CONTINUING WITH COMETS
TALLY ENTRIES 621-630
|621. COMET PANSTARRS C/2015 O1 Perihelion: 2018 February 19.04, q = 3.730 AU
The initial comet that I'll list as I go back on-line with my newly-renamed comet observing effort is something I actually added to my tally over three weeks ago. However, since I like these tally pages to be "decadal," i.e., encompassing even groups of 10, this will be the one that starts it off.
This is one of the many comet discoveries by the Pan-STARRS program in Hawaii that went on-line in 2010, and which at this time can be considered the premier near-Earth asteroid- and comet-hunting survey program in the world. This is, somewhat curiously, the 100th comet discovered by Pan-STARRS (of 155 comet discoveries at this writing, a number that is rapidly increasing all the time). Most of the Pan-STARRS comet discoveries are far too faint for me to observe, or even bother attempting, although the second discovery of the program, C/2011 L4, which was the first one from the program I observed (no. 504), has been the brightest comet of this decade thus far, at least as far as the northern hemisphere is concerned. This latest comet is only the 13th Pan-STARRS comet that I have observed thus far, and indeed there are only nine more that I've even attempted.
Pan-STARRS discovered this comet -- then a relatively dim object of 19th magnitude -- on July 19, 2015, only four days before the 20th anniversary of my discovery of Comet Hale-Bopp C/1995 O1 (no. 199), and just as I was putting the final touches on finishing my autobiography. The initial similarities between the two comets seemed especially remarkable, beginning with the "5 O1" part of their respective discovery designations; this comet's discovery location in southern Capricornus was only a little over 20 degrees east-northeast of Hale-Bopp's discovery location in Sagittarius. The initial calculations also suggested that this comet was at similar distances from the sun and Earth as Hale-Bopp was at the time of its discovery, which suggested in turn that this is a moderately bright comet, intrinsically. Finally, the initial published orbit indicated a perihelion passage in early February 2017 at a heliocentric distance of 1.04 AU, which suggested, if not necessarily a "Great" comet, at least a comet that might become relatively easily visible to the unaided eye, and with good viewing geometry from the northern hemisphere. I speculated a bit on this possibility in my autobiography, although clearly things were too preliminary at that point to make any definitive statements.
Alas, it was not to be. Within a month it was clear that perihelion passage would take place at least a year later than that, at a much larger heliocentric distance. The heliocentric and geocentric distances at discovery (8.42 and 7.42 AU, respectively) were even greater than those initially indicated, although these in turn did verify that the comet is, intrinsically, a bright one. Unfortunately, with the much larger viewing distances involved this rendered any chances of even a somewhat bright comet essentially down to zero.
Comet PANSTARRS is traveling in a fairly steeply-inclined retrograde orbit (i = 127 degrees) and was at opposition (at a heliocentric distance of 4.31 AU) this past June 18. I made my first (unsuccessful) attempt in mid-May, but when I attempted it again on the evening of June 13 I suspected it as an extremely faint and tiny object of magnitude 14 1/2, which I was able to verify as indeed being the comet over the next half-hour due to its motion.
The comet is currently located in southern Hercules a little over three degrees southeast of the star Delta Herculis, and is traveling due westward at a current rate of about 20 arcminutes per day, although this slows down to 10 arcminutes per day by the end of August, at which time it will be located 3 1/2 degrees west of the star Beta Herculis. It reaches its stationary point in early October, when it will be located 3 1/2 degrees west of the star Gamma Herculis (and two degrees north of the Hercules galaxy cluster, Abell 2151), after which its motion turns eastward, although by this time it will be starting to be low in the northwestern sky after dusk. Throughout this entire time the comet will likely remain quite faint, although it does seem to have brightened slightly since I first picked it up; nevertheless, it will still probably gain at most just a few additional tenths of a magnitude in brightness.
