Comet Section        




All eyes were on C/2017 S3 (PANSTARRS) in July as the comet experienced two outbursts and kept observers wondering for how much longer will it survive. As August begins, PANSTARRS is still with us though recent CCD images suggest it is in a weakened or even disrupted state. Luckily, short-period comet 21P/Giacobini-Zinner will provide a nice bright comet to observe this month as it brightens to magnitude 7 by the end of the month.

Bright Comets (magnitude < 10)

21P/Giacobini-ZInner - Assuming C/2017 S3 (PANSTARRS) doesn’t give us another surprise, short-period comet 21P/Giacobini-Zinner should be the brightest comet this August. With an orbital period of 6.5 years, 21P is making its 16th observed return since its visual discovery in 1900 by Michel Giacobini (Nice, France). Though missed at its next return, it was visually re-discovered 2 returns later in 1913 by Ernst Zinner (Bamberg, Germany), hence the double appellation.

This year 21P has an above average return as it will pass 0.39 AU of Earth on September 10 with perihelion also occurs on the that date. Only the returns in 1946 and 1959 were closer to Earth and not until 2078 will 21P be this close again.

21P brightened to magnitude ~8.5 to 9.0 by the end of July and appears to be brightening as expected.. This month the comet continues to ride high in the northern sky. Though best in the morning, it is visible all night long for northern observers. The comet starts the month around magnitude 8.8 and should brighten to 8.0 by the 14th, 7.5 by the 23rd and 7.1 by the end of the month. Visual and CCD observers will enjoy watching 21P pass through the star-rich constellations of Cassiopeia (Aug 1-18), Camelopardalis (18-27), Perseus (27-28) and Auriga (29-31). By the end of the month, 21P will have started to move south enough for some southern hemisphere observers to observe it. September will see its southern motion accelerate allowing observers in both hemispheres an opportunity to enjoy the comet.

CCD images by John D. Sabia from July 12 and 19 show an elongated coma with the beginning of a tail. There also appears to be a sunward feature as well.

T = 2018-Sep-10  q = 1.01 AU   Short-Period comet  Period = 6.5 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01   8.8   23 59  +65 32   1.163   0.593    88M   Cas
2018-08-11   8.2   01 41  +66 19   1.101   0.520    85M   Cas
2018-08-21   7.6   03 31  +61 20   1.054   0.456    82M   Cam
2018-08-31   7.1   04 54  +49 52   1.024   0.410    80M   Aur

C/2016 M1 (PANSTARRS) – Dynamically old, long-period comet C/2016 M1 passes perihelion on August 10 at 2.21 AU. Visual observations by Chris Wyatt and Willian Souza found the comet slowly fading in July from between around magnitude 8.3 – 8.7 to between 8.6 and 9.1. Though it passes perihelion this month, an increasing comet-Earth distance will result in a further fading by another ~0.5 magnitude over the course of the month. As was the case last month, C/2016 M1 is solely a southern hemisphere object this month as it moves through Circinus (Aug 1-19) and Centaurus (19-31).

T = 2018-Aug-10  q = 2.21 AU   Long-Period comet – dynamically old
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01   8.9   15 24  -56 12   2.213   1.640   110E   Cir
2018-08-11   9.1   14 58  -56 57   2.211   1.801    99E   Cir
2018-08-21   9.3   14 40  -57 31   2.214   1.966    90E   Cen
2018-08-31   9.5   14 30  -58 12   2.224   2.128    81E   Cen

C/2017 S3 (PANSTARRS) – Going… going… gone??? The “biggest question mark” of July remains a big question mark in August as well. Dynamically new and intrinsically faint C/2017 S3 (PANSTARRS) was expected to disintegrate at some point either on approach to perihelion or not too long afterwards. Based on recent magnitude estimates and CCD images, the comet may have already disrupted.

