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ALPO Comet News for July 2021

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#1 Carl H.

Carl H.

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Posted 09 July 2021 - 01:49 AM

A Publication of the Comets Section of the Association of Lunar and Planetary Observers
By Carl Hergenrother


The monthly Association of Lunar and Planetary Observers (ALPO) Comet News PDF can be found on the ALPO Comets Section website @ http://www.alpo-astr....org/cometblog/. A shorter version of this report is posted here (minus magnitude estimates and figures). The ALPO Comet Section welcomes all comet related observations, whether textual descriptions, images, drawings, magnitude estimates, or spectra. You do not have to be a member of ALPO to submit material, though membership is encouraged. To learn more about the ALPO, please visit us @ http://www.alpo-astronomy.org. We can also be reached at < comets @ alpo-astronomy.org >.



Sorry for the later than usual report.


We are still in the midst of a bright comet drought. The brightest comets of July should be around magnitude 10.0. These include two comets that ranked among the “brightest” last month, C/2020 T2 (Palomar) and 7P/Pons-Winnecke. Joining them around the magnitude 10.0 level will be another short-period comet, 15P/Finlay, which should reach its brightest at the end of July into early August. Between 11th and 13th magnitude are a number of other comets such as 4P/Faye, 8P/Tuttle, C/2017 K2 (PANSTARRS), and C/2019 L3 (ATLAS). Imagers are encouraged to continue monitoring inbound C/2021 A1 (Leonard). While this comet still has the potential to be a notable object at the end of the year, that hope is fading as the comet has been slow to brighten. Among newly discovered objects, C/2014 UN271 (Bernardinelli-Bernstein) ranks as one of the most interesting discoveries in some time. This object is still 10 years away from an extremely large perihelion distance of 10.95 au. Bernardinelli-Bernstein has already been imaged going back to 2014 and it is possible modestly equipped imagers may be able to follow it as an active object for another 20 years!


Comets Section News

Since June 1, the ALPO Comets Section received 15 images and/or sketches from John Chumack, Michel Deconinck, Jim Filipski, Carl Hergenrother, Martin, Mobberley, Mike Olason, John D. Sabia, and Tenho Tuomi of the following comets: 4P/Faye, 6P/d’Arrest, 7P/Pons-Winnecke, 8P/Tuttle, 15P/Finlay, 57P/du Toit-Neujmin-Delporte, 108P/Schuster, 246P/NEAT, C/2020 J1 (SONEAR), C/2020 T2 (Palomar), C/2021 A1 (Leonard), C/2021 D2 (ATLAS), P/2021 J3 (ATLAS), and an oldie but goodie of C/1975 V1 (West).


Also since June 1, the Section has received 58 magnitude measurements from Michel Deconinck, J. J. Gonzalez, Carl Hergenrother, and Chris Wyatt of comets 7P/Pons-Winnecke, 10P/Tempel, 15P/Finlay, 67P/Churyumov-Gerasimenko, 117P/Helin-Roman-Alu, 246P/NEAT, C/2017 K2 (PANSTARRS), C/2018 U1 (Lemmon), C/2019 F1 (ATLAS-Africano), C/2019 L3 (ATLAS), C/2019 T4 (ATLAS), C/2020 F5 (MASTER), C/2020 J1 (SONEAR), C/2020 R4 (ATLAS), C/2020 S3 (Erasmus), C/2020 T2 (Palomar), and C/2021 A1 (Leonard).


In addition to observations submitted to the ALPO, we also occasionally use data submitted to other sources for our analysis. We acknowledge with thanks the comet observations from the International Comet Quarterly, the Minor Planet Center, the COBS Comet Observation Database, and our own ALPO contributors used in this report.


Comets Calendar for July 2021
July 01 – Last Quarter Moon
July 05 – C/2020 T4 (PANSTARRS) at perihelion (q = 2.19 au, V ~ 17)
July 09 – New Moon
July 10 – 252P/LINEAR at perihelion (q = 1.00 au, V ~ ???, reached ~4th mag in 2016 during an
especially close approach to Earth at 0.036 au, 252P may have been abnormally active during the 2016 return, the current return is poor and located at low elongations, comet may be anywhere between mag 12 and 19 depending on activity level, lack of recent observations suggests 252P has been faint).
July 11 – C/2020 T2 (Palomar) at perihelion (q = 2.05 au, V ~ 10, more below)
July 13 – 15P/Finlay at perihelion (q = 0.99 au, 6.6-year orbit, V ~ 10, more below)
July 15 – C/2021 A7 (NEOWISE) at perihelion (q = 1.97 au, V ~ 14)
July 17 – First Quarter Moon
July 17 – P/2020 V4 (Rankin) at perihelion (q = 5.15 au, 29-year orbit, V ~ 18, low elongation)
July 20 – 413P/Larson at perihelion (q = 2.14 au, 7.2-year orbit, V ~ 17)
July 22 – C/2021 G1 (Leonard) at perihelion (q = 3.42 au, V ~ 21)
July 23 – Full Moon
July 25 – 241P/LINEAR at perihelion (q = 1.92 au, V ~ 17)
July 27 – D/1977 C1 (Skiff-Kosai) at perihelion (q = 2.80 au, 7.5-year orbit, V ~ ?, only seen at
in 1977)
July 31 – Last Quarter Moon


Bright Comets (magnitude < 10.0)
None, though C/2020 T2 (Palomar) is straddling the 10.0 magnitude level with some observers placing it slightly brighter than magnitude 10.0.


