Comet Section        




Comet C/2017 O1 (ASASSN) is the only comet that can be considered bright this month at magnitude ~9 to 10. While pickings are slim for visual observers, CCD observers can image dozens of other comets including outbursting 29P/Schwassmann-Wachmann 1 (magnitude ~ 12.0).

The lack of any bright comets has resulted in few visual magnitude estimates being submitted this August and the majority were non-detections. The Section received visual estimates from Salvador Aguirre and Carl Hergenrother for comets C/2015 ER61 (PANSTARRS) and C/2017 O1 (ASASSN). CCD images were obtained from Gianluca Masi, Mike Olason, Richard Owens, and John D. Sabia for comets (457175) 2008 GO98, 2P/Encke, 49P/Arend-Rigaux, 71P/Clark, 73P/Schwassman-Wachmann 3, 77P/Longmore, 89P/Russell, 139P/Vaisala-Oterma, 145P/Shoemaker-Levy, 174P/Echeclus, 189P/NEAT, 237P/LINEAR, 240P/NEAT, 353P/McNaught, 355P/LINEAR-NEAT, C/2015 ER61 (PANSTARRS), C/2015 O1 (PANSTARRS), C/2015 V2 (Johnson), C/2015 VL62 (Lemmon-Yeung-PANSTARRS), C/2016 M1 (PANSTARRS), P/2016 WM48 (Lemmon), and C/2017 O1 (ASASSN). Most of the above objects were imaged by Mike Olason, a new contributor to the Section. The large number of observed comets is due to his ability to push his 11″ telescope to ~20th magnitude. ¬†An example of his work is the below image of periodic comet 237P/LINEAR, a low activity comet displaying a nice tail from dust released near perihelion. The nearly inactive nucleus is the stellar object near the ‘head’ of the dust tail.

Evening C0mets

C/2015 V2 (Johnson) [Perihelion on 2017 June 12 at 1.64 au]

Comet Johnson is only visible from the southern hemisphere where it is well placed for observation as it moves through Lupus (Sep 1-13) and Norma (13-30). Johnson is now 3 months past perihelion and is in full retreat from the Sun (1.96 to 2.19 au) and Earth (1.85 to 2.34 au). As a result, the comet will continue to fade this month from around magnitude 10.0 to 11.0.

29P/Schwassmann-Wachmann 1 [Perihelion on 2019 March 7 at 5.77 au]

29P has been very active this summer with multiple outbursts being detected. As August begins the comet has just experienced another outburst and is as bright as 12th magnitude. Being located out near the orbit of Jupiter, it doesn’t move much from month to month so it remains near the Capricornus/Aquarius border. This is one comet where rapid changes in brightness and coma morphology make day-to-day (and sometimes even hour-to-hour) monitoring worthwhile.

Morning Comets

C/2015 ER61 (PANSTARRS) [Perihelion on 2017 May 9 at 1.04 au]

C/2015 ER61 (PANSTARRS) reached perihelion back in May and has faded rather rapidly to between magnitude 11.5 to 12.5. The comet will continue to fade as it moves away from the Sun (2.04 to 2.41 au) and its geocentric distance remains steady at (1.66 to 1.67 au). ER61 is a morning object this month moving through Taurus.

C/2017 O1 (ASASSN) [Perihelion on 2017 Oct 13 at 1.50 au]

Recent discovery C/2017 O1 (ASASSN) should be the brightest comet of the month. In the days after discovery, the comet appeared to be undergoing an outburst and was observed to brighten by a few magnitudes. Since then the comet’s rate of brightening has significantly slowed down. It is currently around magnitudes 9.5 and may brighten to between 8.0 and 9.0 by the end of the month as it approaches both the Sun (1.62 to 1.52 au) and Earth (1.06 to 0.77 au). How bright this comet gets is still in question. It is well placed in the morning sky for northern observers as it moves northeastward through Taurus (Sep 19-27) and Perseus (27-30).

John D. Sabia obtained the image of Comet ASASSN shown below on August 26 UT.

As always, the Comet Section is happy to receive all comet observations, whether images, drawings or magnitude estimates. Please send your observations via email to < carl.hergenrother @ >.

- Carl Hergenrother (ALPO Comet Section Coordinator)

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