TALLY ENTRIES 691-700
|691. COMET ATLAS C/2020 R4 Perihelion: 2021 March 1.94, q = 1.028 AU
I have now entered the "home stretch" on the approach to lifetime comet number 700. I do so with a comet discovered by the ATLAS survey on September 12, 2020 (and independently that same night by the Pan-STARRS survey), at which time it was a very dim object of about 19th magnitude. I have had expectations of eventually seeing this comet, although for the most part it has remained quite faint ever since its discovery; however, several days ago amateur astronomer Taras Prystavski in Ukraine reported that, according to images he had taken, it had apparently brightened quite rapidly within the recent past. Upon reading his report I obtained a pair of images via the Las Cumbres Observatory network which showed that the comet was indeed bright enough to be worth attempting visually, however when I did so on the evening of December 13 I was unable to see anything convincing. Over the next couple of days I saw additional images that suggested that the comet was continuing to brighten, and when I tried again on the evening of the 15th I successfully detected it as a vague and diffuse object slightly fainter than 13th magnitude.
This particular Comet ATLAS is traveling in a shallowly-inclined retrograde orbit (inclination 164 degrees, i.e., 16 degrees with respect to the ecliptic) and appears to be an intermediate-period object with an approximate orbital period slightly less than 1000 years. At the time of my initial observation it was fairly low in my southwestern evening sky, at an elongation of 54 degrees in eastern Capricornus (and just a little over two degrees south of Comet 88P/Howell (no. 678)), and with a general westward motion its elongation is dropping by over a degree per day. It is entirely possible that this will be my only observation prior to its disappearing into twilight, since the moon is now moving into the evening sky, and by the time the moon exits the evening sky during the early days of January 2021 the comet's elongation will have dropped to just a little over 30 degrees. In any event it is lost in twilight shortly thereafter.
The comet is in conjunction with the sun at the beginning of February, and for about two weeks around the time will be in the field-of-view of the LASCO coronagraphs aboard SOHO, although it will likely be too faint to be detected. It begins emerging into the morning sky in early March, when it will be located in far western Aquarius; it will be traveling towards the northwest, initially rather slowly, but accelerating as it approaches Earth. Over the coming weeks the comet tracks across Aquila, Hercules, Corona Borealis, Bootes, Canes Venatici, and Leo, reaching a peak northerly declination of +33 degrees in late April -- when it will also be at opposition -- before beginning to curve southward. It is nearest Earth (0.46 AU) on April 23, at which time it will traveling at close to 4 1/2 degrees per day, although it slows down rather rapidly afterwards, to just over 20 arcminutes per day by the end of May (when it will be located a couple of degrees northwest of the star Delta Leonis).
As is always the case with long-period comets, any brightness predictions for Comet ATLAS would be somewhat problemetical, but the fact that it has been around the sun at least once before, combined with its recent rapid brightening, suggests that it could be around 11th magnitude when it appears in the morning sky in early March, and perhaps as bright as 9th or 10th magnitude when it is closest to Earth. I suspect it might begin fading fairly rapidly after that, and will probably drop below visual detectability around the beginning of June.
Meanwhile, my entry into the "home stretch" towards comet number 700 did not end with this comet . . .
INITIAL OBSERVATION: 2020 December 16.07 UT, m1 = 13.3, 1.3' coma, DC = 1-2 (41 cm reflector, 229x)
The cometary activity in the evening sky during this month of December 2020 has been very busy, with well over half a dozen comets that I presently have under observation; it generally takes me two or three nights to get satisfactory observations of all of them. On the evening of December 15, I observed four comets, beginning with my initial observation of the previous entry; 2 1/2 hours later, after observing two previously-added comets, I concluded the evening by adding this particular comet to my tally. This is the 37th time, and the second time in 2020, that I have added two or more comets in the same night.
This is yet another discovery by the ATLAS survey, which found the comet as far back as June 10, 2019, at which time it was about 19th magnitude and located at a heliocentric distance of 8.46 AU. It has brightened steadily since then, and various images and reports I've seen over the past few months suggested it was bright enough for visual attempts, and I began making these around mid-October, although these were unsuccessful. (The rich Milky Way star fields in Cassiopeia that the comet has been traveling through have made attempts rather problematical at times.) I made an additional unsuccessful visual attempt in early December, but on this attempt -- when the comet was located in a "clean" star field -- I managed to detect it as a very faint, almost starlike object (with just a trace of diffuseness) near magnitude 14 1/2.
