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
JPL
The Comet Coma
As the ices of the comet nucleus
evaporate, they expand into a large cloud around the middle part of the comet.
This cloud, called the coma, is the atmosphere of the comet. It can extend for millions of miles. The cloud is very thin, 10,000 times thinner than a cloud in the Earth's atmosphere!
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