TALLY ENTRIES 741-750
|741. COMET 103P/HARTLEY 2 Perihelion: 2023 October 12.51, q = 1.064 AU
I am now entering the "home stretch" as I approach lifetime comet no. 750. I do so with the fourth member of the "Dependable Dozen" to be added to my tally, and that moreover is an old friend, having been discovered in 1986 by Malcolm Hartley at Siding Spring Observatory in New South Wales and that I have seen on three previous returns, most recently in 2010 (no. 477); I discuss its overall history, and my history with it, in the entry for that return. (Incidentally, this current return gives Hartley his 17th appearance in my lifetime tally, which puts him all alone in 7th place for most appearances in the tally.) The 2010 return was an exceptionally favorable one, during which the comet passed just 0.121 AU from Earth -- currently the 17th closest approach to Earth of all the comets in my tally -- and became faintly visible to the naked eye near magnitude 5 1/2 while displaying a large diffuse coma half a degree in diameter. Also during that return, it had a spacecraft visitor: the Deep Impact spacecraft, which had encountered Comet 9P/Tempel 1 in 2005 (no. 367) and later repurposed under the mission name EPOXI, passed 700 km from the nucleus on November 4 of that year. The EPOXI images revealed the nucleus as a peanut-shaped object some 2 km in length with significant jetting activity at both ends and a smooth, inactive plain between those ends.
Images of the nucleus of Comet 103P/Hartley 2 taken by the EPOXI mission on November 4, 2010 during the comet's return that year (no. 477). LEFT: The full nucleus, showing jetting activity at both ends and a smooth, inactive plain between those ends. RIGHT: The "snowstorm" image, a high-resolution view of jetting activity at one of the ends. Both images courtesy NASA.
After 2010 Comet Hartley 2 returned to perihelion in early 2017, under unfavorable viewing geometry; it was recovered as a faint object later that year and followed for several months. As the current return approached I attempted to recover it on a couple of occasions in late March 2023 with the telescopes of the Las Cumbres Observatory network but was unsuccessful; when it was later recovered on April 17 by Francois Kugel at his private observatory in Dauban, France it was clear that it had been too faint for me to have detected it. Following its recovery the comet brightened fairly rapidly, and LCO images I took in late July and early August suggested it was bright enough to attempt visually. On the late evening of August 10 I successfully detected it as a vague, diffuse object of 13th magnitude that exhibited the expected motion during the half-hour that I followed it.
While not quite as good as that of 2010, the current return of Comet Hartley 2 is nevertheless a very favorable one, with a closest approach to Earth of 0.38 AU taking place on September 26. At present it is located in central Andromeda two degrees west of the star Beta Andromedae and is traveling towards the east-northeast at one degree per day (passing one degree north of that star on August 16). Gradually curving more directly eastward and accelerating, the comet enters Perseus and passes 20 arcminutes south of the open star cluster M34 on August 31, then reaches its peak northerly declination of +43 degrees six days later. As it begins a gradual turn southward it enters Auriga in mid-September, and at the time of closest approach to Earth will be two degrees south of the star Theta Aurigae and traveling at slightly over 1 1/2 degrees per day. At that time it should be at least as bright as 8th magnitude and quite possibly as bright as 7th magnitude.
Afterwards, the comet continues traveling towards the southeast -- curving more and more directly southward -- as it enters Gemini in early October, crosses southwestern Cancer late that month, and then enters western Hydra just before the month's end. It passes through its stationary point on December 5, when it will be located some six degrees west-southwest of the star Alpha Hydrae (Alphard); on January 1, 2024 it reaches its farthest south point (declination -14.6 degrees) and begins curving towards the northwest as it enters northeastern Puppis in mid-January and goes through opposition near the end of that month before entering Monoceros at the beginning of February. The comet should remain fairly bright throughout October but should fade afterwards, remaining visually detectable until probably sometime in January.
I should have a pretty good run of observations of Comet Hartley 2 this time around, but once the return is over, that will almost certainly be it for me as far as this long-time friend is concerned. The next return, in 2030 (perihelion early April), is very unfavorable, and while the return after that, in 2036 (perihelion late September, with a closest approach to Earth of 0.63 AU taking place early that month), is relatively good, as I have indicated numerous times in these pages I consider it very doubtful that I will still be observing comets then, even if I am still alive. I hope that my observations over these next few months can provide a fitting farewell as this friend and I prepare to go our separate ways.
