Joe B wrote:

Definitely the mysterious exploding comet. -- Joe

With respect to your earlier question sometime ago, about why comets have round heads, I made a pit stop at the Marriott library and have also bought a couple of books on comet physics. Haven't had the time to type up any notes. The short answer on 17P is that the venting side of the comet faces the Sun. Gitto image of venting Halley's Comet nucleus in 1986 http://seds.lpl.arizona.edu/nineplanets/nineplanets/halley.html The ultra-low density magnetically neutral comet gas moves sunward from the vent at between 500 meters/sec to 1000 meters/sec. The intuitive model of the comet - because venting is on the Sun side - would be that the tail should point towards the Sun. But the counter-intutitive reality is that this comet gas runs into the solar wind - another ultra-low density gas moving between 400 meters/sec to 800 meters/sec but going in the opposite direction. The head-on or slightly oblique collision of these two gas molecules make a bow shock wave on the leading edge of the expanding comet gas and the rebounding molecules travel into orbits that make the characteristic spherical-cone shaped coma. The leading bow shock wave of a comet coma can be seen in some images of 17P, depending on how the images were processed. Sebastian Voltmer 11-12 Animated color 11-2 to 11-12 http://spaceweather.com/comets/holmes/13nov07/Sebastian-Voltmer1.gif This leading bow shock and the denser gas around the psuedo-nucleus also gets ionized by these collisions. Comets also traverse the spiral magnetic fields emanating from the Sun - the Parker spiral - and the magnetic field lines wrap around the comet's magnetic field and trail almost straight-out behind the comet. This is analogous to the Earth's magnetosphere-sheath that also passes through the Sun's spiral magnetic field. The ionized gas around the pseudo-nucleus and at the bow shock travels follow those magnetic lines trailing straight behind the comet and become concentrated in the magnetic current sheet trailing the comet. In 17P, the small trailing sheet or coma psuedo-short-tail is very easily seen. Patrick's 11-14 image http://gallery.utahastronomy.com/v/patrickw/HOLMES14NOV2007-030.JPG.html It is the bow-shock and trailing current-sheet short-tail that gives 17P its defining trilobite or "space bug" look. The diameter of the coma is a function of the intensity of the solar wind - which decreases by the d^2 exponential distance from the Sun. Compare McNaught in Jan. 2007 going inbound (tiny coma, huge tail) with Holmes going outbound in Oct.-Dec. 2007 (huge coma, no tail). Inbound comets have larger comas the further they are from the Sun. The size of the coma decreases as the comet moves closer to the Sun (within the orbit of Earth) due to the increased density and pressure of the solar wind. Conversely, the tail gets bigger because the increased pressure of the solar wind strips more molecules off the venting coma. 17P is outbound between 2 and 3 a.u. Somewhere near 3 a.u. the heat of the Sun will become insufficient to vaporize any water molecules on 17P. At 2.1 to 2.7 a.u., the solar wind is still moving around 400-800 meters second, but is much less dense. As the comet moves further from the Sun, the pressure decreases, there are less solar wind molecules to collide with venting comet gas, and the coma gets bigger before the molecules get turned around in their tajectory into a spherical coma - well alot bigger coma in the case of 17P. Not all vents on a comet point directly at the Sun all the time. Some comet nucleui rotate. This rotation has been seen indirectly in the subtle changes in the magnitude (<0.1 mags) of the psuedo-nucleus. Meech page of rotating comet nucleus light curves http://www.ifa.hawaii.edu/faculty/meech/rot.html A consequence of this rotation are so-called "non-gravitational" changes in a comet's orbit. It is one reason why periodic comets have to be re-acquired on each new orbit. These subtle non-gravitational parameters change the comet's orbit chaotically and result in a slightly different inbound tajectory on the next pass. In closing, 17P's coma is not so mysterious. A big coma on a far away-outbound comet is consistent with dynamics between the relative intensities of inbound venting comet gas and the outboud solar wind. Because of Utah's extended December cloud-out, I have not seen 17P for weeks. Current (12-23) internet list traffic says that 17P is still faintly visible in binoculars before the Moon rises. - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net