Fainter Comets of Interest (generally magnitude 10.0 to 13.0)


C/2020 T2 (Palomar)


Discovered 2020 October 7 at 19th magnitude by the Zwicky Transient Facility (ZTF)

Discovery Telescope: 1.2-m Samuel Oschin Schmidt on Mount Palomar
Perihelion on 2021 July 11 at 2.05 au, inclination = 27.9 deg, eccentricity = 0.9935
Dynamically old long-period comet with orbital period of ~5600 years

C/2020 T2 (Palomar)                                               Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  13 50  +10 02   2.058   1.591   102E   Boo  10.1    47   40
2021-Jul-06  13 55  +07 44   2.056   1.626    99E   Boo  10.2    43   42
2021-Jul-11  14 00  +05 27   2.055   1.665    97E   Vir  10.2    40   45
2021-Jul-16  14 05  +03 11   2.056   1.707    94E   Vir  10.3    37   47
2021-Jul-21  14 12  +00 57   2.058   1.751    92E   Vir  10.4    34   49
2021-Jul-26  14 18  -01 14   2.062   1.798    89E   Vir  10.4    31   51
2021-Jul-31  14 25  -03 22   2.068   1.847    87E   Vir  10.5    29   52
2021-Aug-05  14 32  -05 27   2.076   1.899    85E   Vir  10.6    26   53
Comet Magnitude Parameters --- H = 6.0, 2.5n = 10.0


When C/2020 T2 (Palomar) was discovered on 2020 October 7 at 19th magnitude and 4.4 au from the Sun, it was only expected to brighten to about 14th magnitude at perihelion. Thanks to a rapid rate of brightening, the comet is currently near a peak of 9-10th magnitude.


This month Palomar comes to perihelion on the 11th at 2.05 au. Due to an increasing Earth-comet distance (1.59 to 1.85 au), the comet may start to slowly fade this month as it moves through Boötes (July 1-6) and Virgo (6-31). It is well placed for observation in the evening sky from both hemispheres.
Last month, visual and CCD observations by Michel Deconinck, J. J. Gonzalez, Carl Hergenrother, and Chris Wyatt found Palomar between magnitude 9.9 and 10.6 with a visual coma diameter between 3.8’ and 6’ and CCD diameter as large as 14’. After applying an aperture correction to the data by normalizing the aperture as described in [Morris, C. S. “On Aperture Corrections for Comet Magnitude Estimates”. Publ Astron Soc Pac 85, 470 (1973)], the corrected visual magnitudes fell into a tighter range of 9.7 to 10.2. CCD and aperture corrected visual magnitude measurements submitted to the ALPO have been consistent with CCD magnitudes submitted by Michael Lehmann to the COBS site. A combination of the ALPO and Lehmann observations going back to the start of 2021 can be well fit by the following photometric parameters: H0 = 0.5 and 2.5n = 27.0.


Plotting the same with the addition of pre-2021 CCD photometry submitted to the Minor Planet Center extends the lightcurve back to 2019 December 11, which is 7 months prior to discovery thanks to the large number of pre-discovery observations. The addition of this older data shows the comet was brightening at a 2.5n ~ 12 rate from December 2019 till the start of 2021 when it started to brighten at the quicker 2.5n ~ 27 rate found above. The comet was around 3.0 au from the Sun at the time of the increase in brightening. For July, my brightness prediction assumes a 2.5n ~ 10 rate. If the comet’s lightcurve continues to follow the 2.5n ~ 27 rate after perihelion, it would fade quicker than shown in the ephemeris above.




Discovered on 1819 June 12 by the Jean-Luis Pons
Rediscovered on 1858 March 9 by Friedrich August Theodor Winnecke
Perihelion on 2021 May 27 at 1.23 au, inclination = 22.4 deg, eccentricity = 0.64
Short-period comet with orbital period of ~6.31 years

7P/Pons-Winnecke                                                  Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  23 18  -31 53   1.307   0.467   118M   Scl  10.8    12   82
2021-Jul-06  23 31  -34 54   1.328   0.482   120M   Scl  11.0    10   85
2021-Jul-11  23 42  -37 43   1.351   0.499   122M   Scl  11.2     9   88
2021-Jul-16  23 51  -40 18   1.376   0.518   124M   Phe  11.5     7   90
2021-Jul-21  23 58  -42 40   1.403   0.540   126M   Phe  11.8     6   87
2021-Jul-26  00 03  -44 47   1.432   0.563   127M   Phe  12.1     5   85
2021-Jul-31  00 06  -46 39   1.462   0.589   129M   Phe  12.4     3   83
2021-Aug-05  00 07  -48 15   1.493   0.618   130M   Phe  12.8     2   82
Comet Magnitude Parameters --- H =  9.7, 2.5n =  26.6, offset = +11 days


Comet 7P/Pons-Winnecke was an 8th magnitude object when visually discovered by Jean-Luis Pons on 1819 June 12. At the time, a short-period orbit was calculated for the object. Unfortunately, the orbit was not precise enough to support the comet’s recovery and the comet would have to be re-discovered. That re-discovery occurred thirty-nine years later on 1858 March 9 by Friedrich August Theodor Winnecke. Throughout the 19th and early 20th century, Pons-Winnecke routinely reached 6th magnitude during its better placed apparitions. In 1927 during an especially close approach to Earth (0.04 au), the comet peaked at magnitude 3.5. Unfortunately, it hasn’t had a bright return since 1939 (6th magnitude) and nowadays usually gets no brighter than ~10-11th magnitude. The recent drought of bright apparitions is due to an increase in perihelion distance from 0.76 au in 1841 to a maximum of 1.26 au in 1989. This year’s perihelion occurred on 2021 May 27 at 1.23 au as 7P is still close to its maximum perihelion distance.


