ALPO COMET NEWS FOR JULY 2023
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 the magnitude estimates, images, and other figures contained in the full PDF). The ALPO Comets 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 >.
Summary
July will be a nice month for comet watchers. While there are no “bright” comets, three comets should make nice targets for binoculars or small telescopes. The brightest comet of the month, 7th magnitude C/2021 T4 (Lemmon), will be well placed for southern hemisphere observers as it races from the morning into the evening sky. Northern observers will be able to observe it early in the month though a bright Moon and a decreasing elevation will make it a bit challenging. Northern observers will have 9th magnitude C/2023 E1 (ATLAS) all to themselves as it moves through the northern circumpolar sky. Both hemispheres can observe 9-10th magnitude C/2020 V2 (ZTF) in the morning sky. Imagers are asked to watch one of next year’s potential bright objects, C/2023 A3 (Tsuchinshan-ATLAS), at 16th magnitude in the evening sky.
Last month the ALPO Comets Section received 99 magnitude estimates, images, and sketches of comets C/2023 E1 (ATLAS), C/2023 A3 (Tsuchishan-ATLAS), C/2022 E3 (ATLAS), C/2022 A2 (PANSTARRS), C/2021 T4 (Lemmon), C/2021 S3 (PANSTARRS), C/2020 V2 (ZTF), C/2020 K1 (PANSTARRS), C/2019 U5 (PANSTARRS), C/2019 T4 (ATLAS), C/2019 L3 (ATLAS), C/2017 K2 (PANSTARRS), 364P/PANSTARRS, 276P/Vorobjov, 237P/LINEAR, 126P/IRAS, 77P/Longmore, 71P/Clark, and 12P/Pons-Brooks. A big thanks to our June contributors: J. J. Gonzalez, Jose Guilherme de Souza Aguiar, Christian Harder, Carl Hergenrother, Eliot Herman, John Maikner, Uwe Pilz, Michael Rosolina, and Chris Wyatt.
Request for Observations
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 >.
Photometric Corrections to Magnitude Measurements
We include up-to-date lightcurves for the comets discussed in these reports and apply aperture and personal corrections to the visual observations and only personal corrections to digital observations. All magnitude estimates are affected by many factors, including instrumental (aperture, focal length, magnification, type of optics), environmental (sky brightness due to moonlight, light pollution, twilight, aurora activity, zodiacal light, etc.), cometary (degree of condensation, coma color, strength and type of gas emission lines, coma-tail interface) and personal (sensitivity to different wavelengths, personal technique, observational biases). The first correction used here corrects for differences in aperture [Charles S. Morris, On Aperture Corrections for Comet Magnitude Estimates. Publ Astron Soc Pac 85, 470, 1973]. Visual observations are corrected to a standard aperture of 6.78 cm by 0.019 magnitudes per centimeter for reflectors and 0.066 magnitudes per centimeter for refractors. After applying the aperture correction and if a sufficient number of visual observations are submitted for a particular comet, we also determine personal corrections for each observer for each comet; for digital observations, only a personal correction is applied. A single observer submitting both visual and digital magnitude measurements may also have separate corrections for each observing method. If the magnitudes shown in the text don’t match those plotted in the lightcurves, it is because of the application of these corrections.
Acknowledgments
In addition to observations submitted directly to the ALPO, we occasionally use data from other sources to augment our analysis. Therefore, we acknowledge with thanks observations submitted directly to the ALPO and those submitted initially to the International Comet Quarterly, Minor Planet Center, and COBS Comet Observation Database. In particular, we have been using observations submitted to the COBS site by Thomas Lehmann for our analysis, so we would like to thank Thomas for his COBS observations. We would also like to thank the Jet Propulsion Laboratory for making their Small-Body Browser and Orbit Visualizer available and Seiichi Yoshida for his Comets for Windows programs that produced the lightcurves and orbit diagrams in these pages. And last but not least, we’d like to thank Syuichi Nakano and the Minor Planet Center for their comet orbit elements, the asteroid surveys and dedicated comet hunters for their discoveries, and all of the observers who volunteer their time to add to our knowledge of these fantastic objects.