INITIAL OBSERVATION: 2023 August 11.22 UT, m1 = 12.8, 1.5' coma, DC = 1 (41 cm reflector, 70x)
During my early decades of comet observing, when many of the newly-discovered comets were being found by amateur astronomers who were deliberating hunting for them, it was common that a comet would be bright enough to be visually detectable as soon as it was discovered. With the advent of the comprehensive surveys a quarter-century ago most visually observable comets nowadays are being discovered many weeks, if not months, before they are bright enough to be seen visually, and it is quite rare for me to be able to observe a comet as soon as it is discovered. This particular comet is even more unusual in that not only was it visually observable at discovery, it was actually relatively bright.
The comet was discovered by a Japanese amateur astronomer, Hideo Nishimura, on DSLR images he had taken on August 12, 2023; he later found it on images he had taken the previous morning. This is Nishimura's third comet discovery: his first came almost three decades ago, in July 1994 with Comet Nakamura-Nishimura-Machholz 1994m (no. 189), which he found while visually hunting with large binoculars; it reached a peak brightness near magnitude 8 1/2 the following month while en route to passing 0.4 AU from Earth. His second discovery came just two years ago, with Comet C/2021 O1, which he found with the same DSLR and setup with which he found his current comet; that particular comet was a 10th-magnitude object in the morning sky at the small elongation of 23 degrees. The elongation decreased during the next few weeks and the comet soon disappeared into twilight as it approached perihelion, and I never had a decent opportunity to look for it.
When discovered, this newer Comet Nishimura was located in central Gemini 1 1/2 degrees south of the star Zeta Geminorum at an elongation of 34 degrees, and was reported as being about 10th magnitude. I was informed of it privately not too long thereafter, and soon it also appeared on the Minor Planet Center's Possible Comet Confirmation Page, but unfortunately I was clouded out the following two mornings and was unable to observe it. The next morning -- the 15th -- also started out overcast, but after seeing the skies clear later on I began to set up the telescope for observations, only to watch the skies begin to cloud over again. I then noticed a small patch of clear sky traveling northward along the treeline to my east, and shortly thereafter the sky was clear in the comet's vicinity for approximately one minute. It took me about 30 seconds to locate the comet, and I had an additional 30 seconds to observe it before the clouds covered it back up. Despite the hurried observation the comet was quite easy to see as a moderately condensed object of 10th magnitude.
Calculations have revealed that Comet Nishimura is traveling in a moderately-inclined retrograde orbit (inclination 130 degrees) and approached the inner solar system from behind the sun; indeed, it has been at an elongation of less than 40 degrees since late April. Only within the recent past has it emerged into the morning sky (although it may appear faintly in images taken by the Heliospheric Imager aboard the STEREO-A spacecraft in early July, and also in ultraviolet images taken by the Solar Wind ANisotropies (SWAN) telescope aboard the SOHO spacecraft in early August). It is currently traveling towards the east-northeast at approximately 50 arcminutes per day, and reaches a maximum elongation of just over 35 degrees around August 22, at which time it will be located some seven degrees south of the bright star Pollux and traveling at just over one degree per day. As its daily motion accelerates the elongation soon starts to decrease, dropping below 30 degrees on September 3 and below 25 degrees four days later. (Curiously, on August 27 it will pass less than 8 arcminutes south of Comet 29P/Schwassmann-Wachmann 1 (no. 498), which has recently been detected in a state of outburst, although thus far it has been at too small of an elongation for me to observe.) Since the comet is approaching both the sun and Earth during this time it could, theoretically, brighten quite rapidly, and by the time it disappears into twilight it possibly may be as much as three or four magnitudes brighter than it is now.
The comet's unusually small perihelion distance suggests it could be a fairly bright object as it passes through perihelion, although its elongation will only be twelve degrees at that time and -- unless it is very bright -- it will be buried in twilight and almost certainly unobservable. It will be between Earth and the sun during this time and its phase angle will reach a maximum of 118 degrees on September 15, which suggests the possibility of brightness enhancement via forward scattering of sunlight; however, thus far it has not shown any indication of a dust tail -- just a weak ion tail on images I have seen -- so I am doubtful that such an enhancement will occur.