But first, let’s recap an exciting July. At the end of June, the comet was less than impressive and rather faint at about 12th magnitude. On June 30 or July 1, the comet experienced its first outburst and ultimately peaked around magnitude 9.0 between July 4th and 7th. CCD images showed a brighter and larger comet but little in the way of near-nucleus structure to suggest a break-up was underway. By July 13th the comet had settled down to magnitude 10. A second outburst started by the 15th and resulted in a peak brightness close to magnitude 7 on the 20th. This time CCD images did detect significant near-nucleus features. Two jets perpendicular to the comet-Sun direction were detected. Jets of this type are sometimes referred to as “wings” and have been interpreted as a sign of nucleus splitting. Not every comet displaying “wings” disintegrated [for example, the great comet C/1996 B2 (Hyakutake) displayed "wings" in Hubble images], but many disintegrating comets have displayed these features in the weeks to days prior to complete disruption [examples include C/1996 Q1 (Tabur), C/1999 S4 (LINEAR) and C/2012 S1 (ISON)].. The “wings” were visible from July 15th to the 20th. On the 20th, I was able to observe the comet at magnitude 7.1. It was as an easy object in 10×50 binolculars and wonderful in my 30×125s.

So far, July 20th has been the peak for this comet. Within hours, the “wings” disappeared and a pronounced drop in brightness was observed. A report by Bernd Lütkenhöner (on the comets-ml) announced that the comet is deviating from its orbit suggest that what is left of the nucleus is small enough (or in small enough pieces) to be very sensitive to the solar radiation pressure.. In effect, these pieces are being “blown” radially away from the Sun relative to where the comet should be, just like comet tail particles. Interestingly, the start of these deviations from the expected position began around the time the second outburst ceased. A recent image by Denis Buczynski on July 30 UT ( http://www.britastro…1_0124_dgb.html ) shows a very diffuse comet with almost no central condensation. This image appears to confirm that the nucleus is either highly or totally disrupted.

Assuming there is anything left of C/2017 S3, perihelion will occur this month on the 15th at a rather small perihelion distance of 0.21 AU. Being only 2 weeks from perihelion, it is already at a small elongation of 31 degrees on the 1st. Most observers will lose sight of it during the first week of August as its elongation drops below 25 degrees on the 4th and 20 degrees on the 6th. If the comet is still a going concern it will be visible in the SOHO LASCO C3 FOV between August 24 and September 13. The comet appear around the 7:30 position, passes to the east (left) of the Sun (halfway between the Sun and edge of FOV) and exits the FOV around the 12:30 position. On July 31, the STEREO Hi1-A camera will also provide continuous imaging of what is left of the comet.

T = 2018-Aug-15  q = 0.21 AU   Long-Period comet – dynamically new
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01   ?.?   06 57  +40 06   0.536   0.810    31M   Aur
2018-08-11   ?.?   08 37  +12 47   0.276   0.800    11M   Cnc
2018-08-21   ?.?   10 06  +01 57   0.279   1.204    10E   Sextens
2018-08-31   ?.?   10 53  +06 40   0.539   1.538     4E   Leo

C/2017 T3 (ATLAS) – Too bad dynamically old, long-period comet C/2017 T3 (ATLAS) wasn’t better placed for observation. A triple whammy of a large geocentric distance, and a small elongation, and only being observable from the southern hemisphere has hindered observation of this comet. Now a few weeks past perihelion, C/2017 T3 (ATLAS) has probably peaked in brightness. Chris Wyatt observed ATLAS throughout July and consistently estimated it to be between magnitude 8.9 and 9.4. Located south of the Sun, the comet will remain visible only from the southern hemisphere and even then at low elevations (~10 – 17 degrees).

C/2017 T3 (ATLAS)
T = 2018-Jul-19  q = 0.83 AU   Long-Period comet – dynamically old
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01   9.0   08 04  -20 10   0.860   1.351    39M   Pup
2018-08-11   9.4   09 16  -26 30   0.930   1.390    41M   Pyx
2018-08-21  10.0   10 27  -29 52   1.026   1.506    42E   Ant
2018-08-31  10.7   11 27  -30 50   1.139   1.677    41E   Hyd

C/2018 N1 (NEOWISE) – C/2018 N1 (NEOSWISE) was discovered by NASA’s low-Earth orbiting NEOWISE spacecraft on July 2nd at 16th magnitude. Earth-based observers quickly determined that NEOWISE was much brighter. The comet continued to brighten as it approached perihelion on August 1 at 1.31 AU and closest to Earth on July 27 at 0.31 AU. Chris Wyatt and Juan Jose Gonzalez estimated it to be around magnitude 8.3 to 9.5 by the end of July. During August the comet will head north again as it moves through Scorpius (Aug 1), Ophiuchus (1-4), Scorpius (4-5), Ophiuchus (5-7), Scorpius (7-10) and Libra (10-31). It will quickly fade as it slowly moves away from the Sun and rapidly away from the Earth.