Comet PANSTARRS is in conjunction with the sun (40 degrees north of it) in late November, and not too long thereafter becomes accessible in the northeastern morning sky before dawn. At the beginning of 2018 it will be located 1 1/2 degrees due north of Beta Herculis and traveling slightly eastward of due north at 10 arcminutes per day; this rate increases over time as the comet turns more towards due north and then eventually to the north-northwest. At the time of perihelion passage it will be traveling through the "keystone" of Hercules at 20 arcminutes per day and will be located two degrees southwest of the large globular cluster M13; by then it should be at least half a magnitude brighter than it is now, perhaps somewhere between magnitudes 13 1/2 and 14. The comet continues its northwesterly trek after that, crossing into northeastern Bootes in late March and passing north of declination +50 degrees at the end of that month, and then skirting parts of southwestern Draco before entering eastern Ursa Major shortly after mid-April. It is closest to Earth (3.23 AU) in early April, at opposition during the latter part of that month, and at its maximum northerly declination (+57 degrees) in early May; throughout this time it should be near its peak brightness, perhaps near magnitude 13 1/2.
Now traveling towards the west-southwest, Comet PANSTARRS crosses the Big Dipper's "handle" between the stars Zeta ("Mizar") and Epsilon ("Alioth") Ursae Majoris shortly after mid-May, and in fact passes just 15 arcminutes south of the latter star on May 21. Its motion turns more and more towards the southwest, and it passes one degree east of the star Chi Ursae Majoris in early July. Due to both increasing faintness as well as getting lower in the northwestern evening sky, I will probably lose it sometime around that time.
At this time my life is pretty much as described in my July 6 personal statement. Shortly before I picked up this comet I had returned from the 7th SpaceFest conference that was held in Tucson, Arizona, where I had been invited to speak on the "Planetary Defense" panel on the last day (and where I encountered the young lady from Kosovo whom I referred to at the end of my statement). Shortly before that, on May 29 my nephew Jeremy (son of my older brother Barry) and his wife became the parents of a baby girl, Claire, and thus I now have a grand-niece. It will be interesting to see what kinds of changes and goings-on might take place in my life during the year or so that I will be following this comet; I can't help but notice that I will "celebrate" my 60th birthday (ugh!) right around the time that it is brightest. Life goes on, as the comets continue . . .
INITIAL OBSERVATION: 2017 June 14.21 UT, m1 = 14.5, 0.2' coma, DC = 8 (41 cm reflector, 229x)
As fate would have it, I was able to sneak one more comet into my tally before "going live" with the re-worked Earthrise web site and with "Continuing With Comets." This newest comet is somewhat of an old friend; it was discovered by Mike Van Ness in September 2005 (apparently after an outburst caused by an earlier fragmentation of its nucleus) during the course of the LONEOS program in Arizona, and I obtained a handful of observations of it then as a faint object (no. 381). During its subsequent return in 2011 I observed it as a part of "Countdown" (no. 485) and followed it for 4 1/2 months, although it remained faint, never getting much brighter than 14th magnitude; the secondary fragment that I commented on during the comet's "Countdown" entry remained much too faint for me to detect. Since 2011, a somewhat distant approach to Jupiter (1.17 AU) in October 2014 has decreased the perihelion distance to the present value and shortened the orbital period to 6.12 years. For whatever it's worth, 213P becomes the seventh comet that I have observed at its discovery return and at both of its next two subsequent returns.
The viewing geometry at the current return is rather favorable, but as April of this year approached and there had not been any announced recovery of the comet, I became curious as to whether or not it still existed -- the fragmenting event in 2005 having led to its eventual demise -- or if in fact it had somehow simply been missed (a moderately high southerly declination of -33 degrees perhaps being a factor), especially when ephemeris predictions were suggesting it might almost be bright enough for visual detection. I ended up making a visual recovery attempt on the morning of April 27, but didn't see anything. I would later learn that the comet had been recovered just a few days earlier, on April 22 by Kevin Hills at the Tacande Observatory at La Palma in the Canary Islands; the reported brightness was close to 18th magnitude.
I didn't look for Comet Van Ness during May, suspecting it was likely too faint for any worthwhile attempts, but I began looking for it again during June, especially after reading reports and seeing images that suggested it had brightened quite a bit. I finally suspected it on the morning of June 29, although it was in a somewhat crowded star field, and I was having to contend with smoke from a distant forest fire affecting my sky conditions; under better conditions, and with the comet in a better star field I easily confirmed it the following morning as as rather vague and diffuse object near magnitude 13 1/2.
