ALAN'S COMETS

TALLY ENTRIES 721-730

721. COMET 73P/SCHWASSMANN-WACHMANN 3          Perihelion: 2022 August 25.79, q = 0.973 AU

During the early decades of the 20th Century most of the asteroid discoveries, as well as a non-trivial number of the comet discoveries, were made during the course of photographic survey programs being conducted at various observatories around the world. One such effort was conducted by the German astronomers Arnold Schwassmann and Arno Wachmann at Hamburg Observatory in Bergedorf, Germany, who in addition to several asteroids discovered four comets between 1927 and 1930. The first of these comets, 29P/Schwassmann-Wachmann 1, is a most interesting object that travels in a near-circular orbit between the orbits of Jupiter and Saturn and which undergoes outbursts at irregular intervals. I am currently following my fourth "return" of this comet (no. 498), and during the (northern hemisphere's) autumn of 2021 it underwent a series of outbursts which temporarily raised its brightness to 10th magnitude, the brightest it has been in several decades. At present it is just starting to emerge into the morning sky following conjunction with the sun a month ago; it will be at opposition at the very end of this year, and we'll just have to see what kind of activity it exhibits during this coming viewing season.

Schwassmann and Wachmann discovered their last comet in early May 1930, and at the end of that month it passed just 0.062 AU from Earth and reached a peak brightness near 7th magnitude. It was found to have an orbital period as short as 5.4 years, but its low intrinsic faintness combined with a perihelion distance right around 1.0 AU which keeps it hidden in sunlight for months at a time during unfavorable returns caused the next several returns to be missed. It was expected to have a favorable return in mid-1979, however initial searches for it then were unsuccessful. A comet discovered from Perth Observatory in Western Australia in mid-August of that year was soon identified as being this object, although the predicted time of perihelion passage had been off by over a month.

Comet 73P's 1985 return was very unfavorable and it wasn't recovered, however the following return, in 1990 (perihelion mid-May), was relatively favorable, with a closest approach to Earth of 0.37 AU. I followed it for a little over three months (no. 140) and it reached a peak brightness near 10th magnitude.

The viewing geometry during the comet's return in 1995 (perihelion late September) was mediocre at best, especially for the northern hemisphere, and I had no plans to look for it. However, starting around perihelion passage the comet underwent a couple of outbursts which brought its brightness up to as bright as 5th magnitude during October, and it exhibited a bright dust tail almost one degree long which gave it a telescopic appearance of a miniature "Great Comet;" despite its placement fairly low in my southwestern evening sky I ended up following it for four months (no. 202). Towards the end of that period images with larger telescopes revealed four distinct nuclear fragments, and it is obvious that the outbursts were associated with this splitting of the nucleus; it also became clear that the brightest of these fragments, which was designated as "component C," was the primary component.

The comet next returned in early 2001 (perihelion late January) under distinctly unfavorable viewing geometry, but it maintained its enhanced brightness from 1995 and I was able to obtain a handful of observations of component C in late 2000 (no. 284) as a dim object of 12th magnitude at a very low elevation. Images obtained with larger telescopes revealed that component B from 1995 and an additional fragment, designated component E, were accompanying the primary component as separate objects, but I was unable to detect these visually.

Comet 73P's return in 2006 (perihelion early June) was eagerly awaited by the astronomical community, as it would be passing only 0.08 AU from Earth in mid-May and would be visible under excellent viewing geometry. I was able to follow component C for seven months (no. 385) as it reached a peak brightness of 7th magnitude and exhibited a bright dust tail half a degree long. Component B was bright and visible as well, being separated from the primary by as much as seven degrees when they were closest to Earth, and I ended up following it for three months; it exhibited unstable brightness behavior, however, at one point undergoing an apparent outburst and becoming as bright as 6th magnitude when I could faintly detect it with my unaided eye. Numerous additional components also appeared throughout the apparition, and I successfully detected two of these (components G and R) as separate objects (G on a handful of occasions and R once). Images taken by large telescopes, including with the Hubble Space Telescope, revealed that many of the secondary components (including B and G) were themselves fragmenting, and eventually as many as 66 discrete fragments (designated up through component BS) were identified.

