Comets
Paragraph 1：Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases (water vapor, ammonia, methane, carbon dioxide, and carbon monoxide) that hold together small pieces of rocky and metallic materials. Many comets travel in very elongated orbits that carry them far beyond Pluto. These long-period comets take hundreds of thousands of years to complete a single orbit around the Sun. However, a few short-period comets (those having an orbital period of less than 200 years), such as Halley’s Comet, make a regular encounters with the inner solar system.
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