical orbits which take them very close to the Sun and back out into deep space, far beyond the orbit of Pluto. These comets will take hundreds of thousands or even millions of years to complete one orbit. Other comets will never pass far beyond Pluto's orbit and have orbital periods less than 200 years. These comets with short orbital periods are called periodic comets.
The main body of the comet is called the nucleus and is composed of dust particles trapped in a mixture of ices of water, carbon monoxide, carbon dioxide, methane, and ammonia. The nucleus is typically only a few kilometers in diameter and is the only part of the comet that exists when it is far from the Sun. As the comet approaches the inner solar system, the solar heat generates a halo of evaporated gas and dust which sublimes from the nucleus and spreads out around it, forming the coma. The coma will grow larger in size as the comet gets closer to the Sun. An invisible hydrogen envelope also forms engulfing the nucleus in a huge cloud of neutral hydrogen stretching out through millions of kilometers of space.
There are two types of tails associated with visible comets. The first is composed of microscopic dust particles which usually form a broad and gently curved tail. The second, straighter and thinner, is composed of ionized gas. The radiation pressure and solar wind from the Sun cause the dust and ions to be blown away from the comet's nucleus and coma producing the visible dust and ion tails. Consequently, the tails of comets are always directed away from the Sun.
Glossary
Research
Crni Vrh Observatory: CCD Photometry of Faint Comets
More Information
References
- Bailey, M.E., Clube, S.V.M., Napier, W.M. The Origin of Comets. Pergamon Press, New York, 1990.
- Brandt, J.C., Chapman, R.D. The Comet Book. Jones and Bartlett Publications, Inc., Boston, 1984.
- Chaisson, E., McMillian, S. Astronomy. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1995.
Page Written by: Jennifer R. Schuster
Edited by: Dr. Lucy McFadden
Last Update: Puru Shetty (July 12, 2007)
Contact: pshetty@umd.edu