Joe asked:

That's really nice, Kurt. My question is, why is the coma roughly spherical if it's shooting away from some vent? Thanks, Joe

I don't know, Joe. Speculating, like most explosions, I suspect that there is a differential energy flux from the center to the outside of the event. For example, lunar craters have two halos - the bright Copernician rays represent the high energy ejecta from the initial impact. Crater Copernicus's smaller dark halo represents the eject from the end of the event - where most of the energy of the impact is exponential dissipated. In comets you have the highest-energy ion tail being ejected out the tail, the next lower energy dust tail exiting in a curve. In the coma of 17P we see the higher energy streamers and central condensation being ejected in a linear oval out of the back of pseudo-nucleus. The spherical shell may represent the lower energy dust particles at the edges of the eruption that do not have enough energy to achieve escape velocity from the weak gravity field of the comet. But again, I'm guessing. - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net