      Comet 73P/Schwassmann-Wachmann 3 in 2006 (no. 385). LEFT: Component B (left) and the primary (component C) on June 2. Courtesy John Drummond in New Zealand. RIGHT: Composite infrared image (at a wavelength of 24 microns) obtained between May 4 and 6 by the Spitzer Space Telescope. The primary (component C) is at upper right, component B is towards the lower left, and various additional components as well as an entire train of dusty debris are evident. Courtesy NASA.

The primary, component C, has been observed on every return since 2006, but none of the other fragments from that year have been seen since then. The viewing geometry in 2011 (perihelion mid-October) was relatively unfavorable, and although I did look for it on a handful of occasions I was unable to detect it. The following return, in 2017 (perihelion mid-March), was only marginally better, but I nevertheless managed to observe the comet on a handful of occasions (no. 612) low in my southeastern sky before dawn. Curiously, an additional previously-unseen component, designated as component BT, appeared with it, and for a time was even brighter than the primary. I managed to observe this object as well, although neither it nor the primary ever became brighter than magnitude 12 1/2.

On its present return Comet 73P (component C) was recovered as far back as January 13, 2021 by the Lowell Discovery Telescope in Arizona, at which time it was an extremely faint object near 23rd magnitude. Apparently there were no further observations made for over a year, but beginning in late January 2022 (when it was near 19th magnitude) it has been imaged regularly around the world as it has approached perihelion and brightened; it went through opposition in late February. I took a set of images with the Las Cumbres Observatory network shortly after mid-June, and these suggested the comet was bright enough to attempt visually, however when I was able to make such an attempt near the end of June I couldn't detect anything convincing. (A monsoon season which began early this year has made clear nights relatively few and far between for the past few weeks.) An additional set of LCO images that I took in mid-July suggested that the comet had become distinctly brighter, and on the evening of the 16th I successfully detected it as a small and relatively condensed object slightly fainter than magnitude 12 1/2.

The geometry of the present return could perhaps be described as "moderately favorable," although this primarily applies to observers in the southern hemisphere; for those of us north of the Equator, it is only accessible low in the southwestern sky after dusk since it remains south of the sun until early December. At present it is located in far western Virgo (at an elongation of 57 degrees) just over three degrees north of the star Beta Virginis, and is traveling towards the east-southeast at approximately 50 arcminutes per day. As the comet approaches both the sun and Earth its motion increases, reaching a maximum of 80 arcminutes per day in late September, at which time it is near its closest approach to Earth (0.97 AU) and also when its declination goes south of -30 degrees (reaching a peak just south of -36 degrees in mid-October). Throughout this interval it travels through Virgo (passing just over 20 arcminutes south of the bright star Spica on August 21) then enters Libra in early September and Scorpius shortly before the end of that month, passing one degree south of the large open star cluster M7 on October 11 before crossing into Sagittarius a day later, where it remains before finally crossing into Microscopium in early November.

If the comet behaves more or less "normally," it should brighten by perhaps a magnitude to a magnitude and a half over the next few weeks, thus reaching a peak brightness near 11th magnitude in late August and early September. It is always possible, based upon its past behavior, that it could undergo outbursts or other activity during this time, and meanwhile, although thus far there haven't been any reports of additional companions, the possibility remains that one or much such objects could appear. In any event, based upon its poor placement in the northern hemisphere's sky, our ongoing monsoon season here in New Mexico, and my general slowdown in observational activity, at best I will likely obtain only a handful of additional observations of the comet.