The perihelion distance of Pons-Winnecke will steadily decrease over the coming decades from this year’s 1.23 au to 1.13 au in 2027, 0.98 au in 2039, 0.87 au in 2051, and a minimum near 0.84 au for a large number of perihelia from 2062 through the end of the 21st century. The smaller distances will once again allow close approaches to Earth, in particular, approaches in 2045 (0.21 au), 2062 (0.17 au), 2073 (0.19 au), and 2084 (0.31 au). Check out Kazuo Kinoshita’s Comet Orbit Page entry for 7P for more details on 7P’s past and future orbital evolution at http://jcometobs.web...pcmtn/0007p.htm.


Two visual and three CCD magnitude estimates were submitted to the Comets Section. J. J. Gonzalez visually observed a very diffuse coma as large as 4-5’ in diameter. CCD measurements in June by Carl Hergenrother found a similar brightness to J. J.’s visual measurements but also a significantly larger coma (up to 17.5’ in diameter).


Pons-Winnecke has likely peaked in brightness for this apparition. With increasing heliocentric and geocentric distances, the comet should fade from around magnitude 10.8 to 12.4 as it moves through the morning constellations of Sculptor (July 1-14) and Phoenix (14-31). Its location at southern declinations makes it a difficult object from the northern hemisphere but well placed for southern hemisphere observers.




Discovered on 1843 November 23 by the Herve Faye
Perihelion on 2021 September 9 at 1.62 au, inclination = 8.0 deg, eccentricity = 0.58
Short-period comet with orbital period of ~7.48 years

4P/Faye                                                           Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  01 57  +13 46   1.769   1.900    66M   Ari  12.4    23   32
2021-Jul-06  02 10  +14 37   1.750   1.846    68M   Ari  12.2    25   31
2021-Jul-11  02 24  +15 24   1.731   1.795    69M   Ari  12.0    28   31
2021-Jul-16  02 37  +16 07   1.714   1.745    71M   Ari  11.8    31   31
2021-Jul-21  02 51  +16 47   1.699   1.696    72M   Ari  11.6    33   30
2021-Jul-26  03 05  +17 21   1.684   1.649    74M   Ari  11.5    36   30
2021-Jul-31  03 19  +17 51   1.671   1.604    75M   Ari  11.3    39   30
2021-Aug-05  03 33  +18 16   1.659   1.560    77M   Tau  11.2    41   29
Comet Magnitude Parameters --- H =  3.7, 2.5n =  29.5


Comet 4P was discovered visually by Herve Faye on 1843 November 23 at 5th-6th magnitude. This year’s apparition is the 22nd observed return with the comet only having been missed at perihelia in 1903 and 1918.  At its best returns, as in 1991 and 2006, Faye reached 9th magnitude. 2021 is a moderately good but not great return for Faye and should see it reach magnitude 10.3 at the end of September.

While no observations of 4P/Faye have been submitted to the ALPO in June, three observations submitted to the COBS site by Michael Lehmann showed the comet brightening from 13.4 to 12.6 between June 6 and 22. As with all of Michael’s observations, these are CCD measurements. He also observed the coma increasing in diameter from 4.2’ to 6.2’.


Perihelion occurs on September 8 at 1.62 au followed by a minimum distance to Earth a few months later on December 5 at 0.94 au. Faye is currently a morning object observable from both hemispheres and should brighten from around magnitude 12.4 to 11.2 as its moves through Aries.




Discovered on 1790 January 9 by Pierre F. A. Mechain
Rediscovered on 1858 January 5 by Horace Tuttle
Perihelion on 2021 August 27 at 1.03 au, inclination = 54.9 deg, eccentricity = 0.82
Short-period comet with orbital period of ~13.6 years

8P/Tuttle                                                         Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  05 24  +39 08   1.339   2.220    22M   Aur  12.7     4    0 
2021-Jul-06  05 43  +37 23   1.295   2.180    22M   Aur  12.4     3    0
2021-Jul-11  06 02  +35 25   1.252   2.140    21M   Aur  12.1     3    0
2021-Jul-16  06 20  +33 13   1.212   2.100    21M   Aur  11.7     2    0
2021-Jul-21  06 37  +30 47   1.175   2.062    21M   Aur  11.4     2    0
2021-Jul-26  06 55  +28 07   1.141   2.024    21M   Gem  11.1     1    0
2021-Jul-31  07 12  +25 14   1.110   1.989    21M   Gem  10.7     0    0
2021-Aug-05  07 28  +22 08   1.083   1.955    22M   Gem  10.4     0    0
Comet Magnitude Parameters --- H =  7.0, 2.5n =  20.0, offset = +25 days


Similar to the discovery story of Pons-Winnecke, 8P/Tuttle was discovered during two widely separated apparitions. Pierre François André Méchain made the first discovery in January 1790. Sixty-eight years later, 8P was re-discovered by Horace Parnell Tuttle in January 1858. With a 13.6-year period, 8P/Tuttle is making its 13th observed return. Tuttle’s relatively large semi-major axis of 5.7 au and inclination of 54.9° makes it a Halley-type rather than a Jupiter-family comet.


