We don't really know. There is no good way to 'date' comet material, especially since we don't have any good samples of their material. From the scraps of comet dust that we can collect in the stratosphere. They may pre-date the formation of our solar system, but we don't know whether they were formed inside ancient interstellar clouds, or in the disk of gas and dust that once orbited the infant Sun. They do represent very primitive matter with chemical compositions very similar to gases found in many interstellar clouds today. The NASA Stardust mission will arrive at comet Wild 2 on January 2, 2004 and capture some of its tail material, then alter course and return to Earth where the sample of comet material will be recovered. Soon after it arrives on January 15, 2006 we will know the age and chemistry of at least one comet.
