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In 1999, the Ulysses spacecraft accidentally discovered that comet Hyakutake's tail was half a billion kilometers (more than 300 million miles) long, making it the longest tail ever recorded.
  
space 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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More on comets:
Comet Observation Comet Observation
   Do you want to know when a comet is going to be visible in the night sky? This site provides this information as well as other comet-related data.
Comet Shoemaker-Levy Comet Shoemaker-Levy
   In 1994, Comet P/Shoemaker-Levy 9 collided with Jupiter. This was the first collision of two solar system bodies ever observed. Much of the data about the impact's effect on Jupiter's atmosphere is included in this site.
Comet Hale-Bopp Comet Hale-Bopp
   This unusually bright comet was discovered in 1995. It is the most distant comet ever discovered by amateur astronomers. Hale-Bopp appeared 1000 times brighter than Comet Halley did at the same distance.
Comet Hyakutake Comet Hyakutake
   This comet, discovered in January, 1996 was visible for several months during that year. This site includes information and images on the comet.


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