ALPO COMET NEWS FOR APRIL 2020 A Publication of the Comet Section of the Association of Lunar and Planetary Observers By Carl Hergenrother - 2020-April-1 The monthly ALPO Comet News PDF can be found on the ALPO Comet Section website (http://www.alpo-astronomy.org/cometblog/). A shorter version of this report is posted on a dedicated Cloudy Nights forum (https://www.cloudynights.com/topic/700215-alpo-comet-news- for-april-2020/). All are encourageD to join the discussion over at Cloudy Nights. 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. A lot has changed since over the past month, not only with the state of the world, but with comets. C/2019 Y4 (ATLAS) has asserted itself as the comet of the moment. Currently between 7th and 8th magnitude as April begins, the comet may become a borderline naked eye object under dark skies by the end of the month. It will be interesting to watch how it develops. C/2019 Y4 isn’t the only object of interest. Both C/2017 T2 (PANSTARRS) and C/2019 Y1 (ATLAS) are brighter than 10th magnitude and CCD and large aperture visual observers are encouraged to watch fainter comets 88P/Howell, 210P/Christensen, 249P/LINEAR, and C/2019 U6 (Lemmon). Bright Comets (magnitude < 10.0) C/2019 Y4 (ATLAS) – The Great Comet of 2020, a Great Disappointment, or something in between? For the first time since 2013, a comet has the potential to become something special. Some articles in the popular press are already hyping this comet as you can see from these example headlines. “Comet Atlas could be the brightest comet in decades” “Comet ATLAS: Comet could be as bright as the MOON as it nears the Sun” “Comet of a generation set to light up the night sky” When it was discovered at the end of 2019, C/2019 Y4 was a faint 19th magnitude object. Other than the fact that it shared an orbit and possibly a progenitor with the Great Comet of 1844, the 1 comet looked like it was destined to remain a faint object or even disintegrate as it neared perihelion. An extraordinary rapid brightening through early March brought the comet to around 8th magnitude. It was extrapolating this brightening trend through to perihelion that lead to the predictions that inspired the above headlines. So, where does ATLAS stand as April begins? The comet possesses a large low surface brightness gas coma which is evident visually and in color CCD images (see the image below by Martin Mobberley from March 31). ALPO contributors measured the coma as large as 15’ visually and 22’ in images. Some reports to the COBS website have measured an even larger coma. A condensed inner dust coma and tail is also evident. Unfortunately, the large coma is complicating the measurement of its total brightness. The observation of such a large object is very sensitive to sky conditions, equipment, and observer experience. CCD image of C/2019 Y4 (ATLAS) by Martin Mobberley on 2020 March 31. Magnitude estimates submitteD to the ALPO by Salvador Aguirre, Michel Deconinck, J. J. Gonzalez, and Carl Hergenrother over the past 2 weeks found ATLAS to be between magnitude 7.2 and 8.6 (March 14-31). Personally, I’ve useD 30x125 and 10x50 binoculars to observe ATLAS. On most nights since March 15th, I placed the comet between 7.9 and 8.4. On the evening of March 28, the comet appeared brighter in both of my binoculars. In fact, it was the only night when it was a strong detection in my 10x50s. That night I placed the comet at magnitude 7.2 in the 10x50s and 7.6 in the 30x125s. I’m not sure if the comet was actually brighter, the night was more transparent than usual, or I was biased to the bright side that evening. The fact that most observers didn’t see 2 the comet brighter that night suggests the later reason is correct. Regardless, by the night of 30th, it was back to its usual recent brightness. If we ignore the anomalous night, the comet’s appearance (and brightness) didn’t seem to change much, if at all, over the past 2 weeks. So now the big