Comets are the remainders of material formed in the coldest part of our solar system. Impacts from comets played a major role in the evolution of the Earth, primarily during its early history billions of years ago. Some believe that they brought water and a variety of organic molecules to Earth.
Comets are visible for two reasons.
Dust driven from a comet's nucleus reflects sunlight as it travels through space. Secondly, certain gases, stimulated by the sun, give off light like a fluorescent light bulbs. Over time, a comet may become less active or even dormant. Scientists are anxious to learn whether comets exhaust their supply of gas and ice to space or seal it into their interiors. Controlled cratering like that planned for Deep Impact allows a look deep into the interior of the comet.