Last updated: October 10, 2012
Ephemerides for all of these comets (and many others) can be calculated at the IAU Minor Planet Center's web site. Recent reports of their brightness can be found at the ICQ's magnitude page, the Comet Observation Home Page [currently inactive] and the Yahoo! comet observations group. Discussions of these and other comets are carried out at the Yahoo! comets mailing list group, and links to images are at the Yahoo! comet images group. For basic comet observing information see the appropriate page at this web site.
NOTE ADDED NOVEMBER 21, 2007: There are numerous objects that are apparent asteroids but that travel in distinctly cometary orbits or are otherwise associated with cometary phenomena in some way, and it is entirely possible that these are extinct or dormant comets. When any of these come by I do attempt to observe them, but they are not eligible for adding to my comet tally unless observations show that they are indeed comets, in which case I'll add them to my list retroactively (as has happened once so far). This page will now include any such objects that I'm currently following at the time of any updates.
NOTE ADDED JULY 3, 2008: This page will now highlight in red print those comets that are 9th magnitude or brighter (and that are also easily accessible for observation) at the time of the update in question. Such comets should be detectable from suburban areas with small telescopes, and thus are those that are specifically recommended for "Countdown" participating students.
NOTE ADDED JANUARY 28, 2012: With the addition of comet no. 500 to my tally, "Countdown" now comes to a close. Earthrise will be transitioning to a new program within the fairly near future, probably in about two to three months; during the interim, we will continue to update this page on a fairly regular basis just as we have throughout "Countdown." New additions to my tally will be included as they are added, however this will normally happen without announcement or fanfare.
Comet LINEAR C/2012 K5 (no. 510)
This comet continues to brighten slowly as it approaches the earth and the sun, appearing close to 12th magnitude when I observed it a few nights ago. Visually it exhibits a relatively small and condensed coma, with a distinct "swept-back" appearance that is consistent with the prominent tail that it has shown on recent CCD images. It is starting to get fairly low in my northwestern sky after dusk, and will be in conjunction with the sun (54 degrees north of it) on October 28, thereafter becoming primarily a morning object. The comet is currently located in northern Bootes three degrees north-northeast of the star Gamma Bootis; it is traveling towards the southwest at a relatively slow five arcminutes per day but gradually curves more and more due westerly, reaching a farthest south point of declination +40.6 degrees on October 23. I expect it to continue brightening over the coming weeks.
Comet LINEAR C/2011 F1 (no. 499)
I am probably about finished with this comet, as it is getting quite low in my western sky after dusk (the present elongation being 37 degrees, and decreasing by half a degree per day). It is currently located in southern Serpens Caput half a degree southeast of the star 10 Serpentis (and three degrees east-southeast of the globular star cluster M5) and is traveling towards the southeast at slightly over half a degree per day, passing less than five arcminutes southwest of the star Mu Serpentis and then crossing into northeastern Libra on October 23. The comet continues its unexpectedly slow brightening trend, appearing close to 12th magnitude during my recent observations; while it is still approaching perihelion (now three months away) it is also receding from Earth, and I don't expect any significant change in brightness before it is lost in twilight.
Comet LINEAR C/2010 S1 (no. 494)
This distant comet seems to have brightened slightly after going through opposition in late August, and has appeared as a small and condensed object between magnitudes 13 1/2 and 14 during my recent observations. It is currently located in northern Cygnus three degrees northeast of the bright star Deneb; it is traveling towards the south-southwest at 15 arcminutes per day (gradually curving more due southerly as it approaches its stationary point shortly after mid-November), passing ten arcminutes northwest of the star 55 Cygni on October 16 and forty arcminutes east of Deneb five days later. There should be little change in brightness over the coming weeks, at most perhaps a very gradual fading as the comet recedes from Earth.
Comet Catalina C/2012 J1 (no. 508)
Although still almost two months away from perihelion passage, this faint and relatively distant comet was at opposition last month and is nearest Earth (2.33 AU) on October 16, and thus is probably close to its maximum brightness; it has appeared as a small and condensed object of 14th magnitude during my recent observations, and it will likely maintain this overall brightness and appearance for a few more weeks. The comet is located in southwestern Andromeda, presently four degrees south-southeast of the star 14 Andromedae; it is traveling almost due southward at slightly over ten arcminutes per day, and after going through its stationary point on October 21 it crosses into northeastern Pegasus three days later.