C/2018 N1 (NEOWISE)
T = 2018-Aug-01  q = 1.31 AU   Long-Period comet – dynamically old
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01   9.1   17 53  -31 38   1.308   0.358   139E   Oph
2018-08-11  10.4   15 57  -18 19   1.315   0.641   102E   Lib
2018-08-21  11.4   15 25  -12 47   1.339   0.977    84E   Lib
2018-08-31  12.1   15 13  -10 11   1.378   1.312    71E   Lib

Faint Comets (between magnitude 10 and 13)

37P/Forbes – Short-period comet Forbes was discovered by South African astronomer Alexander F. I. Forbes on August 1, 1929. 2018 marks the comet’s 15th apparition and 12th observed apparition since discovery. During its last return the comet experienced a 4 magnitude outburst some 9 months after perihelion. This year 37P was not expected to become brighter than ~13th magnitude. Recent observations place the comet anywhere from magnitude 10.5 to 13.5 over the past few weeks. Similar to last month, estimates of coma diameter are also all over the place (from 1′ to 5′) suggesting the comet’s perceived brightness may depend on how much of a low surface brightness coma is being detected by each observer. The magnitudes forecast below are splitting the difference between the late June observations. Located in Pisces, Forbes rises late in the evening.

Similar to 21P’s CCD appearance, Charles Bell found 37P to possess a long anti-solar tail and what appears to be a sunward feature.

T = 2018-May-04  q = 1.61 AU   Short-Period comet  Period = 6.4 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  12.4   23 44  +04 03   1.826   0.999   130M   Psc
2018-08-11  12.7   23 41  +05 19   1.872   0.980   139M   Psc
2018-08-21  12.9   23 34  +06 08   1.920   0.976   149M   Psc
2018-08-31  13.2   23 25  +06 29   1.970   0.990   160M   Psc

38P/Stephan-Oterma – Comet Stephan-Oterma is returning for the first time since 1980. This comet has a bit of an interesting backstory. In 1867, it was first sighted by Jérôme E. Coggia (Marseilles, France) who thought he had found an uncatalogued nebula. Over the following nights, followup observations by E. J. M. Stephan (Marseilles, France) uncovered the true nature of the object. For some reason, the discovery announcement cited Stephan as the discoverer with no mention of Coggia. Though Coggia would go on to discover six more comets, 38P was his first. After being missed at its next return in 1904, the comet was photographically rediscovered in 1942 by Liisi Oterma (Turku, Finland).

A side note, Coggia would see his name dropped from another of his discoveries when Comet 1873 V1 (Coggia-Winnecke) was linked with comets 1818 D1 (Pons) and a comet discovered by Forbes in 1928. Comet Pons-Coggia-Winnecke-Forbes was then rebranded 27P/Crommelin after the orbit computer Andrew Claude de la Cherois Crommelin.

Michael Olason and Carl Hergenrother were able to image the comet last month. On July 20th, 38P was measured at magnitude 15.1 which is at its expected brightness. This month, 38P will be a morning object in Taurus as it brightens from 14th to 12th magnitude. Ultimately the comet will peak around magnitude 9.0 to 9.5 in late November after a perihelion on November 10 at 1.59 AU. In 1980, Stephan-Oterma approached to within 0.59 AU of Earth and brightened to between magnitude 8.5 and 9.0. This year’s return will be a little further away at 0.76 AU, hence the slightly fainter maximum brightness.

T = 2018-Nov-10  q = 1.59 AU   Short-Period comet  Period = 38.0 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  14.4   03 33  +02 52   2.025   1.989    77M   Tau
2018-08-11  13.8   03 55  +04 18   1.954   1.839    80M   Tau
2018-08-21  13.2   04 17  +05 43   1.887   1.696    84M   Tau
2018-08-31  12.6   04 40  +07 08   1.825   1.561    87M   Tau

48P/Johnson – 48P/Johnson has a large perihelion distance for a relatively bright short-period comet. Perihelion (2.00 AU) and closest approach to Earth (1.01 AU) occur within a week of each other in mid-August making this as good a return as possible for 48P. This year marks its 11th observed return since it was discovered in 1949 by Ernest L. Johnson on photographs taken at the Union Observatory in Johannesburg, South Africa.