Thank you to everyone who contributed to the ALPO Comets Section!
Clear skies!
- Carl Hergenrother
Comets Calendar
Lunar Phases
July 03 - Full Moon
July 09 - Last Quarter Moon
July 17 - New Moon
July 25 - First Quarter Moon
Comets at Perihelion
Jul 01 - C/2023 E1 (ATLAS) [q = 1.03 au, 85-yr period, V ~ 9, more below]
Jul 05 - 287P/Christensen [q = 3.03 au, 8.5-yr period, V ~ 16, found in 2006, 3rd observed return]
Jul 05 - 126P/IRAS [1.71 au, 9.5-yr period, V ~ 13, discovered by IRAS spacecraft in 1983, also seen at 1996 and 2010 returns, not yet seen at current return]
Jul 08 - P/2012 WA34 (Lemmon-PANSTARRS) [q = 3.07 au, 10.1-yr period, V ~ 21, this will be its first return since discovery, close to Sun at perihelion]
Jul 11 - 39P/Oterma [q = 5.71 au, 19.4-yr period, V ~ 21-22, discovered in 1943 when perihelion was at 3.4 au, close approach to Jupiter in 1963 set up current distant orbit, current return is its 5th observed return]
Jul 12 - 180P/NEAR [q = 2.50 au, 7.6-yr period, V ~ 17-18, discovered in 2000, pre-discovery observations from 1955, 5th observed return]
Jul 12 - 185P/Petriew [q = 0.93 au, 5.5-yr period, V ~ 11, visual discovery in 2001, will be 5th observed return]
Jul 20 - 347P/PANSTARRS [q = 2.22 au, 5.0-yr period, V ~ 19-20, found in 2016, pre-discovery observations from 2003 and 2009 returns]
Jul 31 - C/2021 T4 (PANSTARRS) [q = 1.48 au, V ~ 8, more below]
Photo Opportunities
Jul 03-04 - C/2021 T4 (Lemmon) within 1 deg of 9th mag galaxy NGC 7793
Jul 09-10 - C/2021 T4 (Lemmon) within 1.5 deg of the Grus Quartet of galaxies [10-13th mag galaxies NGC 7552, 7582, 7590, 7599]
Jul 11 - C/2021 T4 (Lemmon) within 20’ of 10th mag galaxy IC 5267
Jul 14 - C/2021 T4 (Lemmon) passes within a few arc min of 11th mag galaxy NGC 7196 and 13th mag galaxy NGC 7200
Jul 16 - C/2021 T4 (Lemmon) passes within a few arc min of 12th mag galaxy NGC 7064
Jul 25 - C/2021 T4 (Lemmon) within 2 deg of bright 5th mag globular cluster NGC 6397
Jul 26 - C/2021 T4 (Lemmon) within 0.5 deg of bright 7th mag open cluster IC 4651
Jul 29 - C/2021 T4 (Lemmon) within 0.5 deg of 8th mag open cluster NGC 6204
Jul 29 - C/2021 T4 (Lemmon) within 0.5 deg of bright 7th mag open cluster NGC 6178
Jul 30 - C/2021 T4 (Lemmon) within 0.5 deg of bright 6th mag open cluster NGC 6169
Jul 30 - C/2023 E1 (ATLAS) within 1.5 deg of large, bright 6th nebula NGC 7023 [Iris Nebula]
Jul 31 - C/2021 T4 (Lemmon) within 0.5 deg of 12th mag planetary nebula IC 4599
New Discoveries and Recoveries
P/2023 M1 (PANSTARRS) - R. Weryk (University of Western Ontario) reported the discovery of P/2023 M1 on June 16 at 20th magnitude with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala. P/2023 M1 is a short-period comet with an 18-year orbital period. Perihelion was back on 2023 December 14 at 2.83 au. July should see M1 at its peak brightness of 19th magnitude. [MPEC 5273, CBET 2023-M65]
C/2023 L1 = P/2008 L2 (Hill) - R. Kresken, F. Ocana, M. Micheli, and L. Conversi report the recovery
of comet P/2008 L2 on CCD images obtained with a 0.56-m f/2.52 astrograph at La Silla, Chile on 2023 June 1, 2, and 8. The comet was reported to be stellar at 18th magnitude. It is already past its 2023 May 9 perihelion at 2.33 au.
