My last post just isn't what I really should have said. Astronomy is a precise science, and I am not a precise person and must admit that my interest level isn't what it used to be, and is not nearly as high as most list members these days. I have a good mental grasp of most processes and heavenly motion but don't care to take it beyond that point most of the time anymore (you can only have the same discussion so many times in a lifetime) even though I can do the math. Adult ADD also makes it very difficult to stick with a subject down to the level of intricate detail, lack of unstructured free time for the past few years also adds to my personal editing process. And sometimes I just assume too much. Uncle Carl is 99.99% correct, though once a particle gets to a certain size (he touches on this briefly) then sunlight pressure gives way to orbital motion. Also any motion imparted at the time of ejection from the nucleus can have a dramatic effect on the particle's direction of motion away from the comet at certain sizes. Again here I confess to having drifted off the excerpt, but were the sun's magnetic field lines addressed? The short answer is: two tails, two different raw materials/mechanisms. Jim, I commend you for digging as deeply as you did and taking the time to possibly violate a copyright for our benefit. I in no way meant to minimize or marginalize your posts, you most definitely are the comet master. I'm just here for the donuts. __________________________________ Do you Yahoo!? Yahoo! Mail - You care about security. So do we. http://promotions.yahoo.com/new_mail