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Inactive comet passing Saturn, computer artwork. Comets are composed of rock, dust, ice and other frozen gases. As they approach the Sun they warm up and gases and other debris are ejected, creating their characteristic tail (coma). In the outer solar system (as seen here) comets remain frozen (inactive) and are extremely difficult to detect from Earth. (KEYSTONE/SCIENCE PHOTO LIBRARY/DETLEV VAN RAVENSWAAY)