Comet 73P's cascading episodes of fragmenting in 2006 and the debris stream it exhibited then led to speculation that Earth might witness a meteor shower this year, probably occurring in late May and coming from a radiant near the star Tau Herculis. Accurate predictions for any such shower were difficult to make, in significant part because it would have to be due to debris leading the comet, as opposed to debris trailing the comet as is usually the case with most showers. It turns out that a shower did occur, on the night of May 30-31 and best visible from North America, although it was brief and not especially strong, with a peak zenithal hourly rate of 30 to 40 meteors per hour. During a brief watch that night -- from my front porch, where a non-trivial fraction of my observable sky is obscured -- I did manage to see a handful of meteors from this shower.

      LEFT: An image of Comet 73P/Schwassmann-Wachmann 3 I took via the Las Cumbres Observatory facilty at Haleakala Observatory in Hawaii on July 15, 2022, slightly less than two days before my first visual observation. RIGHT: The Tau Herculid meteor shower on May 31, 2022, as seen from space: this five-minute exposure was taken by the Yangwang-1 Space Telescope launched in 2021 by the private Chinese firm Orbital Space. Courtesy Zhouxiao Wang and Orbital Space.

As is the case for many of the other "old friend" periodic comets that I have been observing lately, this will very probably be the last return during which I will observe Comet 73P. The next return, in 2027 (perihelion late December), is very unfavorable, but the one after that, in 2033 (perihelion early May), is a relatively good one, somewhat reminiscent of that of 1990; as I have indicated in numerous previous entries, however, I will likely have ceased visual comet observing well before then. The comet will make additional close approaches to Earth in 2054 (0.20 AU) and in 2070 (0.12 AU), but I will leave any observations to the comet observers of those eras.

INITIAL OBSERVATION: 2022 July 17.16 UT, m1 = 12.7, 0.6' coma, DC = 5 (41 cm reflector, 70x)

UPDATE (August 8, 2022): Austrian amateur astronomers Michael Jaeger and Lukas Demetz have discovered two new fragments of Comet 73P, designated as components BU (first detected July 23 although not noticed until the following day) and BV (also first detected July 23), utilizing a remotely-controlled telescope located in Namibia. Both components are leading the primary component by approximately 16 to 22 arcminutes and are both about 19th magnitude, each with a coma approximately 10 arcseconds in diameter and a tail slightly less than an arcminute long. Preliminary orbital calculations indicate that they will pass perihelion roughly 20 hours prior to the primary component. At this time it is too early to tell when they may have separated from the primary, and in all likelihood neither one will probably survive for much longer.

Because of our continuing monsoon here in New Mexico and the comet's low placement in our southwestern sky (which in turn creates only a very short observing window after dusk), and now moonlight as well, I have not seen Comet 73P again since my initial observation on the 16th. Since it will only be getting lower during the coming weeks it is entirely possible that that initial observation will remain my only observation this time around, and more than likely would be the final observation I ever make of this comet.

 

722. COMET ZTF C/2022 E3          Perihelion: 2023 January 12.78, q = 1.112 AU

In the entry for my first "Comet ZTF" (no. 720) I mentioned this incoming comet. It was discovered by the Zwicky Transient Facility survey in California on March 2, 2022, at which time it was described as a 17th-magnitude apparently asteroidal object located in the morning sky at an elongation of 47 degrees. Japanese amateur astronomer Hidetaka Sato managed to take a pair of images the following morning utilizing a remotely-controlled telescope in New Mexico that showed weak cometary activity. It was apparently lost after that, as there were no further observations for the next two weeks, but the ATLAS survey in Hawaii finally picked it up on March 18 (at which time the moon was full), and thereafter numerous observations began to be reported from around the world. I managed to obtain a couple of images of it on March 20 with the Las Cumbres Observatory (LCO) facility at Teide Observatory in the Canary Islands, and the astrometric measurements I made from these are among those included on the comet's discovery announcement that went out the following day.

The comet has traveled northward and has gradually brightened since its discovery, and went through opposition in early July. I've obtained occasional LCO images during the interim, and meanwhile some recent images I've seen -- which show the development of a distinct dust tail -- and various other reports have indicated that it is now bright enough to attempt visually. During a brief respite from our summer monsoon here in New Mexico, on the evening of July 23 I successfully detected it as a small and condensed object of magnitude 13 1/2 that exhibited distinct motion against the background stars in as little as 10 minutes.

