Update on Comet Hartley 2

Comet Hartley 2 has now reached naked eye brightness for those lucky enough to observe from a dark site. For the rest of us the comet is actually a rather difficult object. Under any level of light pollution, the comet is nearly impossible to see with just the naked eye. At my home (Lm = +5.5) the comet is a faint, barely discernible fuzzball in 10×50 binoculars. At least the comet’s location is relatively easy to find due to the large number of bright stars available for star-hopping. Still the comet has not been an impressive sight and I’d guess that most inexperienced observers will have a hard time finding it.

For example, I just went out to catch a glimpse of the comet as it passed by the Double Cluster in Perseus. I  was rather surprised that I could not find it at first. Because it was located near a few bright stars it was swamped by their glow. The proximity to the stars also ruled out any attempt at estimating its brightness. On of these nights I need to drive out to a darker site and see how much of a difference that makes.

The reason for the difficulty in seeing the comet is twofold. One, the comet is large with recent CCD images finding the comet’s light spread over a coma 36’+ across. Two, any amount of light pollution really does a number on low surface brightness objects. Since most of us live and observe near city lights, objects like Hartley 2 can literally be lost against the bright sky.

The comet is much easier to see in CCD images and I continue to observe it with the LightBuckets 0.2-m (8″) astrograph. The two images below are from the nights of Oct. 2 and 6 UT, respectively. Due to the increasingly rapid motion of the comet I was able to median combine the 19 individual images from Oct. 6 UT in such a way as to make the stars (mostly) go away. The coma is fairly circular and there is little fine detail in the coma. A narrow dust(?) tail points just west of south (PA ~200-205) and can be traced up to 12′ from the nucleus. This is still within the outer coma so this comet does not (yet at least) display the ‘lollypop’-type appearance typical of 5th to 6th magnitude comets.

Recent visual magnitude estimates place the comet between magnitudes +5.3 and +6.5. My own visual estimate from last night (Oct. 7.22 UT) gave a magnitude of +5.9. CCD derived measurements from the LightBuckets 0.2-m come in a little fainter at +6.3. The following table presents a summary of my visual and CCD observations in ICQ .

Oct. 07.22, m1= 5.9, Diam=24' (Visual, 10x50 binoculars)
     06.27, m1= 6.3, Diam=36' (0.20-m reflector + CCD + Astrodon G filter)
     04.17, m1= 6.5, Diam=16' (Visual 10x50 binoculars)
     02.43, m1= 6.7, Diam=38' (0.20-m reflector + CCD + Astrodon G filter)
     02.30, m1= 6.5, Diam=20' (Visual 10x50 binoculars)
Sep. 25.31, m1= 7.3, Diam=23' (0.20-m reflector + CCD + Astrodon G filter)
     15.28, m1= 8.2, Diam=24' (0.20-m reflector + CCD + Astrodon G filter) 
     12.23, m1= 8.1, Diam=26' (0.20-m reflector + CCD + Astrodon G filter)
     03.33, m1= 9.8, Diam=8'.6 (0.61-m reflector + CCD + Astrodon G filter)
Aug. 05.37, m1=11.9, Diam=4'.4 (0.32-m astrograph + CCD + Astrodon G filter)
June 17.41, m1=16.8, Diam=0'.6 (0.32-m astrograph + CCD + Astrodon G filter)

As the graphs below show, the brightness trend has not changed over the past month. All but a single visual estimate has fallen below the expected brightness of the comet (based on its last well observed return in 1997). So unless we are all missing a large fraction of the coma and the comet is actually brighter than is being reported, Hartley 2 is on pace to peak at magnitude +4.5 to +5.0 in two weeks.