Comets are dirty cosmic snowballs -- small, fragile, irregularly shaped chunks of rock, dust and frozen gases that hold clues to the origins of the solar system.
These icy bodies are thought to be nearly pristine examples of the original material from which the sun and planets were born 4.6 billion years ago. Their occasional collisions with Earth are thought to have seeded our planet with the basic ingredients for life.
With the launch of NASA's seven-year Stardust space mission in 1999, scientists are hoping to pick up some samples of this ancient cosmic dust from a comet discovered in 1978 by Swiss astronomer Paul Wild (pronounced "vildt").

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