      Images of Comet ZTF I've obtained via the Las Cumbres Observatory (LCO) network. LEFT: "Stacked" images (total exposure time 10 minutes) taken from the LCO facility at Teide Observatory in the Canary Islands on March 22, 2022. The comet is the small, essentially starlike object in the center. RIGHT: A single 3-minute exposure I took via the LCO facility at McDonald Observatory in Texas on July 24, 2022, two hours after my first successful visual observation.

Comet ZTF is traveling in a steeply-inclined retrograde orbit (inclination 109 degrees) and is presently located in eastern Hercules some three degrees southwest of the star Theta Herculis, traveling due westward at slightly over half a degree per day; it crosses the "Keystone" of that constellation from August 10 through the 22nd (passing two degrees south of the bright globular star cluster M13 on the latter date) and then crosses into Corona Borealis in early September. After that it begins a gradual turn towards the south, crossing into Serpens Caput during the latter part of October and being at its stationary point in early November; shortly after the middle of that month it is in conjunction with the sun (44 degrees north of it).

After it re-emerges into the morning sky afterwards the comet begins traveling almost due northward through Corona Borealis, Bootes, and Draco, entering northern circumpolar skies during the latter part of January 2023 and reaching its farthest north point (declination +80.3 degrees) on January 30 when it crosses into Camelopardalis. It is closest to Earth (0.284 AU) on February 1, when it will be located near a declination of +71 degrees and traveling towards the west-southwest at slightly over 6 1/2 degrees per day. Afterwards it starts slowing down and begins traveling almost due southward through Auriga, Taurus (passing three degrees east of the center of the Hyades star cluster on February 15), and (beginning in early March) Eridanus. In late April the comet crosses into Lepus, is in conjunction with the sun (42 degrees south of it) around the time of the June solstice, and over the next couple of months gradually starts emerging into the southern hemisphere's morning sky, although by then it will be a distant and (likely) dim object.

As I indicated in the earlier entry, this Comet ZTF's relatively close approach to Earth early next year suggests the possibility that it could become visible to the unaided eye, although as is always the case with previously-unseen long-period comets brightness predictions can be problematical. A couple of encouraging signs for this comet are the fact it is already exhibiting a distinct dust tail (at least, on CCD images), and the fact it appears not to be a "new" comet from the Oort Cloud, the most recent calculations indicating a previous return approximately 48,000 years ago. A cautiously optimistic scenario suggests the comet should reach visibility in ordinary binoculars by sometime in December 2022, and will be a naked-eye object of perhaps 5th magnitude -- conceivably, 4th magnitude -- around the time of its closest approach to Earth. For observers in the northern hemisphere it will be conveniently located in far northern skies at that time, although unfortunately moonlight will interfere (full moon being on the night of February 5-6). After its passage by Earth the comet will likely fade fairly rapidly, dropping below binocular visibility by perhaps mid-March.

While there is no reason to expect Comet ZTF will be a "Great Comet," it appears that we should at least have a somewhat bright comet that will briefly grace our skies in early 2023. It will likely be one of the last naked-eye comets, although hopefully not the very last such object, that I will observe before the "retirement" that I have discussed so much in previous entries.

I add this particular comet to my tally under somewhat bittersweet personal circumstances. My older son Zachary is relocating back to Australia (where he lived for four years previously, although he has been back here in New Mexico for the past two years), perhaps indefinitely. With my current health issues, my ability to travel any significant distance is limited, and while I suppose it is theoretically possible that I might travel to Australia again (especially if a very bright comet or other notable astronomical event appears which is only visible from the southern hemisphere), I consider it very unlikely that I will do so, thus any future occasions when I might see him again will almost certainly be restricted to those times when he visits back here. I can only wish him well on his future endeavors.

INITIAL OBSERVATION: 2020 July 24.15 UT, m1 = 13.5, 0.4' coma, DC = 7 (41 cm reflector, 70x)

 

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