Comet ATLAS is traveling in a moderately-inclined direct orbit (inclination 48 degrees) and is at a present heliocentric distance of 4.9 AU and still over 12 1/2 months away from perihelion passage. (Among long-period comets on my tally, this is the 9th-longest interval before perihelion when I have first picked up a comet.) It is currently located about one degree southwest of the star Beta Cassiopeiae (the western-most star of the "W" of Cassiopeia) and is traveling slowly towards the southeast. It spends the next few weeks tracking across the southern part of that constellation, before crossing into northwestern Perseus (and turning more directly eastward) in early March 2021. It gradually sinks lower into the north-western evening sky, and after passing slightly less than one degree north of the star Alpha Persei in early May it is in conjunction with the sun (30 degrees north of it) a week after the middle of that month.
The comet starts to emerge into the morning sky around early August, at which time it will be located a few degrees north-
|Comet ATLAS C/2019 L3 on the evening of December 15, 2020 -- less than an hour after my initial observation -- as imaged from the Las Cumbres Observatory facility at Haleakala Observatory in Hawaii.|
east of the star Beta Aurigae. It travels towards the east-southeast from there, crossing into Lynx in early September, and then begins to turn more directly southward, passing through its stationary point in early November and crossing into Gemini in mid-December. The comet is at opposition in early January 2022, around the same time that it is passing through perihelion, and will be located some four degrees southeast of the star Theta Geminorum and traveling towards the southwest at slightly over 15 arcminutes per day. Over the next few weeks it curves more towards the south and passes through its other stationary point in early March before eventually resuming its motion towards the east-southeast; it crosses into Canis Minor in early May and disappears into evening twilight over the next month. After being in conjunction with the sun near the end of July the comet emerges back into the morning sky towards the end of September, when it will be located a couple of degrees south of the star Alpha Hydrae; from that point it tracks southeastward through Hydra, eventually turning southward and passing through its stationary point in early December, crossing into Antlia during the middle of that month, and going through opposition again in mid-February 2023.
Based upon my observations of previous large-q long-period comets, I suspect Comet ATLAS will brighten slowly over the coming months, perhaps being close to 13th magnitude by the time it disappears into evening twilight around May. It may be close to that same brightness when it emerges into the morning sky in August, brightening to a peak brightness near 12th magnitude when it is near perihelion before fading back to 13th magnitude by the time it disappears again into the dusk. If the comet is still visually detectable when it emerges into the morning sky during the latter months of 2022 it will probably be no brighter than 14th magnitude, and I suspect it will fade beyond visual range by perhaps the beginning of 2023.
In my introductory comments to "Continuing With Comets" I mentioned that my plans at that time (mid-2017) included continuing with my high level of observational activities at least until I reached my 500th separate comet. It so happens that this Comet ATLAS is separate comet no. 491, which means that I am now on the "home stretch" towards that goal (in addition to being on the "home stretch" to lifetime comet no. 700). There aren't any "repeat" periodic comets that I expect to add to my tally until about mid-May 2021, so any additions that I might make over the next few months will likely be new separate comets as well. I will probably reach both goals sometime around the middle of 2021, and . . . whether or not I might start slowing down after that point, as I hinted I might do in those introductory comments, is the proverbial bridge that I will cross when I arrive there.
INITIAL OBSERVATION: 2020 December 16.18 UT, m1 = 14.4, 0.3' coma, DC = 7-8 (41 cm reflector, 229x)
The new year of 2021 has now arrived. From a national and international perspective the new year arrives at a time of great anxiety, in significant part due to the global coronavirus pandemic that has as of now claimed over 2 million lives worldwide and 400,000 lives here in the U.S. The recent development of vaccines brings hope that this calamity will eventually begin to recede, although even at best it will be several months before things can start returning to some sense of "normal," and in any event whatever new "normal" emerges is likely to be somewhat different from the pre-pandemic one. Meanwhile, here in the U.S. there is a profound feeling of shock and dismay following the insurrection at the U.S. Capitol on January 6 and its aftermath; while I would like to think that some national healing can begin following the inauguration of the new President later this week, that process will likely be a long and at times painful one.