The current return is relatively poor with Tuttle arriving at perihelion nearly on the opposite side of the Sun from Earth. Even with such poor placement and a minimum distance to Earth on September 12 of 1.81 au, Tuttle is expected to brighten to magnitude 8.5 in September.


It will be very difficult to observe Tuttle this month as it will be at a low elongation all month (21-22 deg). The comet will become better placed by late August though that is only true for southern hemisphere observers. Tuttle’s large inclination results in it spending most of its time post-perihelion far south of the ecliptic, as a result the comet won’t be visible to northern observers until it has faded and become a difficult CCD target.




Discovered 1886 September 26 by the William Henry Finlay
Perihelion on 2021 July 13 at 0.99 au, inclination = 6.8 deg, eccentricity = 0.72
Short-period comet with orbital period of ~6.56 years

15P/Finlay                                                        Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  02 44  +12 51   1.009   1.109    56M   Ari  10.5    13   27
2021-Jul-06  03 07  +15 13   0.998   1.127    55M   Ari  10.3    15   25
2021-Jul-11  03 30  +17 22   0.993   1.148    54M   Tau  10.2    16   22
2021-Jul-16  03 52  +19 17   0.992   1.173    53M   Tau  10.1    18   20
2021-Jul-21  04 14  +20 57   0.998   1.199    52M   Tau  10.1    20   18
2021-Jul-26  04 35  +22 24   1.008   1.227    52M   Tau  10.0    22   16
2021-Jul-31  04 56  +23 36   1.024   1.256    52M   Tau  10.0    24   15
2021-Aug-05  05 16  +24 36   1.044   1.286    52M   Tau   9.9    25   13
Comet Magnitude Parameters --- H =  9.6, 2.5n =  15.7, offset = +20 days


15P/Finlay was discovered in 1886 by William Henry Finlay at the Royal Observatory at Cape of Good Hope in South Africa. This apparition marks the 16th observed return of 15P. Its best return was in 1906 when it passed 0.27 au from Earth and reached 6th magnitude. During its previous return in 2014/2015, 15P experienced two outbursts of 2-3 mag outburst with the brightest reaching 7th magnitude.


Chris Wyatt visually observed 15P at magnitude 11.1 on July 6 with his 0.40-m f/4 reflector. The comet had a moderately condensed 2.9’ coma. CCD observations by Michael Lehmann and Jose Chambo were submitted to the COBS site and found the comet brightening to magnitude 10.9 by June 24. Aperture correcting Chris’ July 6 estimate finds the comet may have been as bright as 10.5 on July 6 which closely matches the ephemeris prediction above (the photometric parameters used above are from Seiichi Yoshida).


Perihelion occurs this month on 13th at 0.99 au. The comet should continue to brighten to a maximum close to 9.9 in early August. The comet is currently a morning object in Aries (July 1-9) and Taurus (9-31).




Discovered 2017 May 21 by the Pan-STARRS survey with the Pan-STARRS1 1.8-m on Haleakala
Perihelion on 2022 December 19 at 1.80 au, inclination = 87.6 deg, eccentricity = 1.00008
Dynamically old long-period comet

C/2017 K2 (PANSTARRS)                                             Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  17 33  +42 33   5.983   5.519   112E   Her  13.2    87    7
2021-Jul-06  17 28  +42 06   5.942   5.489   111E   Her  13.2    88    8
2021-Jul-11  17 24  +41 34   5.900   5.462   110E   Her  13.1    88    8
2021-Jul-16  17 20  +40 58   5.858   5.439   109E   Her  13.1    89    9
2021-Jul-21  17 16  +40 18   5.816   5.420   108E   Her  13.1    90   10
2021-Jul-26  17 12  +39 33   5.774   5.405   106E   Her  13.1    88   10
2021-Jul-31  17 09  +38 45   5.732   5.393   104E   Her  13.0    85   11
2021-Aug-05  17 06  +37 53   5.690   5.385   102E   Her  13.0    82   12
Comet Magnitude Parameters --- H =  3.6, 2.5n =  7.6


Get ready to hear about this comet every month in these pages for quite some time. C/2017 K2 (PANSTARRS) was discovered on 2017 May 21 by the Pan-STARRS1 1.8-m on Haleakala. At discovery the comet was around 21st magnitude and located at once seemed like an extreme 16.1 au from the Sun (that is until the discovery of C/Bernardinelli-Bernstein, more on that one later). Pre-discovery observations were found back to May of 2013 when the comet was 23.7 au from the Sun. For comparison Uranus has a semi-major axis of 19.2 au.


C/2017 K2 (PANSTARRS) is about one and a half years out from its 2022 December 19 perihelion at 1.80 au when it should peak around 7th magnitude (if its current 2.5n ~ 7.6 brightening trend continues which it probably won’t). Over the past few weeks, Michel Deconinck, J. J. Gonzalez, and Chris Wyatt have observed C/2017 K2 between magnitude 12.2 and 13.5. With the PANSTARRS still over 5 au from the Sun and Earth, it is a small object with all three visual observers reporting a small moderately condensed coma of up to 1.5’ in diameter.


C/2017 K2 is a northern object in Hercules though still observable from the southern hemisphere as shown by Chris Wyatt’s observations from Australia. The comet will continue to slowly brighten throughout the remainder of 2021 and all of 2022.