It is not at all certain that Comet Nishimura will survive its perihelion passage. If it does, it recedes from perihelion on the far side of the sun from Earth, with the elongation not going above 30 degrees until early November and not above 40 degrees until after the middle of that month; since it will be well south of the sun any observations would be restricted to the southern hemisphere. For what it's worth, the comet will enter southern circumpolar skies in early January 2024, but whatever might be left of it by then will almost certainly be very faint by that time. In all likelihood, then, any additional observations of Comet Nishimura that I am able to obtain within the next couple of weeks will wrap up its visibility as far as I am concerned.
When I first mentioned the "Dependable Dozen" a few entries back I noted that I only needed to add two additional comets not on that list to all but ensure I would reach comet no. 750 by the end of next year. With the addition of Comet Nishimura to my tally that has now happened. Thus, provided I am able to keep actively observing over the next 16 months and successfully observe the remaining eight comets of the "Dependable Dozen," I should easily achieve that remarkable milestone.
INITIAL OBSERVATION: 2023 August 15.45 UT, m1 = 10.2, 3' coma, DC = 4-5 (41 cm reflector, 70x)
UPDATE (August 20, 2023): A revised (although probably still somewhat preliminary) orbit published earlier today places perihelion half a day earlier than the initial orbit and slightly increases the perihelion distance and inclination (to 133 degrees). (The appropriate updated infomation is now reflected in the header for this entry.) The general scenario described above remains basically unchanged, although the elongation now drops below 25 degrees on September 6, the maximum phase angle is now 122 degrees and will be reached on September 14, and the elongation at perihelion is now 13 degrees. (The closest approach to Comet 29P on August 27 is now just over 9 arcminutes.) This revised orbit also suggests the comet is traveling in an elliptical orbit with an approximate period of 300 years, although at this time it is still too early to attach much significance to this. If this is later found to be valid, this would suggest a higher probability that the comet will survive perihelion, and -- once a more precise orbital period is determined -- might help in identifying any previous returns in historical records.
I have now had some clear mornings, and had a successful observation of Comet Nishimura on August 19 (although the window between the comet's rising above the trees to my east and the onset of dawn is still fairly short). It appeared marginally brighter than it did during my initial observation.
MOST RECENT OBSERVATION: 2023 August 19.46 UT, m1 = 9.6, 3.5' coma, DC = 4-5 (41 cm reflector, 70x)
UPDATE (September 6, 2023): Comet Nishimura has now been identified in images taken by the Pan-STARRS survey back in January 2023, and these have helped establish its orbital period as being close to 440 years. This would suggest that the previous return took place back in the late 16th Century, but thus far none of the records of comets that appeared around that time (nor, for that matter, around the times of earlier returns) can be convincingly identified as corresponding to this comet.
Meanwhile, the comet itself has brightened steadily as it has approached perihelion, although it is now rather rapidly sinking lower into the dawn sky. The weather around here has been quite good during the past few days, and after waiting for the recent full moon to dwindle a bit I observed the comet yesterday morning; although from my normal observing site it didn't rise above the trees until dawn was well underway, by motoring to a nearby site I easily detected it in 10x50 binoculars shortly after it rose above a nearby hill. In the binoculars it appeared as an almost starlike object of 6th magnitude, and I could see a suggestion of the ion tail that is quite prominent in recent images I have seen.
This will more than likely be my final observation of Comet Nishimura. Now traveling through the "head" of the constellation Leo, it is presently at an elongation of 25 degrees and is traveling just southward of due east at three degrees per day, and accordingly disappears into the dawn within the next couple of days. While, in theory, it should be accessible in the evening sky around the time of perihelion, as I mentioned above it will be at an elongation of only 13 degrees, so unless it becomes much brighter than expected it will be buried too deeply in twilight to be detectable. For what it's worth, the fact that it has been around before suggests a reasonable likelihood that it will survive perihelion passage this time around as well. On the other hand, thus far there has been no strong evidence of any dust tail, which in turn suggests a low dust content, and thus little likelihood of enhanced brightness due to forward scattering of sunlight.
MOST RECENT OBSERVATION: 2023 September 5.47 UT, m1 = 6.1, 4' coma, DC = 8 (10x50 binoculars)
When I entered a state of "semi-retirement" from visual astronomical activities two years ago I didn't have a clear and specific definition of what that entailed, and in the time that has elapsed since then I have continued to be somewhat flexible in how I have approached that. One distinct element, however, is that I no longer try to chase down every faint, distant, nondescript comet that might be in the sky, although there have been a couple of comets that perhaps have challenged that. At first glace, this comet might appear to be another such comet, but there are enough intriguing elements to its story that I eventually felt it worthwhile to make an attempt for it.