With a 14.8-yr orbital period, it was last at perihelion during its 2008 discovery apparition. Our ALPO Solar Section coordinator Rik Hill discovered P/2008 L2 on 2008 June 12 with the 0.68-m Catalina Schmidt telescope. In 2008, it peaked at 15th magnitude. Though it is not expected to get brighter than 17th magnitude this time. [CBET 5270]
C/2023 K1 (ATLAS) – The ATLAS, Asteroid Terrestrial-Impact Last Alert System, 1-m telescope at Sutherland, South Africa, found C/2023 K1 on 2023 May 27 at 17th magnitude. The comet should peak at 16th magnitude this month on its way to a 2023 September 7 perihelion at 2.04 au. [CBET 5268, MPEC 2023-L19]
C/2023 H5 (Lemmon) – The Mount Lemmon Survey discovered C/2023 H5 (Lemmon) as an asteroidal object on 2023 April 19 at 20th magnitude. C/2023 H5 is a distant object ~7.5 au from the Sun at discovery and 2 years from a 2025 June 30 perihelion at 4.31 au. It should be a 16th magnitude object for most of 2025 in the northern circumpolar sky. [CBET 5272, MPEC 2023-M44]
In addition to the above comets, several apparently asteroidal objects on cometary orbits were also found. Here are some of the more interesting recent examples.
2023 MF4 – This 19-20th magnitude object was discovered by the Pan-STARRS1 telescope on 2023 June 19. Pre-discovery observations from 2022 and April, May, and June of this year were also found. A quick look at all photometry submitted to the MPC seems to follow an asteroidal brightening trend suggesting the object is inactive. MF4 is a Damocloid object with perihelion on 2024 April 22, at 2.79 au, and an orbital period of ~160 years. If it remains inactive, a peak brightness at 18-19th magnitude should occur in September 2024. Assuming an albedo of 0.04, its absolute magnitude of 14.4 yields a diameter of 8.8 km.
2023 JW18 – Yet another Pan-STARRS1 find, 2023 JW18 was found on 2023 May 1 at 22nd magnitude. JW18 has an orbital period of ~114 years and a perihelion occurring on 2023 October 13, at 4.53 au. The object is already close to its peak brightness. Assuming an albedo of 0.04, its absolute magnitude of 14.6 yields a diameter of 8.8 km.
2023 JM3 – Keeping with the trend, the Pan-STARRS1telescope also found 2023 JM3. First seen on 2023 May 15, at 21st magnitude, JM3 is still over a year out from its 2024 August 19 perihelion at 4.25 au when it should peak around 20th magnitude. The comet has a ~44-year orbital period. Assuming an albedo of 0.04, its absolute magnitude of 14.5 yields a diameter of 8.4 km.
2023 GC4 – And let’s make it 4 asteroidal objects on cometary orbits found by Pan-STARRS1. GC4 was discovered on 2023 April 6, at 21st magnitude. There have only been a few observations of this object submitted to the MPC, but they suggest an object brightening faster than expected for an inactive asteroid. So perhaps GC4 is an active comet. We should know more as it approaches perihelion on 2023 December 24, at 2.68 au.