The comet has already been the subject of intense investigation with no less than 8 papers already published on it as well as dedicated HST observing campaigns. We’ll talk about some of these results in the months to come.


C/2019 L3 (ATLAS)


Discovered 2017 May 21 by the Pan-STARRS survey with the Pan-STARRS1 1.8-m on Haleakala
Perihelion on 2022 January 9 at 3.55 au, inclination = 48.4 deg, eccentricity = 1.0016
Dynamically new long-period comet

C/2019 L3 (ATLAS)                                             Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  05 14  +49 17   3.959   4.795    31M   Aur  12.2    13    0
2021-Jul-06  05 23  +49 04   3.939   4.759    32M   Aur  12.2    15    0
2021-Jul-11  05 33  +48 50   3.920   4.720    34M   Aur  12.1    16    0
2021-Jul-16  05 42  +48 35   3.902   4.677    36M   Aur  12.1    18    0
2021-Jul-21  05 51  +48 19   3.883   4.632    38M   Aur  12.0    20    0
2021-Jul-26  05 59  +48 02   3.866   4.584    40M   Aur  12.0    22    0
2021-Jul-31  06 08  +47 44   3.848   4.533    42M   Aur  11.9    25    0
2021-Aug-05  06 16  +47 25   3.831   4.479    45M   Aur  11.8    27    0
Comet Magnitude Parameters --- H = -2.4, 2.5n = 18.8


C/2019 L3 has recently passed solar conjunction as it passed north of the Sun last month. July will see the comet become an easier object to observe at least for northern observers (it won’t be visible to southern observers in July) as it moves through Auriga (18-30).


J. J. Gonzalez spied L3 at magnitude 11.2 on June 10 with a 2.5’ coma. CCD photometry submitted to the COBS site has the comet a magnitude fainter than J. J. with the CCD measurements falling in the 12.2 to 12.4 range.

C/2019 L3 arrives at perihelion on 2022 January 9 at 3.57 au. The large perihelion distance means C/2019 L3 could remain a visual object well into 2022 and possibly even 2023. If the comet brightens at a conservative 2.5n = 8 rate, it could reach magnitude 10.0 at the end of 2021.


C/2021 A1 (Leonard)


Discovered 2022 January 3 by Greg Leonard of the Catalina Sky Survey with the 1.5-m on Mount Lemmon
Perihelion on 2022 January 3 at 0.61 au, inclination = 132.7 deg, eccentricity = 1.00002
Dynamically old long-period comet

C/2021 A1 (Leonard)                                              Max El
    Date      R.A.   Decl.     r       d    Elong  Const  Mag   40N  40S
2021-Jul-01  10 28  +51 35   3.091   3.614    51E   UMa  16.8    33    0 
2021-Jul-06  10 28  +50 40   3.029   3.601    48E   UMa  16.7    30    0
2021-Jul-11  10 28  +49 45   2.966   3.582    45E   UMa  16.7    28    0
2021-Jul-16  10 29  +48 51   2.903   3.559    43E   UMa  16.6    25    0
2021-Jul-21  10 31  +47 59   2.840   3.531    40E   UMa  16.5    23    0
2021-Jul-26  10 33  +47 07   2.775   3.497    38E   UMa  16.4    21    0
2021-Jul-31  10 35  +46 17   2.711   3.458    36E   UMa  16.3    19    0
2021-Aug-05  10 37  +45 28   2.645   3.412    34E   UMa  16.2    17    0
Comet Magnitude Parameters --- H = 10.3, 2.5n = 7.6


C/2021 A1 (Leonard) was found on 2021 January 3 by Greg Leonard with the Mount Lemmon 1.5-m reflector. At discovery, it was around 19th magnitude and located 5.1 au from the Sun.


C/2021 A1 has the potential to become a nice object at the end of the year. It has a few things going for it including 1) a relatively small perihelion of 0.62 au on 2022 January 3, 2) a close approach to within 0.233 au from Earth on December 12, and 3) a phase angle that reaches a maximum of 160 degrees at the time of close approach. The high phase angle may result in a few magnitudes of enhanced brightness due to forward scattering of light by cometary dust. Working against it will be a small solar elongation at the time of maximum brightness (a minimum elongation of 15 deg). Also in the minus column is the comet’s slow rate of brightening. The comet has been brightening at a rate of 2.5n ~ 7.6. Extrapolating that into the future produces a peak brightness around magnitude 6.3. Even with 2-3 magnitudes of dust forward scattering enhancement, Leonard may be a very difficult object to observe when at its best.


C/Leonard is an evening object near 16-17th magnitude located in Ursa Major. Imagers are strongly encouraged to monitor C/2021 A1 over the coming months.


