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Comets are nomadic bodies of ice, rock and organic and inorganic volatile compounds left over from the formation of our solar system about 4.5 billion years ago. Their orbits sometimes take them from the cold and dark outermost reaches of the solar system to close passes by the Sun. When comets venture into the more intense sunlight of the inner solar system, the ice on the comet nucleus begins to evaporate and escape, carrying small solids with it. These form the comet's tails, which can often be seen from Earth. Generally, comets come from the Kuiper belt located beyond Neptune, and from the Oort Cloud, which marks the outside edge of the solar system.
For several billion years, comets probably struck Earth and caused major changes to Earth's early atmosphere and climate. Their impacts with Earth may have delivered the first carbon-based molecules to our planet. As some of the least changed objects in our solar system for billions of years, they may yield important clues about the formation of our solar system. JPL's Deep Space 1 spacecraft took the highest-resolution pictures ever of a comet's nucleus, adding volumes of information to comet science in just one day.
For more information, visit NASA's Solar System Exploration site.
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