From my own perspective, I guess I am fortunate that I am in both a physical location and at a place in my life where, to a non-trivial degree, I am largely unaffected by the goings-on in the larger world (although the pandemic has introduced some distinct changes in how I conduct my social life, and I have very seldom left my residence during the past several months). Now that I am finished with "Ice and Stone 2020" I have been taking some "down time" as I contemplate how I might want to proceed from here; I would like to think that there are some things I can do that can help address some of the underlying educational issues that, among other things, contributed to the horrifying events at the U.S. Capitol, but for the time being I am unclear in my mind as to what these might be. I will keep working at it . . . I can't let myself not do so.
Meanwhile, I continue to remain active in comet observing in this new year, as I approach the goals (that I've mentioned in previous entries) of my 700th overall comet and my 500th separate comet. I carried over quite a few of the comets I was following late last year, and meanwhile I have already begun adding new comets to my tally. My first tally addition of 2021 is an object discovered on January 4, 2021 by the Zwicky Transient Facility (ZTF) survey in California, at that time a faint 19th magnitude apparent asteroid which, however, began brightening rapidly and exhibiting cometary activity within a few days. At around that same time German amateur astronomer Maik Meyer determined that the ZTF comet is in fact identical to a comet that had been detected in May 2016 by researcher Scott Ferguson in images taken by the Heliospheric Imager and one of the coronagraphs aboard the STEREO-A spacecraft. The comet appeared in over a hundred STEREO images over a two-day period that allowed the calculation of an uncertain orbit with an indicated period of 7.7 years; despite attempts by several observers, no one succeeded in recording it in any ground-based images. Meyer's identification of the ZTF comet with the STEREO comet -- which had been assigned the designation P/2016 J3 -- shows that the comet's true orbital period is a significantly shorter 4.67 years.
Comet STEREO was already fairly low in the southwestern evening sky -- elongation 44 degrees -- and getting lower at the time of its re-discovery by ZTF. Over the subsequent days some of the image reports suggested it might have been as bright as 14th magnitude, and I made a visual attempt on the evening of January 8; while at times I thought I might possibly be glimpsing "something" I was unable to convince myself that I was detecting any valid suspect. Additional reports afterwards suggested that the comet was continuing to brighten fairly rapidly, and when I tried again on the evening of January 12 I successfully observed it as a small and somewhat condensed object near magnitude 13 1/2; by that time the elongation had shrunk to 36 degrees and in addition to the comet's low elevation I also had to contend with moderately bright zodiacal light.
This will almost certainly be my only observation of Comet STEREO. The moon has now emerged into the evening sky, and meanwhile the comet's elongation continues to decrease, having now dropped below 30 degrees and dropping below 25 degrees on January 22. Following perihelion passage on the 25th the comet is closest to Earth (0.50 AU) on February 3 and is in conjunction with the sun (28 degrees south of it) four days later. It is conceivable that observers in the southern hemisphere may be able to detect it in the morning sky starting around mid-February, when it will be traveling through the constellations of Indus and Telescopium near a declination of -50 degrees; it will likely fade quite rapidly and its period of visibility will be rather short.
I also have no expectations of seeing Comet STEREO on any future returns. The next two returns (perihelion passages in mid-September 2025 and late May 2030) are unfavorable, and while the return in 2035 (perihelion mid-February) is fairly similar to this year's, it would still be a difficult object to observe, and even if I am still alive at that time I will very likely have "retired" from visual comet observing long before then. Especially due to its unusual history and its overall elusiveness I am glad I was able to grab at least one visual observation of this comet before all is said and done.
OBSERVATION: 2021 January 13.07 UT, m1 = 13.6, 0.8' coma, DC = 3 (41 cm reflector, 229x)
For the 10th time during my years of visual comet observing, I have now retroactively added an object to my lifetime comet tally (although on three of those occasions I did so while the object in question was still under observation). This particular object, which I discussed in some detail in an earlier tally entry, was discovered on November 12, 2018 by the Pan-STARRS survey in Hawaii, at which time it was a very faint stellar-appearing object of 21st magnitude. Calculations soon revealed that it was located 4 AU from the sun and traveling in an orbit reminiscent of a long-period comet -- although, since it didn't exhibit any cometary activity at the time, it was assigned an "A/" designation -- with an inclination of 165 degrees (i.e., 15 degrees with respect to the ecliptic, but retrograde) and an approximate orbital period of 1340 years. Shortly before perihelion passage it would be passing by Earth (minimum distance 0.37 AU on August 18, 2019, at which time it would also be at opposition) as it and Earth rapidly flew past each other while going in opposite directions.