New Discoveries, Recoveries and Other Comets in the News


New Periodic Comet Numbering


421P/2009 U4 = P/2020 H10 (McNaught) [Ref: MPC 130596]


P/2021 L5 = P/2021 S2 (La Sagra) – Michael Rudenko (of the Minor Planet Center) found recovery observations of this comet in astrometry obtained with the 1.8-m Pan-STARRS1 telescope on Haleakala on 2021 June 6. Rob Weryk (University of Western Ontario was then able to go back and find pre-recovery observations back to 2020 April 26 when the comet was 23rd magnitude in April 2020. Independent of the Pan-STARRS observations, Hirohisa Sato recovered P/La Sagra on 2021 July 3 with an iTelescopes 0.43-m at Siding Spring in Australia. It was measured at 19th magnitude with a small condensed 6” coma in Sato’s images. P/La Sagra arrives at perihelion on 2021 October 31 at 1.36 au when it should be around magnitude 17. [Ref: MPEC 2021-N47, CBET 4994]


P/2021 L4 (PANSTARRS) – Richard Wainscoat (University of Hawaii) reported the discovery of a 20-21st magnitude comet by the Pan-STARRS1 1.8-m on 2021 June 14. P/2021 L4 is a Jupiter-family comet with a 5.64-year period and is already nearly two years past a 2019 September 17 perihelion at 2.75 au. Although pre-discovery observations have been found back to 2021 June 3, the lack of observations closer to perihelion suggests an object either in outburst or possessing a strong seasonal post-perihelion offset in activity. This is the 251st comet to be named after the Pan-STARRS survey. [Ref: MPEC 2021-M77, CBET 4986]


C/2021 L3 (Borisov) – Gennady Borisov has discovered the 11th comet to bear his name. Borisov used his 0.65-m f/1.5 astrograph at the MARGO observatory near Nauchnij, Crimea to find C/2021 L3 at 20th magnitude on 2021 June 8 at a far northern declination of +71 degrees. A long-period comet with a distant perihelion distance of 8.44 au (T = 2022 March 3), the newest Comet Borisov is unlikely to get much brighter than 19th magnitude. [Ref: MPEC 2021-M75, CBET 4985]


P/2021 L2 (Leonard) – Greg Leonard of the Catalina Sky Survey found his 12th named comet on 2021 June 8 at 20th magnitude with the Mount Lemmon 1.5-m. P/2021 L2 is a Jupiter-family comet with a 2021 July 24 perihelion at 1.94 au. Peak brightness is expected around 19th magnitude. [Ref: MPEC 2021-M74, CBET 4984]


P/2021 L1 = P/2006 S4 (Christensen) – C. Jacques reported the recovery of P/2006 S4 (Christensen) on images taken on 2021 June 5 and 6 by E. Pimental, J. Barros, and himself with the SONEAR 0.45-m f/2.9 reflector. The comet was 18th magnitude at recovery. P/Christensen is on a 15.9-year orbit and reaches perihelion on 2022 January 13 at 3.11 au. The comet peaked at 17-18th magnitude in 2006 and expected to reach the same brightness this summer. [Ref: MPEC 2021-M34, CBET 4979]


C/2021 K3 (Catalina) – The Catalina Sky Survey uses a number of telescopes in southern Arizona in their search for potentially hazardous near-Earth asteroids and comets. Discoveries with the 0.7-m Catalina Schmidt on Mount Bigelow north of Tucson are either named “Catalina” or after the observer responsible for the discovery. Comets named “Lemmon” were found with a 1.5-m Cassegrain on Mount Lemmon, just a bit further up the road than the Catalina Schmidt. While the Catalina Schmidt and Lemmon telescope are almost exclusively used by the Catalina Sky Survey, they also acquire time on other telescopes at the University of Arizona.


C/2021 K3 (Catalina) was discovered on 2021 May 20 at 21st magnitude with the University of Arizona’s 2.3-m Bok Cassegrain on Kitt Peak. This high-q comet comes to perihelion on 2022 February 1 at 5.23 au. Peak brightness at 20th magnitude should occur around opposition July/August 2021 and June/July 2022. This is the 62nd comet to bear the name Catalina. [Ref: MPEC 2021-M87, CBET 4991]


The Bok telescope is named after Bart J. Bok, former director of the University of Arizona’s Steward Observatory, discoverer of Bok globules, and even the focus of a biography by former ALPO Comets Section Recorder David Levy. Bok was also the discoverer of comet, C/1949 N1 (Bappu-Bok-Newkirk). The story of C/1949 N1 is interesting in its own right. The first named discoverer M. K. Vainu Bappu is considered the father of Indian astronomy. A nice write-up on the comet, its discovery, and some of the repercussions of the discovery can be found at https://iucaa-rpl.we...179300/0116.pdf.


C/2021 K2 (MASTER) – This apparently asteroidal object was discovered on 2021 May 23 with a double 0.4-m f/2.5 reflector operated by the "Mobile Astronomical System of the Telescope-Robots" or MASTER survey at the South African Astronomical Observatory. The comet was 18th magnitude at discovery and will likely peak at 17th magnitude this month as it approaches a distant q = 5.47 au perihelion on 2021 September 8. MASTER has 4 other comets named after it. [Ref: CBET 4975 & MPEC 2021-L89]


C/2021 G3 (PANSTARRS) – This 22nd magnitude large-q long-period comet was discovered on 2021 April 1 with the Pan-STARRS1 1.8-m on Haleakala. Perihelion will be on 2021 October 18 at 5.18 au. The comet is not expected to become much brighter than 21st magnitude. [Ref: 2021-M86, CBET 4990]


A/2021 G2 - An apparently asteroidal object was discovered on 2021 April 11 at 19th magnitude with the ATLAS 0.5-m f/2 astrograph on Mauna Loa. Pre-discovery observations have been found back to November 2020. The object is currently 9.7 au from Sun and over 3 years from its. 2024 September 8 perihelion at 4.98 au. CBET 4988 reports that multiple observers have detected a coma making A/2021 G2 an active comet. It will presumably be redesignated C/2021 G2 (ATLAS). [Ref: MPEC 2021-M79, CBET 4988]