A/2023 H4 – For A/2023 H4, the 1.8-m Pan-STARRS2 telescope on Haleakala made the discovery; on 2023 April 23 at 21st magnitude. The object will unlikely get much brighter as it is already close to its August 8 perihelion at 2.12 au. [MPEC 2023-L26]
Comets Brighter than Magnitude 10
C/2023 E1 (ATLAS)
Discovered 2023 March 1 by the Asteroid Terrestrial-Impact Last Alert System program from Sutherland, South Africa.
Halley-family comet
Orbit (from Minor Planet Center, MPEC 2023-N01)
C/2023 E1 (ATLAS)
Epoch 2023 Feb. 25.0 TT = JDT 2460000.5
T 2023 July 1.10689 TT Rudenko
q 1.0265962 (2000.0) P Q
n 0.01157920 Peri. 105.89382 +0.06328453 +0.98427854
a 19.3501214 Node 164.57539 -0.97142770 +0.02288262
e 0.9469463 Incl. 38.31471 +0.22874284 -0.17513461
P 85.1
From 1290 observations 2022 Dec. 25-2023 June 29, mean residual 0".5.
Ephemerides (produced with Seiichi Yoshida’s Comets for Windows program)
C/2023 E1 (ATLAS) Max El
(deg)
Date R.A. Decl. r d Elong Const Mag 40N 40S
2023-Jul-01 14 39 +79 33 1.027 0.646 72E UMi 9.0 50 0
2023-Jul-06 15 43 +80 32 1.030 0.616 73E UMi 9.0 49 0
2023-Jul-11 17 04 +80 36 1.039 0.583 75E UMi 8.9 49 0
2023-Jul-16 18 25 +79 22 1.056 0.548 78E Dra 8.9 51 0
2023-Jul-21 19 30 +76 42 1.077 0.513 82E Dra 8.9 53 0
2023-Jul-26 20 16 +72 42 1.105 0.478 87E Dra 8.9 57 0
2023-Jul-31 20 48 +67 22 1.137 0.444 94M Cep 9.0 63 0
2023-Aug-05 21 10 +60 38 1.174 0.415 102M Cep 9.1 70 0
Comet Magnitude Formula (from ALPO and COBS data)
m1 = 9.8 + 5 log d + 17.0 log r
where “t” is date of perihelion, “d” is Comet-Earth distance in au, and “r” is Comet-Sun distance in au
Next year two Halley-family comets return to perihelion, with 12P/Pons-Brooks and 13P/Olbers returning for the first time since 1954 and 1956, respectively. Though only discovered in March of this year with the Asteroid Terrestrial-impact Last Alert System (ATLAS) 0.5-m f/2 Schmidt reflector at Sutherland, South Africa, C/2023 E1 (ATLAS) is making its first return after 85 years, with its last (unseen) perihelion being back in 1937.
E1 is not as bright as next year’s 12P and 13P returns at 4th and 7th magnitude. With a perihelion on 2023 July 1 at 1.03 au and a close approach to Earth on August 18 at 0.37 au, E1 should reach its peak brightness this month.
Several visual observers (Juan Jose Gonzalez Suarez, Christian Harder, Carl Hergenrother, Michael Rosolina, and Uwe Pilz) were able to see the comet in June, with most finding the comet between magnitude 9.4 and 10.6. The coma was described as diffuse (DC = 2-3) and relatively large (between 3’ and 9’ in diameter). Images taken by Dan Bartlett, Eliot Herman, and Chris Schur found a gassy blue-green round coma with a long, thin gas tail. The gas-rich nature of the coma was confirmed visually by Uwe Pilz, who noted it was greatly enhanced when observed with a Swan Comet filter.
As was the case last month, C/2023 E1 is a far northern object and only visible from the northern hemisphere as it moves through Ursa Minor (Jul 1-13), Draco (13-29), and Cepheus (29-31). Since April, a consistent brightening rate of 2.5n ~ 17 has been observed. If this rate continues past perihelion, it suggests the comet may fade quickly. Luckily the comet will be approaching the Earth in July, so any fading due to an increasing Sun-comet distance should be balanced by the decreasing Earth-comet distance, keeping the comet around magnitude 9 all month.
