The passage of Comet Machholz near the Pleiades a couple of weeks ago reminded me of a project I spent a number of years working on in the late 1980s and early 1990s—the International Halley Watch Atlas of Large-Scale Phenomena— a compendium of Comet Halley images. The whole thing began when applied for a job as a research assistant on an IHW team at the university. I spent the next several years measuring and studying many, many images of Comet Halley, teasing out details about its plasma tail structures. Our interest was to chart the changes in the plasma tail as the comet sampled various “regimes” of the solar wind, and use those changes to understand how the solar wind affected the charged particles in the plasma tail. It got to the point that I could look at an image of Comet Halley and tell you exactly when it was taken (date and time) and tell you something about whether or not its plasma tail was about to disconnect or was rebuilding itself after a disconnection event (when the plasma tail would break off and then re-form in response to changes in polarity in the solar wind stream).
So, it was with a great sense of memory and history that I looked at this image (and many others) of Machholz as it passed near the Pleiades star cluster in the January sky. For one thing, it was clear that the comet had just undergone a disconnection event. The new tail was sprouting out from the coma and the remnants of the old tail were streaming out in a clump highlighted against the Pleiades. (The dust tail, by contrast, is the yellowish streak that seems to point roughly downward in this image.)
Even cooler, I ran across this image from Gerald Rhemann, one of Austria’s best-known comet chasers and astrophotographers. I became quite familiar with Gerald’s work (along with his collaborator Michael Jaeger) during my Halley years, and in my subsequent role as coordinator for observations for the Ulysses Comet Watch network in the early 90s. It’s a double blast from the past—comet plasma tails and the fantastic work of a photographer whose work I’ve long admired. The scene is stunning, combining my old research interest with one of my favorite star clusters in the winter sky. Sometimes astronomy doesn’t get any better than this!