A/2021 E4 – This is yet another apparently asteroidal object on a cometary orbit. Discovered on 2021 March 7 with the Mount Lemmon 1.5-m, A/2021 E4 is currently 21st magnitude and 5.2 au from the Sun. Pre-discovery observations were found back to December 2020. Perihelion is on 2022 April 25 at 4.68 au. The object should peak at a little brighter than magnitude 19.0. [Ref: MPEC 2021-M78]


P/2021 A12 = P/2008 CL94 (Lemmon) - A team led by University of Central Florida comet researchers Charles Schambeau and Yan Fernandez recovered P/2008 CL94 at a faint 22nd magnitude with the 8-m Gemini North telescope on Mauna Kea. Observations made on 2021 January 9 and 15 showed a very condensed object with a slight tailward extension. When originally discovered on 2008 February 8 by the Mount Lemmon Survey, the comet was 20th magnitude and ~1.5 years passed perihelion. Observations a year later in 2009 found a brighter 18-19th magnitude object. The current return sees perihelion on 2021 September 19 at 5.42 au. [Ref: 2021-M89, CBET 4992]


C/2014 UN271 (Bernardinelli-Bernstein) – On June 19, the Minor Planet Center announced the discovery of asteroid 2014 UN271 on Minor Planet Electronic Circular 2021-M52. The discovery by Pedro Bernardinelli and Gary Bernstein was the result of a search for slow moving Trans-Neptunian objects (TNO) in images taken with the Dark Energy Camera (DECam) on the Blanco 4-m at Cerro Tololo Inter-American Observatory in Chile. DECam observed UN271 on 20 separate nights between 2014 October 20 and 2018 November 8. Additional observations by Pan-STARRS and follow up astrometrists have extended the arc to 7 years (from 2014 August 14 to 2021 June 22). The resulting orbit on CBET 4989 shows a truly remarkable object. At discovery, it was 22nd magnitude and located at 29.0 au from the Sun. For comparison, the average distance to the Sun for Neptune is 30.1 au. On 2021 July 1, it will still be 20.1 au from the Sun, a little further away than Uranus’ average distance.


Though originally announced as an asteroid, recent observations and new analysis of older observations confirmed the cometary nature of this object. In ATel 14759, Tony Farnham (U. of Maryland) was able to co-add 976 30-minute exposures (over 20 days! of exposure!) taken with the TESS spacecraft in September and October 2018 to detect cometary activity at 23.8 au. It is very possible that the comet was active at even larger distances, something that is not as unprecedented as you might think considering the above-mentioned C/2017 K2 (PANSTARRS) may have been active as far out as 35 au [Jewitt, D. et al. Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2. Astronomical J 161, 188 (2021)] and C/2012 S2 (ISON) at 20 au [Sekanina, Z. Comet C/2012 S1 (ISON), CBET 3722, eds. D. Green].


Perihelion is still an astonishing 10 years away! When it arrives at perihelion on 2031 January 22, it will be 10.95 au from the Sun, comparable in distance to Saturn (9.6 au). C/Bernardinelli-Bernstein is a long-period comet with an original semi-major axis that is slightly hyperbolic suggesting it is on its first trip this “close” to the Sun.

As is usual for such an interesting object, there have been a lot of articles published in the media about Bernardinelli-Bernstein. Most focus on the size of the nucleus. When the object was still considered asteroidal, one could estimate a nuclear size between ~60 and 200 km depending on its albedo. Now that we know it is cometary and some (perhaps significant) fraction of its brightness is due to a dust coma, the diameter of the nucleus is probably being overestimated. Hopefully a repaired Hubble or soon-to-be-launched JWST will shed some light on the true size of Bernardinelli-Bernstein.


C/Bernardinelli-Bernstein is currently located at a southern declination of -51 deg at ~19th magnitude. Its inclination of 95 deg will carry it further south through 2027. Though still at a declination of -38 deg at perihelion, it will be moving north at that time and become a progressively easier object to observe from the northern hemisphere in the years after perihelion. How bright this comet will get is an open question and one we can spend the next 10 years discussing. If we use the magnitudes reported to the MPC in June as a starting point and assume 2.5n parameters, here is what we get for peak brightness:


2.5n Peak

 8.0 15.9
10.0 15.5
15.0 14.1
20.0 12.8


It is possible that Bernardinelli-Bernstein could become a visual object for observers with modestly large apertures. Time will tell. [Ref: MPEC 2021-M52, 2021-M83, CBET 4983, 4989]



As always, the Comet Section is happy to receive all comet observations, whether textual descriptions, images, drawings, magnitude estimates, or spectra. Please send your observations via email to the Comets Section < comets @ alpo-astronomy .org >, Comets Section Coordinator Carl Hergenrother < carl.hergenrother @ alpo-astronomy .org > and/or Comets Section Acting Assistant Coordinator Michel Deconinck < michel.deconinck @ alpo-astronomy .org >.


Thank you to everyone who contributed to the ALPO Comets Section!

Stay safe and enjoy the sky!

- Carl Hergenrother

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#2 Zorbathegeek


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Posted 09 July 2021 - 05:32 PM

I notice that M. Masek has just estimated C/2014 UN271 to be 17.4 and 17.7 magnitude. 