The comet's story starts with its discovery as a 21st-magnitude stellar-appearing object on May 9, 2022 by the Pan-STARRS survey. A handful of additional observations were obtained over the subsequent three weeks, and in the absence of any additional observations or information it was assigned the asteroidal designation 2022 JK5. There the matter stood until April 3, 2023, when the ATLAS survey's telescope in Chile discovered an 18th-magnitude apparent comet. This object briefly appeared on the Minor Planet Center's Possible Comet Confirmation Page but was removed once it was identified as being identical to the previously-discovered Pan-STARRS "asteroid." It was neverthless some two magnitudes brighter than the predictions based upon the previous year's observations, and several observers -- myself being among them -- noted its apparent cometary appearance on images. (In my case, images I took via the Las Cumbres Observatory's facility at Cerro Tololo Inter-American Observatory in Chile on April 11 showed it as being somewhat diffuse compared to stars of similar brightness, although I really couldn't tell much else.) It was formally announced as a comet -- under the previously-assigned asteroidal designation -- on April 25; my astrometric measurements were included on the Minor Planet Center's discovery announcement, and my "cometary" observations were mentioned on that and on the discovery announcement issued by the Central Bureau for Astronomical Telegrams.
Comet PANSTARRS in traveling in a shallowly-inclined (inclination 17 degrees) distinctly elliptical orbit with an approximate orbital period of 280 years. It was close to perihelion passage at the time its discovery was announced, but was still in the process of emerging into the morning sky then and has been approaching Earth ever since. Various images I have seen, including some LCO images I took in mid-July, have shown a distinct brightening since that time, and another set of LCO images I took on August 20 showed it as being bright enough to suggest it should be detectable visually. Early the following morning I successfully observed it, at 13th magnitude.
Images of Comet PANSTARRS I have taken via the Las Cumbres Observatory network. LEFT: A "stacked" set of images I took on April 11, 2023, via the LCO facility at Cerro Tololo Inter-American Observatory in Chile. The comet is the slightly diffuse object to the lower left of center. These images provided the evidence of cometary nature cited in the comet's discovery announcements. RIGHT: A single 3-minute exposure on August 20, 2023, taken via the LCO facility at the South African Astronomical Observatory. I obtained my first visual observation slightly less than ten hours later.
Now almost four months past perihelion passage, the comet is presently at its closest approach to Earth (2.01 AU) and will be at opposition shortly after mid-September. It is currently located in northern Sculptor some 2 1/2 degrees northeast of the star Delta Sculptoris and is traveling in a generally westerly direction at a relatively slow six arcminutes per day, and since it stays nears its present declination of -27 degrees for the next several weeks it remains fairly low in my southern sky throughout the near-term foreseeable future. I will probably look for the comet again in September once the upcoming full moon clears away from the evening sky, but I suspect it will probably begin fading, and it may well end up as a "one-time wonder." Regardless of whether or not I see it again, I am glad I was able to grab at least one observation of this visitor from the outer solar system.
INITIAL OBSERVATION: 2023 August 21.27 UT, m1 = 13.2, 1.4' coma, DC = 3 (41 cm reflector, 70x)
"Hello, once more!" to one of the most famous of all comets, and which remains the comet with the shortest-known orbital period, at 3.3 years (although there are "active asteroids" with shorter orbital periods that I consider as "comets" for tally purposes). I discuss its overall history in its "Comet of the Week" presentation for "Ice and Stone 2020" and some of my own personal history with it in the tally entries for its returns in 2007 (no. 402), 2013 (no. 531), and its most recent previous return in 2020 (no. 683) -- this particular return was most unusual for me in that it is the only time that I have obtained post-perihelion observations of P/Encke from the northern hemisphere. The current return is the 65th during which it has been observed, and is the 14th return that I have personally observed. This is the 5th of the "Dependable Dozen" comets that I mentioned in a recent previous tally entry.
After its emergence into the morning sky following its conjunction with the sun earlier this year, I successfully obtained a couple of images of P/Encke on June 25 via the Las Cumbres Observatory network; it appeared as a very faint, essentially stellar object of 18th or 19th magnitude. By mid-August LCO images were beginning to show a very faint and vague coma, which was significantly more pronounced in additional images I obtained a week and a half later. I made a couple of unsuccessful visual attempts during late August; my failure to detect it lends support to some suspicions I've had that P/Encke has undergone some intrinsic fading during the recent past (although it's fair to note that the rich Milky Way star fields in Auriga that it was traveling through at the time didn't help in seeing it). After the full moon at the end of August I made my next attempt for P/Encke on the morning of September 7, and despite the Third-Quarter moon less than 25 degrees away I successfully detected it as a vague, marginally condensed object slightly brighter than 12th magnitude.
