Image is courtesy of article.wn.com
A comet is a remnant of our early solar system. They are generally composed of rock and ice trapped in an elliptical orbit around our Sun and pass by planets on their journey.
As they fly by the planets, solar particles from our Sun hit the comets that produce instant out-gassing where ice is transformed directly to water vapour. This out-gassing produces the enormous Comet tail which is visible as they pass by the planets.
Another comet just passed Mars at an incredibly close distance. The comet was named Siding Spring and was visible near Mars as it reached its closest distance on October 19, 2014. It was only 136,000 km away from Mars (only 1/3 the distance from Earth to the Moon).
Here is the NASA site where you can see spectacular images: NASA Site
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