Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In the ancient times, people were both awed and alarmed by comets, perceiving them as "long-haired" stars that appeared unpredictably and unannounced in the sky. Today we know that comets are small Solar System bodies composed of a mixture of frozen gases, dust and rocky particles. As the comet approaches the Sun on its elliptical orbit, it develops a cloud of diffuse material around it, called “coma”, and enormous tails of luminous material that extend for millions of kilometers. Learn more about comets, their discovery, structure, classification, as well as the ambitious space missions to explore them in this book.