NASA - National Aeronautics and Space Administration

Comets are formed of dust and icy gases. They travel highly elliptical orbits that bring them close to the Sun, then swing them back, deep into space. Usually the farthest point of their journey is beyond the distance of Pluto's orbit. Close to the heat of the Sun, comets develop tails millions-of-miles in length, some creating easily viewable spectacles as they visit the vicinity of Earth.


Image above: Composite image of comet Wild 2. Image credit: NASA/JPL-Caltech.
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In centuries past, comets seemed to appear suddenly, generating excessive reactions from people who did not understand them. They were blamed for everything from the death of kings to the demise of great empires to the creation of plagues. The passage of Halley's Comet in 1066, for example, appeared to foretell of the English loss at the Battle of Hastings. More recently, during its 1910 passage, the infant science of spectroscopy discovered toxic gases in Halley's Comet. Fearing that the Earth would pass through the poisonous tail, charlatans sold comet insurance policies and homeopathic medicines for the coming "comet fever."

To probe the mysteries of comets, the NASA Stardust spacecraft will fly close to a comet and, for the first time ever, bring material back to Earth for analysis by scientists worldwide. This cometary material, it is hoped, will help unlock some of the secrets of these small bodies that are often described as "dirty ice balls" hurling through space. By learning something about these little bits of a comet, scientists hope it may be possible to discover more far reaching secrets about the materials that make up our universe and possibly more about its origins. A big question that scientists hope to better answer is whether comets brought water to the Earth.