Comet 168P/Hergenrother (no. 511)
The outburst that this comet apparently underwent last month continues to evolve and, to some extent, strengthen; when I observed it a few nights ago it appeared as a small and relatively condensed coma, almost as bright as magnitude 9 1/2, and exhibited a faint but nevertheless distinct broad tail extending a few arcminutes towards the south. It is currently traversing the northeastern "corner" of the "Great Square" of Pegasus, being presently located 1 1/2 degrees north of the star Psi Pegasi and 3 1/2 degrees southwest of the "northeastern corner" star Alpha Andromedae; it is traveling towards the north-northwest at 40 arcminutes per day (decreasing to 20 arcminutes per day by the end of this month), passing 1 1/2 degrees east of the star 78 Pegasi on October 16 and crossing into southwestern Andromeda seven days later, at which time it passes 45 arcminutes east of the above comet. The comet was at opposition and was nearest Earth last month, and passed through perihelion a week and a half ago; I would expect it to fade over the coming weeks as it recedes from the sun and Earth and as the outburst disperses, although it is conceivable that it could continue to exhibit more outburst-related activity.
Comet 260P/McNaught P/2012 K2 (no. 509)
Having passed through perihelion almost a month ago and having been nearest Earth (0.58 AU) one week ago, this comet is at opposition on October 18; its brightness has held steady at just fainter than 13th magnitude during my most recent observations but I would expect a gradual fading to commence over the next few weeks. It is currently located in eastern Andromeda a little over ten arcminutes west of the star 56 Andromedae (and one degree southwest of the center of the large open star cluster NGC 752); it is traveling towards the north-northwest at slightly over 20 arcminutes per day (but curving more westerly and slowing down to just over ten arcminutes per day by month's end), passing just under a degree east of the star Tau Andromedae on October 22 and a similar distance east of the star Upsilon Andromedae five days later.
Comet McNaught C/2011 R1 [not observed yet]
Recent reports from the southern hemisphere about this comet continue to be somewhat spotty, but suggest that its present brightness is somewhere around 12th magnitude; it passes perihelion on October 19 but is receding from Earth, and thus should commence a gradual fading over the coming weeks. It is currently located in eastern Centaurus three degrees west-southwest of the star Sigma Lupi and is traveling towards the northeast at just under half a degree per day (slowing down to just over 20 arcminutes per day by month's end); it crosses into southwestern Lupus on October 11, then passes half a degree northwest of the star Rho Lupi nine days later and a similar distance southeast of the star Alpha Lupi five days after that. The comet is in conjunction with the sun (31 degrees south of it) in early November and thereafter becomes a morning object.
Comet PANSTARRS C/2011 L4 (no. 504)
I lost this comet below my southwestern horizon a month ago but it has remained accessible for southern hemisphere observers since then; it is, however, starting to get low in their sky as well (present elongation 37 1/2 degrees) and they will lose it as well before too long. It is currently located in southeastern Libra four degrees southeast of the star Sigma Librae and is traveling towards the east-southeast at somewhat over ten arcminutes per day. The comet had brightened to about 12th magnitude by the time I could no longer follow it, and according to the reports I've read from the southern hemisphere it has, as expected, brightened a little since then (by perhaps half a magnitude) and should continue doing so. It will -- hopefully -- be much, much brighter by the time I am able to see it again when near perihelion next March.
Comet McNaught C/2009 F4 [not observed]
The recent reports from the southern hemisphere about this distant comet have, like the above Comet McNaught, been spotty as well, but suggest that its brightness is still near 14th magnitude, similar to what it has exhibited for the past several months. It is currently located in northern Dorado 2 1/2 degrees south of the star Alpha Doradus and is traveling at slightly under ten arcminutes per day, presently towards the west-southwest but gradually curving more due westward and it crosses into northeastern Reticulum on October 23 and reaches a maximum southerly declination of -57.7 degrees three days later. There should be little, if any, change in the comet's brightness over at least the next few weeks.