C/2021 T4 (Lemmon)
Discovered 2021 October 7 by the Mount Lemmon Survey
Dynamically new long-period comet
Orbit (from Minor Planet Center, MPEC 2023-N01)
C/2021 T4 (Lemmon)
Epoch 2023 Feb. 25.0 TT = JDT 2460000.5
T 2023 July 31.51639 TT Rudenko
q 1.4833030 (2000.0) P Q
z -0.0000321 Peri. 329.80287 +0.28278202 -0.90355220
+/-0.0000011 Node 257.87430 -0.80094238 -0.40709191
e 1.0000476 Incl. 160.77621 -0.52775528 +0.13367721
From 1223 observations 2021 Aug. 7-2023 June 21, mean residual 0".6.
1/a(orig) = +0.000007 AU**-1, 1/a(fut) = +0.000949 AU**-1.
Ephemerides (produced with Seiichi Yoshida’s Comets for Windows program)
C/2021 T4 (Lemmon) Max El
(deg)
Date R.A. Decl. r d Elong Const Mag 40N 40S
2023-Jul-01 00 08 -28 30 1.544 0.903 106M Scl 8.9 10 79
2023-Jul-06 23 45 -34 35 1.526 0.765 117M Scl 8.4 9 85
2023-Jul-11 23 04 -42 35 1.511 0.648 129M Gru 8.0 6 87
2023-Jul-16 21 49 -51 24 1.499 0.567 140M Ind 7.7 0 78
2023-Jul-21 19 45 -56 00 1.491 0.542 144E Tel 7.6 0 74
2023-Jul-26 17 42 -51 58 1.485 0.578 135E Ara 7.7 0 78
2023-Jul-31 16 29 -43 58 1.483 0.666 122E Nor 8.0 5 86
2023-Aug-05 15 49 -36 40 1.485 0.788 110E Lup 8.4 9 87
Comet Magnitude Formula (from ALPO and COBS data)
m1 = 7.1 + 5 log d + 10.5 log r
where “t” is date of perihelion, “d” is Comet-Earth distance in au, and “r” is Comet-Sun distance in au
C/2021 T4 (Lemmon) should be the brightest comet this month. Due to its location at far southern declinations, it will be very well placed for southern hemisphere observers. For those of us up north, it will be hugging the southern horizon as its highly retrograde (160 deg) orbit carries it from the morning sky to opposition to the evening sky through the constellation of Sculptor (July 1-8), Grus (8-15), Indus (15-19), Telescopium (19-24), Ara (24-29), Scorpius (29-30), and Norma (30-31).
For observers in most of Europe and northern North America, it will be below the horizon for the second half of July, unfortunately, when it will be at its brightest. Observers in southern North America might be able to spot it above their southern horizon. However, it will compete against the usual hazy air of summer and the increasingly more common forest fire smoke.
Visual observers (Jose Guilherme de Souza Aguiar, Carl Hergenrother, and Chris Wyatt) watched T4 brighten from magnitude 12.1 to 10.0 throughout June. By the end of the month, the moderately condensed coma (DC = 3-5) reached 3’-4’ in diameter.
The combination of its closest approach to Earth on July 20 at 0.54 au and perihelion on July 31 at 1.48 au brings T4 to its brightest in July. With an original semi-major axis of +0.000007 au-1, the comet appears to be making its first approach close to the Sun. Despite being a dynamically new comet, it has brightened at a healthy rate of 2.5n ~ 10.5 over the past 10 months. If this continues, T4 should start the month at magnitude 8.9, reach a maximum brightness around 7.6 at mid-month and then fade to 8.0 by the end of the month.
