If you've seen one comet, you've seen them all, right? Wrong - according to recent laboratory research and historical observations, comets could have formed at different times during the evolution of the solar nebula, and may reveal their age by the structure of their dust grains. If this idea holds, NASA may one day be able to "time travel" through different epochs of the solar system's formation merely by visiting comets that formed during different periods. The research also indicates that theories of comet formation and the dynamics of a protostellar nebula may need to be revised.
Comet images from the The Hubble Space Telescope
A Sungrazing comet image from the Solar and Heliospheric Observatory
(High-resolution Postscript file)
The Jet Propulsion Laboratory Comet Page
A little more detail: (Stardust mission page)
The Oort cloud page in the University of Arizona Nine Planets Tour
