ALPO COMET SECTION NEWS FOR SEPTEMBER 2015
2015-September-01
Southern hemisphere observers will continue to hold a monopoly on viewing September’s brightest comet, C/2013 US10 (Catalina). At 6th magnitude, it is the only comet expected to get brighter than ~10th magnitude this month. Due to the small number of bright comets, this report will dig deeper and highlight some fainter, but interesting comets.
Evening Comets
C/2013 US10 (Catalina) [Perihelion on 2015-Nov-15 at 0.82 AU from the Sun]
The brightest comet in the sky, C/2013 US10 (Catalina) is now magnitude 6.5 and should brighten by another half magnitude or more by the end of the month. It will only be visible to southern observers as it moves through Norma (Sep 1), Circinus (1-6), Lupus (6-23) and Centaurus (23-30). Catalina sees its heliocentric distance decrease from 1.56 to 1.17 AU but its geocentric distance increase from 1.21 AU to 1.61 AU during September. If the comet continues to brighten at its current rate it may be a faint naked eye object from November through January. By late November, it will once again be visible to northern comet watchers.
C/2014 Q1 (PANSTARRS) [Perihelion on 2015-Jul-06 at 0.31 AU from the Sun]
Back in July, C/2014 Q1 peaked at around 4th magnitude. Unfortunately it was never an easy comet to see visually from the northern hemisphere. The latest magnitude estimates from mid-August placed the comet at magnitude 9.5 and steadily fading. This month PANSTARRS should fade from 10th to 12th magnitude as it moves away from both Sun (1.38 to 1.89 AU) and Earth (1.76 to 2.39 AU) against the stars of Centaurus. Due to its far southern location in the sky, views of the comet are still limited to southern observers.
C/2014 Q2 (Lovejoy) [Perihelion on 2015-Jan-30 at 1.29 AU from the Sun]
The other bright ‘Q’ comet of the past year is C/2014 Q2 (Lovejoy). Much fainter than at its 4th magnitude peak, Lovejoy will be around 11th magnitude this month as it slowly moves south through Boötes (Sep 1-6), Hercules (6-12) and Corona Borealis (12-30). Its distance from the Sun (3.15 to 3.46 AU) and Earth (3.27 to 3.76 AU) continues to increase as it retreats into the outer solar system.
22P/Kopff [Perihelion on 2015-Oct-25 at 1.56 AU from the Sun]
Comet Kopff’s 2015 perihelion passage is its 16th since its discovery in 1919. In fact it has never been missed at an apparition since discovery. This month Kopff should reach 10th-11th magnitude as it moves through Libra and approaches the Sun (1.65 to 1.58 AU) while moving away from Earth (1.89 to 1.99 AU).
Morning Comets
67P/Churyumov-Gerasimenko [Perihelion on 2015-Aug-13 at 1.24 AU]
A little fainter than most comets in this report at 12th magnitude, 67P is definitely a comet of interest. As the target of ESA’s Rosetta mission, we know more about this comet than any other. Rosetta’s ring-side seat has witnessed an outbursting jet and an ejected boulder just in the past few weeks. Ground-based observers can watch 67P from a safe distance of 1.8 AU as the comet glides through Gemini (Sep 1-5), Cancer (5-29) and Leo (29-30).
141P/Machholz [Perihelion on 2015-Aug-25 at 0.76 AU from the Sun]
141P is one of many discoveries by former ALPO Comet Section Coordinator Don Machholz. A short-period comet with a 5.25 year period, 141P is making its 5th perihelion passage since its 1994 discovery. During 1994 the comet was actually a multiple comet with 5 components (component D was even observed to split during the apparition). The two brightest components (the primary A and secondary D) made a visually striking double comet in small telescopes. Components A and D were re-observed in 1999 but by 2005 only component A was visible. Due to poor observing conditions, no components were seen in 2010. This year the primary (A) has been seen as well as another component (no orbit has been published yet for this object so whether it is component D or a new component is still TBD).
141P’s split personalities didn’t begin in 1994. Research by Zdenek Sekanina found that components B through E split from the primary during the period of 1987 to 1991. Other research suggests 141P (or its progenitor) may have been breaking up for some time as it is related to both the Alpha Capricornid meteor shower and comet 169P/NEAT (a weakly active comet on an orbit with a 4.2 year period).
The comet was predicted to get as bright as 10th magnitude this year but had been severely lagging that prediction. Over the past few weeks the component A rapidly brightened and is now around 11th magnitude. The new (currently undesignated) component is a few magnitudes fainter at ~14. Comet Machholz is already past perihelion and will be moving away from the Sun (0.77 to 0.98 AU) and Earth (1.06 to 1.38 AU) this month. Though the comet(s) should fade, split comets are notorious for exhibiting erratic behavior so close attention is warranted. It is a morning object as it moves through Gemini (Sep 1-3), Cancer (4-26) and Leo (26-30).
New Discoveries
C/2015 O1 (PANSTARRS) was mentioned in last month’s report. Its initial orbit suggested it might become a nice bright object though there were suspicions that its orbit was incorrect. As it turns out, those suspicions were correct as C/2015 O1 will reach perihelion at a rather distant 3.73 AU in early 2018. The comet is currently located at over 8 AU from the Sun. It may become as bright as 13th magnitude in 2018.
C/2015 P3 (SWAN) is the fourth comet discovered in data taken with the SWAN instrument on the SOHO spacecraft. Michael Mattiazzo of Australia detected the comet in images taken on July 28. Similar to the other 2015 SWAN finds, C/2015 P3 has a perihelion distance of less than 1 AU (0.72 AU on July 27). The brightest magnitude estimates placed it around 10th magnitude. The comet is rapidly fading and is already fainter than 12th magnitude.
P/2015 P4 (PANSTARRS) was discovered on August 14 at 21st magnitude. It is a short period comet with a period of 15 years. Perihelion will occur next year on January 19 at 2.5 AU from the Sun. The comet will remain faint and may not get brighter than 19th-20th magnitude.
P/2015 Q1 (Scotti) was found by former ALPO Comet Section Assistant Coordinator Jim Scotti on August 18 with the Spacewatch 0.9-m on Kitt Peak. Discovered at 19th magnitude the intrinsically faint comet will not get much brighter as it passes perihelion on September 7 at 1.73 AU. It is also a short period comet with a 5.4 year period.
As always, the Comet Section is happy to receive all comet observations, whether images, drawings or magnitude estimates.
- Carl Hergenrother (ALPO Comet Section Coordinator)
