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Learn about planets outside our solar system through Exoplanets and Alien Solar Systems by Tahir Yaqoob, Ph.D., a book in our online store book collection.
This image is a schematic of a comet. The center part of the comet, or nucleus, is represented by the flame. The solar wind particles are shown as green dots with arrows. And the ionized particles are shown as green/red dots with arrows. Neutral particles are shown by the other dots (without arrows).
Click on image for full size

The Comet Coma

As the ices of the comet nucleus evaporate, they expand rapidly into a large cloud around the central part of the comet.

This cloud, called the coma, is the atmosphere of the comet and can extend for millions of miles. The cloud is very thin, however, with only a 100 particles in a cubic centimeter. By comparison, a cloud in the Earth's atmosphere might contain millions of particles per cubic centimeter.

Just as in any atmosphere, there are electrically charged particles present, called ions, as well as electrically neutral particles. Ions are formed when the cloud of neutral particles collides with the solar wind. A continual stream of neutral particles is produced as long as the nucleus is evaporating, and these neutral particles are continually converted to ions. These ions are what help form the comet tail.

Last modified November 10, 2003 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA