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Artists concept of the stardust spacecraft flying throug the gas and dust from comet Wild 2. Credit: NASA/JPL
As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on the surface of the nucleus, spewing gas and dust particles into space. This gas and dust forms the comet’s coma. Radiation from the sun pushes dust particles away from the coma. These particles form a tail called the dust tail. At the same time, the solar wind — that is, the flow of high-speed electrically charged particles from the sun-converts some of the comet’s gases into ions (charged particles). These ions also stream away from the coma, forming an ion tail. Because comet tails are pushed by solar radiation and the solar wind, they always point away from the sun.
Scientists learned much about comets by studying Comet Halley as it passed near Earth in 1986. Five spacecraft flew past the comet and gathered information about its appearance and chemical composition. Several probes flew close enough to study the nucleus, which is normally concealed by the comet coma. The spacecraft found a roughly potato-shaped nucleus measuring about15 kilometers long. The nucleus contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar.
There are interesting articles about comet coma here and here. We have a couple of great articles here on Universe Today. One covers comets in general and the other talks about some facts about comets. Astronomy Cast has a good episode about losing contact with rovers and shooting lasers into space.
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