C/2020 V2 (ZTF)
Discovered 2020 November 2 by the ZTF survey
Dynamically new long-period comet
Orbit (from Minor Planet Center, MPEC 2023-N01)
C/2020 V2 (ZTF)
Epoch 2023 Feb. 25.0 TT = JDT 2460000.5
T 2023 May 8.56971 TT Rudenko
q 2.2278428 (2000.0) P Q
z -0.0004177 Peri. 162.43150 +0.69787213 +0.59390410
+/-0.0000002 Node 212.37212 +0.53387549 -0.05876956
e 1.0009305 Incl. 131.61089 +0.47744261 -0.80238648
From 4350 observations 2020 Apr. 18-2023 June 19, mean residual 0".5.
1/a(orig) = -0.000142 AU**-1, 1/a(fut) = -0.000380 AU**-1.
Ephemerides (produced with Seiichi Yoshida’s Comets for Windows program)
C/2020 V2 (ZTF) Max El
(deg)
Date R.A. Decl. r d Elong Const Mag 40N 40S
2023-Jul-01 03 09 +14 59 2.311 2.837 49M Ari 9.4 9 23
2023-Jul-06 03 10 +13 44 2.327 2.767 54M Ari 9.4 13 26
2023-Jul-11 03 11 +12 22 2.344 2.693 59M Ari 9.3 16 29
2023-Jul-16 03 12 +10 54 2.362 2.616 64M Ari 9.3 20 33
2023-Jul-21 03 12 +09 17 2.382 2.537 69M Cet 9.3 24 36
2023-Jul-26 03 12 +07 31 2.402 2.456 75M Cet 9.2 28 39
2023-Jul-31 03 10 +05 36 2.424 2.374 80M Cet 9.2 32 42
2023-Aug-05 03 09 +03 28 2.447 2.293 86M Cet 9.1 35 45
Comet Magnitude Formula (from ALPO and COBS data)
m1 = -1.4 + 5 log d + 15.9 log r [up to T-580 days
m1 = 3.2 + 5 log d + 10.1 log r [between T-580 and T-220 days]
m1 = 4.0 + 5 log d + 8.6 log r [T-220 days and onward, assumed]
C/2020 V2 (ZTF) is now once again being observed following solar conjunction in early May. Due to the comet’s large perihelion distance of 2.23 au, its brightness isn’t as sensitive to the perihelion date (May 8). After peaking around magnitude 9.1 in January, it is expected to reach a second peak around magnitude 9.0 in August/September. The slightly brighter second peak is due to the comet being a little closer to Earth in August/September (1.9 vs 2.1 au). V2 should have been around magnitude 9.5 in June, but most visual observers found it about 1 magnitude fainter. As a result, the above prediction may be too bright. It is now visible from both hemispheres in the morning sky as it moves through Aries (Jul 1-17) and Cetus (17-31).
Comets Between Magnitude 10 and 12
C/2017 K2 (PANSTARRS)
Discovered 2017 May 21 by the Pan-STARRS survey with the Pan-STARRS1 1.8-m on Haleakala
Dynamically new long-period comet
Orbit (from Minor Planet Center, MPEC 2023-H131)
C/2017 K2 (PANSTARRS)
Epoch 2023 Feb. 25.0 TT = JDT 2460000.5
T 2022 Dec. 19.68872 TT Rudenko
q 1.7968936 (2000.0) P Q
z -0.0004373 Peri. 236.20152 +0.01818934 +0.04921873
+/-0.0000001 Node 88.23602 -0.18087333 +0.98247050
e 1.0007857 Incl. 87.56336 -0.98333819 -0.17980330
From 11224 observations 2015 Nov. 23-2022 Sept. 27, mean residual 0".5.
1/a(orig) = +0.000059 AU**-1, 1/a(fut) = +0.001151 AU**-1.
