Fragmenting Nuclei
The nuclei of some bright comets have fragmented around perihelion passage, and high magnification is necessary for documenting any immediate changes near the nucleus. For example, in late 1995, the periodic comet 73P/Schwassmann-Wachmann 3 split into three fragments, all visible in a small telescope (Sky & Telescope: March 1996, page 11). Using a 4-inch refractor from Volcano, Hawaii, I noticed that the comet's nuclear region appeared extremely elongated at low power, but the three faint secondary nuclei were clearly visible inside this glow at 200x.
Brad D. Wallis, a member of the Cassini imaging team at JPL, obtained a sequence of nightly images of the nucleus of Comet Hale-Bopp at about 02:50
Universal Time March 24th to 30th. He used a 12½-inch f/5.85 Ritchey-Cretien telescope and an SBIG ST-7 CCD camera with a red filter; each image is the sum of six 5-second exposures. Wallis processed the images to highlight the comet's prominent dust fountain and hoods.
Sky & Telescope then assembled the frames into an animation that shows the nucleus rotating and the dust spiraling out into space.
When dust and gas are liberated from the comet's nucleus, they flee in fantastic fountains. Through the telescope, you'll see this activity as
jets, linear rays that flow in the Sunward direction. A jet can appear as a single ray or a fan of several arcing rays. High power should reveal any strong sources near the nucleus. If jets are suspected, record their positions, switch to moderate magnification, and study the shape of the inner coma. If the coma is very bright, the jets might be lost in the glow, but the material they have ejected can surround the inner coma in a series of
parabolic hoods. The farther a jet extends from the nucleus, the lower its surface brightness becomes, so moderate and low magnifications should be used to trace out its greatest extent. Generally, long jets will have a strong linear base that gradually curve toward the antisolar direction.
One of the most curious features within a jet is a knot of material that can easily be misidentified as a secondary nucleus. Secondary nuclei usually migrate tailward over several nights. So be leery of bright spots in the Sunward direction; these are probably clumps of gas and dust shooting off the nucleus within the jets. Many times the knots are brighter than the jets themselves, so a bright knot in a faint or invisible jet will certainly cause alarm! Comet Hale-Bopp exhibited several intense knots, especially in one strong and persistent north-pointing jet, during its passage around the Sun.
Sky & Telescope's Dennis di Cicco and Stuart Goldman obtained this close-up of Comet Hale-Bopp on March 30, 1997, using a Meade 16-inch LX200 f/10 telescope and an SBIG ST-7 CCD camera. A composite of two 2-second exposures, the field shown is about 5½ arcminutes wide. Left: In this view, the image has been processed to reveal a jet spiraling off the rotating nucleus. Right: Here the image has been processed to highlight multiple concentric hoods.
On nights when the comet is visible for several hours, try making a series of drawings of the inner nucleus. I use a pocket sketchpad and render the nucleus each hour on separate sheets of paper. Such multiple drawings are an excellent way to show the evolution of the features.
