Скачать или смотреть Teach Astronomy - Comet Nucleus

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The current model of a cometary nucleus is called the dirty iceberg model, first proposed 50 years ago by Fred Whipple. The icy nucleus is a composite with rock, and comets can break up under gravitational forces or under the pressure of expanding frozen gases. Shoemaker-Levy 9 near Jupiter was an example of a comet breaking up under the gravity force of the giant planet. The icy material in a comet nucleus is composed primarily of frozen water, methane, ammonia, and carbon dioxide. In 1986 a fleet of spacecraft got very close to comet Halley, and Giotto in particular was able to take beautiful pictures of the comet nucleus showing that it was in fact very dark and covered with dust as dark as black velvet reflecting only four percent of the light from the Sun. Different comet nuclei may be darker, caused by dust material, or brighter, caused by frozen gases.