Ephemerides (produced with Seiichi Yoshida’s Comets for Windows program)
C/2017 K2 (PANSTARRS) Max El
(deg)
Date R.A. Decl. r d Elong Const Mag 40N 40S
2023-Jul-01 05 59 -11 34 2.957 3.720 35M Lep 10.7 0 11
2023-Jul-06 06 04 -11 01 3.002 3.761 36M Lep 10.8 0 14
2023-Jul-11 06 09 -10 31 3.046 3.798 36M Mon 10.9 0 16
2023-Jul-16 06 14 -10 05 3.091 3.831 37M Mon 10.9 0 18
2023-Jul-21 06 18 -09 43 3.136 3.858 39M Mon 11.0 0 21
2023-Jul-26 06 22 -09 23 3.181 3.881 40M Mon 11.1 0 23
2023-Jul-31 06 26 -09 07 3.226 3.899 42M Mon 11.1 0 25
2023-Aug-05 06 30 -08 53 3.271 3.912 44M Mon 11.2 0 27
Comet Magnitude Formula (from ALPO and COBS data)
m1 = 4.4 + 5 log d + 6.2 log r [Before perihelion]
m1 = 4.5 + 5 log d + 7.2 log r [After perihelion]
where “t” is date of perihelion, “d” is Comet-Earth distance in au, and “r” is Comet-Sun distance in au
As has been the case for many months, C/2017 K2 (PANSTARRS) is only visible from the southern hemisphere. Now 7 months from a December 2022 perihelion at 1.80 au, K2 is slowly fading as it recedes into the depths of the outer solar system. Surprisingly, its brightening/fading rate has been relatively constant for a comet that has been observed for years. Since perihelion, it has been fading a little faster than it brightened.
Chris Wyatt observed K2 on June 10 at magnitude 10.8 with a moderately condensed 2’ coma. July should see a fading to magnitude 11.1 as its moves through the morning constellations of Lepus (Jul 1-6) and Monoceros (6-31).
Fainter Comets of Interest
C/2023 A3 (Tsuchinshan-ATLAS)
Discovered on January 9 at the Purple Mountain Observatory's XuYi Station and on February 22 by ATLAS
Dynamically new long-period comet
Orbit (from Minor Planet Center, MPEC 2022-N01)
C/2023 A3 (Tsuchinshan-ATLAS)
Epoch 2023 Feb. 25.0 TT = JDT 2460000.5
T 2024 Sept. 27.64391 TT Rudenko
q 0.3911872 (2000.0) P Q
z -0.0004926 Peri. 308.48937 +0.36139360 +0.90086794
+/-0.0000230 Node 21.55724 +0.91856209 -0.29968943
e 1.0001927 Incl. 139.11850 -0.16011983 +0.31404331
From 1693 observations 2022 Apr. 9-2023 June 30, mean residual 0".3.
1/a(orig) = -0.000228 AU**-1, 1/a(fut) = -0.000200 AU**-1.
Ephemerides (produced with Seiichi Yoshida’s Comets for Windows program)
C/2023 A3 (Tsuchinshan-ATLAS) Max El
(deg)
Date R.A. Decl. r d Elong Const Mag 40N 40S
2023-Jul-01 14 11 +03 09 6.141 5.716 110E Vir 16.7 45 47
2023-Jul-06 14 09 +03 02 6.093 5.747 105E Vir 16.7 42 47
2023-Jul-11 14 07 +02 53 6.045 5.781 100E Vir 16.7 39 47
2023-Jul-16 14 06 +02 43 5.997 5.816 95E Vir 16.6 37 47
2023-Jul-21 14 05 +02 31 5.949 5.852 90E Vir 16.6 34 47
2023-Jul-26 14 04 +02 18 5.901 5.888 85E Vir 16.6 32 47
2023-Jul-31 14 03 +02 04 5.852 5.924 81E Vir 16.6 29 45
2023-Aug-05 14 03 +01 49 5.804 5.958 76E Vir 16.6 27 44
Comet Magnitude Formula (from ALPO and COBS data)
m1 = -16.6 + 5 log d + 35.0 log r [Through T-650 days]
m1 = -2.3 + 5 log d + 19.0 log r [Between T-650 and T-490 days]
m1 = 6.6 + 5 log d + 8.0 log r [After T-490 days, assumed]
where “t” is the date of perihelion, “d” is Comet-Earth distance in au, and “r” is Comet-Sun distance in au
2024 is looking to be an exciting year for bright comet observers. In addition to the aforementioned returning Halley-family comets, 12P/Pons-Brooks and 13P/Olbers, the recently discovered long-period comet C/2023 A3 (Tsuchinshan-ATLAS) holds some promise to become a naked eye object in September and October of 2024.