question, how bright will C/2019 Y4 (ATLAS) get? The prediction below assumes an n = 3 rate of increase which is the average for a long-period comet. If ATLAS brightens at this rate it could be a borderline naked eye comet from a dark site by the end of April and perhaps a 3rd-4th magnitude object before it is lost in the glow of twilight from the northern hemisphere. At perihelion, it could brighten to 1st magnitude. ATLAS’ large phase angle at perihelion may result in another magnitude of brightness due to forward scattering. Of course, at that time the comet will be very close to the Sun and even at 1st magnitude may not be visible to most observers. Now this is just a nominal prediction and the comet may be brighter or fainter. In fact, its recent brightness trend suggests that it is underperforming the n = 3 rate. There is still a possibility that the comet will disintegrate as it approaches perihelion just like C/2012 S1 (ISON), the last “Great Comet” that wasn’t. Alternately, those of a certain age may remember C/1989 X1 (Austin) which Didn’t disintegrate but seemed to experience a slow fizzle as it experienced an intrinsic fading over much of its apparition. Even if ATLAS does reach 0th or 1st magnitude at perihelion, the majority of observers (including probably most of the public) will see the comet as no brighter than 3rd magnitude. A nice comet, no doubt, and one I’m looking forward to seeing, but not a Great Comet. We’ll know more as April develops and the ALPO encourages all visual and CCD observers to keep an eye on ATLAS. C/2019 Y4 (ATLAS) T = 2020-May-31 q = 0.25 au Max El Long-Period comet - dynamically old (deg) Date Mag R.A. Decl. r d Elong Const 40N 40S 2020-04-01 7.8 07 55 +68 31 1.475 1.045 92 Cam 61 0 2020-04-06 7.6 07 20 +68 01 1.383 1.029 85 Cam 59 0 2020-04-11 7.3 06 49 +67 03 1.288 1.012 79 Cam 56 0 2020-04-16 7.0 06 22 +65 44 1.191 0.995 73 Cam 51 0 2020-04-21 6.7 05 59 +64 08 1.090 0.974 66 Cam 46 0 2020-04-26 6.3 05 38 +62 16 0.986 0.951 60 Cam 41 0 2020-05-01 5.9 05 18 +60 06 0.879 0.922 53 Cam 35 0 2020-05-06 5.3 04 59 +57 28 0.768 0.890 47 Cam 28 0 C/2017 T2 (PANSTARRS) – With all the attention on C/2019 Y4, it might be easy to forget there are two other bright comets visible in the evening, C2017 T2 (PANSTARRS) and C/2019 Y1 (ATLAS). The former has been bright enough to be seen visually in small apertures since late last year. April brings the comet within a month of its May 4th perihelion at 1.62 au. During March, J. J. Gonzalez and Carl Hergenrother observed T2 to be between magnitude 8.4 and 8.7 with a coma diameter of 3-5’ This month should see more of the same as the comet slowly brightens another 0.3 magnitudes. The comet is well placed in the northern circumpolar sky as it moves through Cassiopeia (Apr 1- 10) and Camelopardalis (10-30). Southern observers will have to wait till late June or July to catch another glimpse of PANSTARRS. The comet should still be between 8-9th magnitude at that time. 3 Lightcurve of C/2017 T2 (PANSTARRS). Data consists of visual and CCD observations submitted to the ALPO Comet Section by SalvaDor Aguirre, Michel Deconinck, J. J. Gonzalez, Carl Hergenrother, Raymond Ramlow, and Chris Wyatt. CCD image of C/2017 T2 (PANSTARRS) by John D. Sabia on 2020 March 16. 4 C/2017 T2 (PANSTARRS) T = 2020-May-04 q = 1.62 au Max El Long-Period comet - dynamically new (deg) Date Mag R.A. Decl. r d Elong Const 40N 40S 2020-04-01 8.5 03 03 +68 47 1.679 1.768 68 Cas 40 0 2020-04-06 8.5 03 19 +70 18 1.662 1.762 67 Cas 39 0 2020-04-11 8.4 03 41 +71 49 1.647 1.753 67 Cam 39 0 2020-04-16 8.4 04 09 +73 17 1.635 1.743 66 Cam 39 0 2020-04-21 8.3 04 43 +74 37 1.626 1.731 66 Cam 40 0 2020-04-26 8.3 05 26 +75 41 1.620 1.718 67 Cam 41 0 2020-05-01 8.2 06 17 +76 16 1.616 1.705 67 Cam 43 0 2020-05-06 8.2 07 13 +76 13 1.615 1.692 68 Cam 44 0 C/2019 Y1 (ATLAS) – It may seem a bit confusing but there is another C/2019 Y# (ATLAS) comet bright enough for small aperture observers.