Most observers have been consistently placing this comet around magnitude 13.0 which is ~2 magnitudes fainter than expected based on it previous returns. The only dissenter being J. J. Gonzalez who estimates it at the much brighter magnitude of 9..8 to 10.5. J. J. also is seeing a much larger coma (1′ vs 6′), so again there is a question as to whether a very low surface brightness extended coma is being missed by some observers. 48P should peak in brightness this month as it passes through perihelion and opposition.

T = 2018-Aug-12  q = 2.00 AU   Short-Period comet  Period = 6.5 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  11.2   22 43  -21 20   2.007   1.055   151M   Aqr
2018-08-11  11.2   22 42  -23 24   2.005   1.024   159M   Aqr
2018-08-21  11.1   22 39  -25 22   2.006   1.014   163M   PsA
2018-08-31  11.2   22 35  -27 02   2.010   1.026   161E   PsA

364P/PANSTARRS – 364P/PANSTARRS is an example of a low activity comet. Similar to other comets of this type (such as 162P/Siding Spring, 169P/NEAT, 209P/LINEAR, 249P/LINEAR and 300P/Catalina), 364P is only active at small heliocentric distances. There is even some research that suggests that some of these objects originated in the Main Belt rather than the Kuiper Belt like most short-period comets.

Discovered in 2013 as P/2013 CU129, 364P is making its second observed return. This comet has a rather short period (4.9 years) and is only active for a few months around perihelion. Perihelion occurred in late June at 0.80 AU from the Sun. While it should rapidly fade this month, this is actually a good thing in that it will allow CCD imagers the opportunity to directly image its bare nucleus by September. While mainly a southern object, it should be visible (though still low) for northern observers by mid-month.

T = 2018-Jun-24  q = 0.80 AU   Short-Period comet  Period =  4.9 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  12.9   04 35  -33 56   0.997   0.256    78M   Eri
2018-08-11  14.2   03 25  -35 12   1.092   0.288    98M   For
2018-08-21  15.4   02 26  -34 32   1.194   0.330   115M   For
2018-08-31  16.4   01 36  -32 37   1.299   0.384   132M   Scl

C/2016 R2 (PANSTARRS) – Early 2018’s “blue comet” has remained observable even as it went through solar conjunction due to its path well north of the Sun. Juan Jose Gonzalez observed this comet on July 9th and placed it at magnitude 11.3. Though now three months past perihelion, the comet should continue its slow fade as it steadily climbs higher in the morning sky for northern observers.

T = 2018-May-02  q = 2.60 AU   Long-Period comet – dynamically old
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  11.4   09 21  +52 38   2.748   3.514    35M   UMa
2018-08-11  11.5   09 53  +52 17   2.784   3.520    37M   UMa
2018-08-21  11.6   10 24  +51 40   2.822   3.526    39M   UMa
2018-08-31  11.7   10 55  +50 49   2.864   3.531    42M   UMa

Other Comets of Interest

Two low activity comet come to perihelion this year. (3552) Don Quixote is still designated an asteroid even though Spitzer Space Telescope observations from 2009 showed a faint coma and tail. This month Don Quixote is slowly fading from its peak brightness. (944) Hidalgo is still inbound and will peak in brightness at 14.3 in November. Unlike Don Quixote, Hidalgo has shown no cometary activity so far. Both objects are located close to each other in the morning sky.

(944) Hidalgo
T = 2018-Oct-26  q = 1.95 AU   Extinct comet       Period = 13.8 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  15.4   03 48  +31 45   2.128   2.308    66M   Per
2018-08-11  15.3   04 08  +35 40   2.090   2.171    71M   Per
2018-08-21  15.1   04 28  +39 49   2.057   2.042    76M   Per
2018-08-31  15.0   04 51  +44 10   2.027   1.921    81M   Aur

(3552) Don Quixote
T = 2018-May-07  q = 1.24 AU   Extinct comet       Period =  8.8 yr
Date         Mag    R.A.   Decl.     r       d    Elong  const
2018-08-01  16.2   02 44  +29 03   1.637   1.450    81M   Ari
2018-08-11  16.2   02 53  +33 37   1.714   1.433    87M   Per
2018-08-21  16.2   02 59  +37 59   1.794   1.416    93M   Per
2018-08-31  16.3   03 01  +42 08   1.876   1.402   100M   Per

As always, the Comet Section is happy to receive all comet observations, whether images, drawings, magnitude estimates, and even spectra. Please send your observations via email to < carl.hergenrother @ >.
- Carl Hergenrother (ALPO Comet Section Coordinator)

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