C/2023 A3 (Tsuchinshan-ATLAS) was discovered at 19th magnitude at the Purple Mountain Observatory’s XuYi Station in China on 2023 January 9. The IAU traditionally uses the anglicized name “Tsuchinshan” for the Purple Mountain Observatory. After discovery, it was placed on the MPC NEOCP but was removed at the end of January due to a lack of follow-up. The ATLAS program independently discovered the comet on February 22 at 18th magnitude. Hence the name “Tsuchinshan-ATLAS.”
Before making predictions about Tsuchinshan-ATLAS and its future behavior, let’s start with what we know. Perihelion occurs on 2024 September 28 at a relatively small 0.39 au. At that time, the comet will be close to the Sun in the sky. That means competing with a rapidly brightening dawn sky in the northern hemisphere. Conditions will be better from the Southern hemisphere, with the comet rising a few degrees above the horizon before the start of astronomical twilight. On the date of perihelion, the comet will only be 22 degrees from the Sun and drawing closer to the Sun in the sky, making it a challenging observation from both hemispheres.
On October 10, a minimum solar elongation of 3 degrees will be reached, with the comet being almost directly between the Earth and Sun and observable in SOHO LASCO coronagraph images. Its phase angle will also reach a maximum of 173 degrees, possibly resulting in a few magnitudes of enhanced brightness due to the forward scattering of dust. Two or three days after that, northern observers with flat horizons and clear skies may be able to pick up the comet low in the evening sky. Southern hemisphere observers will need to wait a few more days to see Tsuchinshan-ATLAS against a dark sky. By then, the phase angle will have decreased, so forward scattering will be less of a factor.
Observability around the time of perihelion and at high phase angles is a mixed bag. Working against the comet is its placement against a bright sky close to the horizon [think C/2020 F3 (NEOWISE) during the first days of July 2020]. Working for the comet is its large phase angle [examples include C/2006 P1 (McNaught), C1975 V1 (West), C/1927 X1 (Skjellerup-Maristany), C/1910 A1 (Great January Comet), C/1861 J1 (Great Comet of 1861), 1P/Halley at many of its non-1986 returns]. One major caveat, Tsuchinshan-ATLAS must be a dust-rich object for forward scattering by dust at large phase angles to boost its brightness appreciably.
Working against the comet is the fact that it appears to be dynamically new. Though dynamically new comets can become nice, or even Great, comets [for example, the above-mentioned C/2006 P1 (McNaught)], cometary history is strewn with once-promising dynamically new comets that severely disappointed early predictions [C/1941 (Cunningham), C/1973 E1 (Kohoutek), C/1989 X1 (Austin)]. Being dynamically new also increases the chance of the comet disintegrating [we all remember C/2012 S1 (ISON)].
A combination of photometry submitted to the Minor Planet Center, COBS, and the ALPO found the comet brightening at a rapid rate of 2.5n ~ 35 between April 2022 and January 2023 and 2.5n ~ 19 between January and May 2023. Since May, the rate of brightening seems to have slowed considerably. For the ephemeris above, I assumed a 2.5n = 8.0 rate going forward. This makes the comet a magnitude 16.5 to 17.0 object this month as it moves through Virgo in the evening sky.
We still have a lot of time to watch Tsuchinshan-ATLAS evolve and brighten. Time will tell…
Edited by Carl H., 03 July 2023 - 01:33 PM.