This year's return is a moderately favorable one for the northern hemisphere, and is almost identical to its return in 1990 (no. 145), this year's perihelion date being only six days earlier than that year's. At present P/Encke is located in northern Gemini some four degrees west-northwest of the star Castor and is traveling somewhat southward of due east at a little over 1 1/2 degrees per day; it passes 45 arcminutes north of that star on September 11. Over the next few weeks it curves more towards the southeast and accelerates, crossing into Cancer on September 15 and being closest to Earth (0.90 AU) on September 24, at which time it is traveling over two degrees per day as it crosses into Leo and makes a quick sojourn through the "head" of that constellation before traversing the "lion's body" and then crossing into Virgo on October 11. By that time it is rapidly disappearing into the dawn, with the elongation dropping below 30 degrees on October 7 and below 20 degrees eight days later. Historically, P/Encke tends to brighten quite rapidly as it approaches perihelion, and it should be close to 8th magnitude, conceivably even 7th magnitude, by the time it is lost in twilight. Realistically, I will probably only get, at most, three or four more observations of it before it's gone for me.
A little over three days after perihelion passage P/Encke enters the field-of-view of the LASCO C3 coronagraph aboard SOHO, and remains there for the next two weeks. Afterwards it stays at a small elongation, and in fact it re-enters the C3 field-of-view in early January 2024 and remains there for almost a month, although it will probably be far too faint to be detectable in C3 images. The comet finally emerges into the morning sky by the latter part of March but will almost certainly be a very faint object by that time.
I pointed out in the tally entry for its 2007 return that when I first saw P/Encke in late 1970 I was 12 years old, and was just starting out on my "career" of comet observing; it was only the fourth comet, and the first short-period one, that I ever observed, and this came just 9 1/2 months after my very first comet observation. I mentioned within that entry some of the interesting and important times in my life that happened to coincide with some of the various returns of P/Encke that I have observed; to that list I might add that it was while I was observing it during its return in 2017 (no. 610) that I met and began dating my current partner Vickie. And now, at age 65, I am observing P/Encke once again, and as before this return is coinciding with some interesting times in my life. I have recently obtained some rather important astronomical observations that I will mention in this space at the appropriate time, and I may be participating in some Earthrise-related endeavors within the near future that I will also mention if and when appropriate. On the personal side of things, in two weeks my younger son Tyler and his long-time partner Sara are getting married, and meanwhile my older son Zachary (presently residing in Australia) has recently informed me that he and his long-time partner Karina are expecting a child in February -- which will make me a biological grandfather for the second time. So, while Comet Encke continues to make its repeated passages through the inner solar system, life and other goings-on here on Earth continue for me, and for everyone else.
As for whether or not P/Encke and I will continue to check in on each other . . . Based upon the "retirement" timeframe of the end of 2024 that I have mentioned repeatedly in these pages, this would appear to be my final observed return of this comet. However, I have always maintained the caveat that, depending upon my health and other life circumstances, I could conceivably "come out of retirement" on occasion for bright or otherwise interesting comets. It so happens that P/Encke's next return, in 2027 (perihelion early February), is a rather favorable one, when it will be well placed in the evening sky. The comet's return in 2030 (perihelion early June) is impossibly placed for observation from the northern hemisphere, although observers in the southern hemisphere should get a good view of it as it passes 0.27 AU from Earth six weeks after perihelion passage. The return in 2033 (perihelion mid-September) is observable from the northern hemisphere but the viewing geometry is only mediocre, at best. The return after that, in 2037 (perihelion early January), is a very favorable one virtually identical to that first return of mine in 1970-71, the respective perihelion dates being only two days apart; the comet passes 0.38 AU from Earth in late 2036 and should be easily detectable in the evening sky. Whether or not I'll still be alive, and willing and able to "come out of retirement," at age 78 remains to be seen, of course, but if it were to come to pass it would seem to be a fitting bookend to this rather interesting "career" that I have had, and that Comet Encke has shared with me from almost the very beginning.
INITIAL OBSERVATION: 2023 September 7.41 UT, m1 = 11.7, 1.8' coma, DC = 1-2 (41 cm reflector, 229x; moonlight)
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