A/2018 V3 was in conjunction with the sun during the latter part of April 2019, and after it began emerging into the morning sky I started attempting to take images of it with the Las Cumbres Observatory network around the end of May. At first these attempts were unsuccessful, but on June 28 I finally was able to record it as a very faint, starlike object. I continued taking images of it during the ensuing weeks as it slowly brightened, although it continued to remain completely stellar in appearance. I made my first visual attempt on the morning of August 12 and managed to glimpse it as an extremely faint starlike object of 15th magnitude. Over the next week and a half -- during which time I had to contend with moonlight (full moon being on the 15th) and typical monsoon conditions -- I was able to observe it on three additional occasions, the last of these being on the evening of August 22; on each of these it appeared at 15th magnitude and remained stellar in appearance. During this time it was traveling rapidly west-southward through Aquarius, Piscis Austrinus, Microscopium, and eastern Sagittarius, reaching a peak apparent speed of 5 1/2 degrees per day when it was closest to Earth and a peak southerly declination of -31 degrees a few days later. After my final visual observation A/2018 V3 began traveling through the dense star fields of the Sagittarius Milky Way and I didn't look for it any further; I did, however, continue taking images with the LCO network, with my last set being taken on September 24, by which time it had become distinctly fainter and was still continuing to exhibit a stellar appearance.
Reports from all other observers also indicated that A/2018 V3 remained asteroidal throughout its apparition, and thus was never an active comet -- and accordingly ineligible for inclusion in my tally -- despite its distinctly cometary orbit. There the matter stood until just a few days ago, when in mid-January 2021 a team of astronomers led by Caroline Piro at the University of Hawaii published a paper announcing results from their study of the object. Their observations indeed indicated that A/2018 V3 remained entirely asteroidal in behavior around the time of its perihelion and passage by Earth -- although photometric studies indicated that it has the very dark surface that is expected of a cometary nucleus -- however a set of "stacked" images taken with the 3.6-meter Canada-France-Hawaii Telescope (CFHT) at Mauna Kea Observatory in Hawaii taken on July 13, 2020 (at which time A/2018 V3 was over ten months past perihelion and located at a heliocentric distance of 4.08 AU) reveals a distinct elongation in roughly the east-west direction, consistent with the direction expected of a cometary tail. A detailed analysis of the brightness profile of this image is consistent with a tiny level of activity at that time (with less than 0.01% of the nucleus' surface being active). While the evidence is perhaps not extremely compelling, in my mind it is strong enough that, for tally purposes, I can consider A/2018 V3 as a comet; indeed, there have been other objects that have been formally designated as "comets" that have not been much more active than this.
LEFT: An image of A/2018 V3 I took on August 20, 2019 via the Las Cumbres Observatory facility at the South African Astronomical Observatory. RIGHT: Figure 2 from the paper by Piro et al. (2021). It shows "stacked" images of A/2018 V3 (left column) and representative field stars (right column) taken with the 3.6-meter Canada-France-Hawaii Telescope (CFHT) at Mauna Kea Observatory in Hawaii on July 7, 2019 (top row) and July 13, 2020 (bottom row). A/2018 V3's asteroidal appearance on the former date and elongated appearance on the latter date is obvious.
A/2018 V3 is apparently a representative of a recently-identified class of solar system object that have been dubbed "manx" objects, after the tailless cats indigenous to the Isle of Man. These are objects in long-period orbits typical of comets, but are at most only very weakly active; these include the "comets" I mentioned in the previous paragraph. Only a relatively small handful of these objects have thus far been identified, and the role they play in the evolution of the solar system as well as their implication for planetary defense -- both of which are discussed in the Piro et al. paper -- are active areas of research at this time. As the "deeper" survey programs continue to come on-line and operate during the years ahead they will likely continue to detect manx objects, although since these tend to remain very faint, unless they come close to the sun and/or Earth as A/2018 V3 did it is unlikely that I'll be seeing very many of them.