#3 Rich (RLTYS)

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Posted 10 July 2021 - 03:48 PM

Once again another great report! waytogo.gif

#4 RNSpeed


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Posted 12 July 2021 - 02:32 PM

Wow, lots of interesting information. Thanks for posting it

#5 Tapio


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Posted 12 July 2021 - 03:12 PM

Too bad C/2014 UN271 is so south now. Otherwise would have imaged it.

#6 Carl H.

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Posted 12 July 2021 - 08:17 PM

I tried to image C/2014 UN271 with some of the iTelescopes in Australia but am currently batting 0-for-3. I was able to get a shot of 19P/Borrelly which should become a nice 8-9th magnitude object in January. I measured it at V = 18.1 with a 0.4' coma and 0.3' tail.



Edited by Carl H., 12 July 2021 - 08:17 PM.

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#7 Redbetter


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Posted 13 July 2021 - 11:42 PM

I finally got a look at 15P/Finlay on Monday morning.  I had to wait for it to clear some trees but caught it just before the sky brightened too much.  I observed it at 156x with the 20" and some with the 110 refractor at 48 and 86x.  With it so low and the sky already beginning to brighten somewhat, I could not determine the shape of the coma and extent well.  The coma was of reasonable surface brightness with brighter center, but without a stellar nucleus.   I rated it as "medium" size overall, larger than ~4 arc minutes.  It was probably brighter than 11th magnitude from what I saw, perhaps 10.  


I was getting some indication of some elongation and possible fanning, but it wasn't strong enough for me to sketch it or to determine orientations...I was just glad to be seeing the comet at all.  I had no idea what to expect and haven't seen any recent images of it.

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#8 Carl H.

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Posted 25 July 2021 - 01:03 PM

New Bright (but hard to observe) Comet!


Hideo Nishimura of Gansho-ji, Kakegawa, Japan, has discovered a 9-10th magnitude comet at an amazingly small solar elongation of 23 degrees. Nishimura found the comet on July 21 with a Canon EOS 6D digital camera and 200-mm f/3.2 lens. At discovery, the new comet was located ~2 degrees from 8P/Tuttle. This is Nishimura's 2nd discovery. His first was the visual discovery C/1994 N1 (Nakamura-Nishimura-Machholz). 


C/2021 O1 (Nishimura) is a long-period comet with perihelion on August 12 at 0.80 au. The comet is unfortunately poorly placed for its entire apparition (at least when it is expected to be reasonably bright). It might have been observable from the southern hemisphere in June but was at a low elevation of ~7 deg at the start of astronomical twilight and around 12th magnitude.  Nishimura discovered the comet during a short window of opportunity for northern observers when it was visible at only a degree or three above the horizon before the start of astronomical twilight. Solar elongation is slowly shrinking to a minimum of 6 degrees in mid-October. It will once again become visible to northern observers in December though the comet will likely be very faint by then. Southern observers will have to wait till January.


[Ref: CBET 5004, MPEC 2021-O47]

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#9 Zorbathegeek


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Posted 25 July 2021 - 07:33 PM

Congratulations to Hideo, that's a pretty impressive feat. 



#10 Aquarellia



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Posted 28 July 2021 - 02:54 AM

New Bright (but hard to observe) Comet!


Hideo Nishimura of Gansho-ji, Kakegawa, Japan, has discovered a 9-10th magnitude comet at an amazingly small solar elongation of 23 degrees. Nishimura found the comet on July 21 with a Canon EOS 6D digital camera and 200-mm f/3.2 lens. At discovery, the new comet was located ~2 degrees from 8P/Tuttle. This is Nishimura's 2nd discovery. His first was the visual discovery C/1994 N1 (Nakamura-Nishimura-Machholz). ./...

Indeed, very difficult to observe

I'm just trying to catch this new comet this morning during 1 full hour with the Moon and before dusk with my 125mm bino. I may have seen it 2 or 3 times with averted vision but really no certainty.



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#11 Carl H.

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Posted 29 July 2021 - 01:25 PM

Mike Olason was able to image C/2021 O1 (Nishimura) from the Tucson area.



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#12 TheUser


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Posted 29 July 2021 - 02:01 PM

is there 63" aperture?

#13 Carl H.

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Posted 30 July 2021 - 03:17 PM

is there 63" aperture?

The aperture Mike quoted in his image is the photometric aperture. In other words, his brightness measurement was made within a 63 arc second circular aperture centered on the comet. The telescope used was a C-11 SCT.


It is a little confusing since in astronomy the word aperture can have multiple meanings.

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#14 Octans


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Posted 30 July 2021 - 06:22 PM

Another potentially interesting comet for the Northern Hemisphere on the PCCP with perihelion of ~0.3 au next year: https://cneos.jpl.na.../object/P11ibiE

Good chance it disintegrates on the way in, particularly if it turns out to be small and dynamically new as most of these do, but if it's big enough to survive, observing conditions will be very favorable next May following perihelion when the dust tail should be most prominent. If so, the comet should be at least mag 5 around that time as it emerges out of the evening twilight, and could look comparable to or better than C/2014 Q1 from a few years ago, which was likely just big enough to survive at a similar perihelion distance, with somewhat worse observing geometry: https://apod.nasa.go...d/ap150723.html

#15 Octans


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Posted 01 August 2021 - 07:03 PM

Now officially C/2021 O3 (PANSTARRS): https://www.minorpla...K21/K21P05.html

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#16 Carl H.

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Posted 05 August 2021 - 06:09 PM

The August 2021 ALPO Comet News forum is now up at https://www.cloudyni...or-august-2021/.

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

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