The comet is presently located in south-central Sagittarius approximately three degrees southeast of the star Zeta Sagittarii (the southwestern star of the "handle" of the "teapot") and is traveling towards the west-northwest at 12 arcminutes per day. It is at opposition on July 8 and is closest to Earth (1.053 AU) ten days later, during which time its motion is turning more northerly, and it passes within 40 arcminutes southwest of the above star (and one degree northeast of the globular star cluster M54). Afterwards, it travels northward through the "handle," being at its stationary point on August 15 and then turning northeastward, passing half a degree south of the star Omicron Sagittarii in mid-September and a similar distance south of the star Pi Sagittarii a week later, and remaining in northeastern Sagittarius until near the end of October. It is probably close to its maximum brightness right now, possibly gaining a couple of tenths of a magnitude by late July and early August, but probably fading beyond the range of visual detectability not too long after perihelion passage.
More than likely, this will be the last return at which I obtain observations of 213P/Van Ness. The viewing geometry at the next return, in 2023 (perihelion mid-November) is not especially good, and while an approach to Jupiter of 0.57 AU in October 2026 will decrease the perihelion distance to 1.93 AU, neither of the next couple of returns are all that favorable, and by the time of the second one (2036) I am quite certain I will have "retired" from comet observing, anyway.
CONFIRMING OBSERVATION: 2017 June 30.30 UT, m1 = 13.4, 1.6' coma, DC = 2 (41 cm reflector, 229x)
In the late 19th and early 20th centuries astro-photography was coming into its own, and among many other things most of the brighter main-belt asteroids were discovered then -- at such a rate that one astronomer of that era referred to them as the "vermin of the skies" (although the discovery rate of asteroids then was only a small fraction of what it is today). One of the more prolific asteroid discoverers of that era was the German astronomer August Kopff, who was a student, and later professor, at Heidelberg University, earning his Ph.D. there in 1906. Overall he discovered 68 asteroids as well as two comets, both in 1906, with one of these being the periodic comet now known as 22P/Kopff which I've observed on several returns, including my fourth return in 2009 as a part of "Countdown" (no. 448) and more recently in 2015 (no. 572).
One of Kopff's earlier asteroid discoveries was an object he found on February 21, 1906, and which passed perihelion the following January. It was found to be traveling in a low-eccentricity (0.16) orbit with an orbital period very close to five years, and recent studies have indicated that it is approximately 113 km (70 miles) in diameter. The asteroid was assigned the permanent number (596) and Kopff named it Scheila, in honor of a British student and acquaintance of his at the University.
For over a century after its discovery Scheila behaved normally for a main-belt asteroid, and was regularly observed when near opposition. However, on December 11, 2010, Steve Larson with the Catalina Sky Survey in Arizona reported that Scheila has apparently experienced a comet-like outburst, being approximately one magnitude brighter than the ephemeris prediction and exhibiting an extended coma roughly 2 by 5 arcminutes across. Other observers soon confirmed Scheila's cometary appearance, and an examination of earlier Catalina images suggested that the outburst had occurred around the beginning of December, although even during November it was running slightly brighter than predicted.
|(596) SCHEILA FOLLOWING ITS OUTBURST IN DECEMBER 2010. Left: Imaged by Martin Mobberley in England, December 13, 2010. Used with permission. Right: Image obtained with the Hubble Space Telescope by David Jewitt (UCLA) and Max Mutchler (Space Telescope Science Institute) December 27, 2010. Image courtesy NASA/ESA.|
|After reading of Scheila's outburst I successfully located it during the early morning hours of December 12, when it was located a few degrees north-northeast of the "head" of the constellation Leo; it appeared slightly fainter than magnitude 13 1/2, and although I couldn't quite convince myself I was seeing any kind of coma, there did seem to be a tiny hint of "fuzziness."
Scheila seemed a few tenths of a magnitude brighter when I next saw it six mornings later, but by then its appearance was completely stellar -- indeed, I never again saw any associated diffuseness that was anywhere near convincing. Overall, it continued to brighten slightly as it approached opposition at the beginning of February 2011, and was near 13th magnitude around that time -- reasonably consistent with the ephemeris prediction. It began fading thereafter -- again, consistent with its ephemeris-predicted brightness -- and I followed it until the evening of June 20, by which time it had faded to 15th magnitude and was getting rather low in the western sky after dusk.