INITIAL OBSERVATION: 2019 August 12.43 UT, m1 = 15.1, 0.0' coma, DC = 9 (41 cm reflector, 229x)
FINAL OBSERVATION: 2019 August 23.20 UT, m1 = 15.1, 0.0' coma, DC = 9 (41 cm reflector, 229x)
I discuss the NEOWISE mission, the follow-up endeavor of the Wide-field Infrared Survey Explorer (WISE) spacecraft, in an earlier tally entry. Including this comet, NEOWISE has thus far contributed seven comets to my lifetime tally, by far the best of these being C/2020 F3 (no. 676) which became a spectacular naked-eye object last July. One month later NEOWISE discovered another comet (C/2020 P1) which, with a perihelion passage in late October and a small perihelion distance of 0.34 AU, also showed some promise of becoming somewhat bright. Initially located in southern circumpolar skies, it brightened somewhat slowly, eventually reaching about 9th magnitude before disappearing into sunlight in early October. Unfortunately, it apparently disintegrated as it passed through perihelion, and when it emerged into the northern hemisphere's morning sky towards the end of that month nothing remained except a thin wisp of material, which moreover stayed at a fairly small elongation from the sun. I never made any attempts for it.
The beginning of 2021 has been a very productive time for NEOWISE discoveries; at this writing three NEOWISE comets have already been announced and a fourth one currently on the Minor Planet Center's Possible Comet Confirmation Page (PCCP) is awaiting a formal announcement. The first of these was discovered on January 3, somewhat deep in southern skies near a declination of -51 degrees; the initial reports suggested it was somewhat bright (around 15th magnitude), and I successfully took some images of it on the 6th via the Las Cumbres Observatory facility at Cerro Tololo Inter-American Observatory in Chile, the astrometric measurements of these (performed by my British colleague Richard Miles) appearing on the discovery announcement that was issued on the 10th. Some reports of visual observations by observers in the southern hemisphere suggested it was bright enough for me to attempt visually, although I had to wait until it had traveled far enough north to make such an attempt worthwhile (in addition to having to contend with the rich Milky Way star fields that it is currently traveling through). On the morning of January 15 (when its declination was just north of -46 degrees) I successfully observed it as a small and slightly condensed object of 13th magnitude, with this same overall appearance being exhibited when I saw it again three mornings later. The visual reports from the southern hemisphere, where it is significantly higher in the sky than it is from my location, indicate that it may be up a magnitude or so brighter than this and that the coma is also somewhat larger than what I have been able to detect.
This particular Comet NEOWISE is traveling in a steeply-inclined retrograde orbit (inclination 107 degrees) and is currently just a few days away from perihelion passage. It is presently located in northern Vela near the star Psi Velorum and is traveling towards the northwest at slightly over 1 1/2 degrees per day, although as it goes through opposition in late January and passes closest to Earth on February 3 (minimum distance 0.51 AU) it accelerates (to a peak rate of 3 1/2 degrees per day around the time of closest approach before slowing down) and curves more towards the north-northwest. Throughout this time it travels roughly parallel to the Galactic Equator (and thus remains within rich Milky Way star fields) as it passes through the constellations of Pyxis, Puppis, Monoceros, Orion, and eastern Taurus, before entering Auriga during the fourth week of February, where it remains for the next 2 1/2 months. When nearest Earth it may be a half- to a full magnitude brighter than it is now, and thereafter will probably fade beyond the range of visual detectability by sometime in March.
As for the other recently-discovered NEOWISE comets, C/2021 A4 passes through perihelion shortly after mid-March and appears to be an intermediate-period object with an orbital period near 400 years; thus far it has remained quite faint, but it possibly may become visually detectable when it passes 0.44 AU from Earth shortly before mid-February. C/2021 A7, which passes through perihelion in mid-July, conceivably could become visually detectable in April and/or May but will be low in my southwestern sky after dusk. According to a very preliminary orbit the apparent NEOWISE comet still on the PCCP passes through perihelion in mid-March and will pass 0.6 AU from Earth in mid-February (at which time it will be in northern circumpolar skies), but thus far it appears to be an intrinsically dim object and it may very well remain too faint for visual observations. As always, of course, any of these comets that I do manage to observe will show up in my tally as future entries.
INITIAL OBSERVATION: 2021 January 15.44 UT, m1 = 13.0 (extinction corrected), 1.0' coma, DC = 3 (41 cm reflector, 229x)
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