Following its outburst Scheila was, obviously, extensively studied by both numerous ground-based telescopes and by space-based platforms like the Hubble Space Telescope and the Swift Gamma-Ray Burst Mission. An analysis of all this data indicates that Schelia's coma was made up entirely of dust, i.e., no gaseous emissions were present, and this in turn suggests that the outburst was almost certainly due to an impact from a previously-unknown asteroid some 35 meters in diameter.
Such an explanation would suggest that future outbursts would be quite unlikely, yet I have continued to monitor Scheila on a semi-regular basis ever since, usually about once a month or so during the months around opposition. Following its conjunction with the sun in early October 2011 I picked it up again in the morning sky shortly before the end of December as a stellar object slightly brighter than magnitude 14.5, and I observed it on a handful of occasions up through late May 2012 when it had just passed through perihelion and was also near opposition; at that time it was 12th magnitude. I did not observe it any further during that viewing season, in significant part because, as I have indicated in "Counting Comets" and elsewhere, I was dealing with the breakup of my relationship, and for a while almost lost interest in observing. I finally picked it up again in late September 2013 when it was near opposition and magnitude 13 1/2, and observed it on a handful of occasions up through late December, by which time it had faded to 15th magnitude.
In 2014 I picked up Scheila for the first time in early October, when it was just brighter than 15th magnitude. By mid-November it has brightened by a full magnitude as it approached its opposition at the end of that month; meanwhile, it was also approaching aphelion, which took place on November 17 at a heliocentric distance of 3.409 AU. I obtained my final observation of the 2012 "return" on the evening of November 15, when it was 41 hours away from aphelion. I have continued to follow Scheila since that time, but those observations are described in the following entry.
Ever since I first observed Scheila following its outburst in December 2010 -- during which time I included it on the "update" pages of "Countdown," at least through the end of that particular viewing season -- I have devoted considerable thought as to whether or not it I could legitimately consider it a "comet" for tally purposes. Some of the earlier reports do in fact describe it as a "main belt comet" (although the term "active asteroid" has become more in vogue since that time), however once it became clear that the 2010 outburst was almost certainly due to an impact as opposed to any kind of intrinsic activity, Scheila's inclusion as a "comet" seemed rather questionable. In late 2014 as I was working on my autobiography I gave heavy consideration to Scheila, as well as to a couple of other "asteroids" I have observed that have exhibited cometary behavior, as to whether or not I could consider them as "comets" for tally purposes (and with the thought in mind that the IAU Committee on Small Body Nomenclature had not assigned them "dual nature" status); while I did determine that a legitimate case could be made for including the other two objects -- (3200) Phaethon and (3552) Don Quixote (both of which I discuss in my "special note" in the "Continuing with Comets" main page -- I concluded that I could not make a strong enough argument for including Scheila as such.
However, as I also point out in that "special note," the object provisionally designated as P/2010 A2, which was discovered in January 2010 by the LINEAR program in New Mexico, has also been determined as being the result of any impact between two asteroids. It was recovered as a very faint and completely stellar object earlier this year, and was assigned the cometary designation P/2017 B5, despite the nature of the activity it exhibited seven years ago and its current appearance. I pointed out in my "special note" that, if the IAU's Committee on Small Body Nomenclature were to assign a permanent periodic comet number to this object, then an argument could be made that any object that exhibits cometary activity, regardless of the mechanism that produced that activity, can be considered a "comet" for tally purposes, even if its true nature is closer to that of an asteroid.
And, indeed, this is now what has happened. In the July 2017 batch of the Minor Planet Circulars, the object originally designated as P/2010 A2 has now been assigned the permanent periodic comet number 354P. I thus am now adding Scheila to my comet tally retroactively, with my first observed "return" (which contained the actual outburst) constituting this entry, and the current return constituting the following entry.
INITIAL OBSERVATION: 2010 December 12.32 UT, m1 = 13.8, <0.1' coma, DC = 8.5 (41 cm reflector, 70x)
FINAL OBSERVATION: 2014 November 16.22 UT, m1 = 13.8, 0.0' coma, DC = 9 (41 cm reflector, 70x)
I discuss this object's discovery, observational history, 2010 outburst, my early observations, and my reasons for adding it to my comet tally retroactively in the previous entry. I also discuss there how I obtained my final observation of that "return" only 41 hours before aphelion. I didn't wait long to get started on the new return; I obtained my next observation only two nights later, i.e., on the evening of November 17, when it was only seven hours past aphelion; it appeared completely stellar at slightly brighter than 14th magnitude, and was located in central Taurus five degrees northeast of the Hyades star cluster.
As I had been doing ever since it underwent its outburst, I have continued to monitor Scheila on a semi-regular basis ever since it passed through aphelion, usually once a month or so during the timeframe surrounding opposition; as has continued to be true ever since the outburst subsided, my measured brightnesses have always been within two or three tenths of a magnitude of its ephemeris prediction. It was at opposition near the end of November 2014 and I followed it until early February 2015, by which time its brightness had dropped to magnitude 14 1/2. Following conjunction with the sun in late June 2015 I picked it up again (at magnitude 14 1/2) in early November and followed it through late May 2016, by which time its brightness was again near magnitude 14 1/2; when near opposition in early February it was near magnitude 13 1/2.
After its conjunction with the sun in early October 2016 I picked up Scheila for the first time during the current viewing season on the morning of February 25, 2017, as a stellar object slightly fainter than magnitude 13 1/2. It brightened steadily after that as it approached perihelion in mid-May and opposition at the end of that month; I consistently measured brightnesses near 12th magnitude between mid-May and mid-June. It has started to fade somewhat now, and was 13th magnitude when I most recently observed it a couple of nights ago.
Scheila is presently located in the "head" of the constellation Scorpius, 1 1/2 degrees north of the star Pi Scorpii (the southernmost of the three stars that mark the "head); it is traveling almost due southward at a slow five arcminutes per day and reaches its stationary point on July 17 at which time its motion turns toward the southeast, passing half a degree northeast of the above star around July 28 and half a degree south of the star Tau Scorpii on September 7. It passes less than ten arcminutes north of the globular star cluster M62 on September 25, by which time it will have faded to 14th magnitude, and thereafter travels due east across eastern Scorpius at approximately 20 arcminutes per day, crossing into western Sagittarius on October 27 and becoming lost in the rich Milky Way star fields as it sinks lower into the southwestern sky.
Scheila is in conjunction with the sun in early February 2018 and begins emerging into the morning sky around April; it is at opposition in late September, when it will be near magnitude 13 1/2 and located in southwestern Cetus (passing half a degree north of the bright star Beta Ceti, or Diphda, during the middle of that month). After disappearing into evening twilight in early 2019 it is again in conjunction with the sun in late April of that year; it emerges into the morning sky around July, and should be between magnitudes 13 1/2 and 14 when it goes through aphelion in late November shortly before it is at opposition. As it was when it was at the previous aphelion, it will then be located in central Taurus a few degrees north-northeast of the Hyades.
Provided that I remain active in observing that long -- and my current expectations are that I likely will -- then I will probably continue to monitor Scheila up through its next perihelion passage (late May 2022) and quite possibly for at least another year or two beyond that. I honestly do not expect any repeats of the outburst in underwent in 2010 -- but, one never knows . . .
Scheila's retroactive addition to my tally induces a couple of retroactive changes into my cometary statistics and notable happenings. I commented in my statistical analysis of comets 1-600 that I observed a record number of comets -- 38 -- during the year 2015; with Scheila now being a comet for tally purposes, that total is now 39. It also means that I have now already recorded my 7000th visual comet observation; that distinction goes to the observation I made of Comet PANSTARRS C/2015 ER61 (no. 611) on the morning of June 30, 2017.
INITIAL OBSERVATION: 2014 November 18.28 UT, m1 = 13.8, 0.0' coma, DC = 9 (41 cm reflector, 70x)
FIRST "POST-TALLY ADDITION" OBSERVATION: 2017 July 13.19, m1 = 13.1, 0.0' coma, DC = 9 (41 cm reflector, 70x)
By long-standing tradition, and for obvious reasons, the IAU has limited names on comets to three different discoverers. Even so, some of the resulting comets' names have been rather unwieldy and tongue-twisting; a glance through a list in comet catalogues, or even the list of comets I observed in my earlier years, bears this out. Over the past couple of decades, and especially as more and more comet discoveries began to be made by teams, the IAU has made a strong effort to limit the number of different names on a comet to two. Nowadays, with the various surveys patrolling the skies and finding very faint comets, two independent discoveries of any given comet are rather rare, and thus only a relative few of the comet discoveries over the past decade or so even have two names.
With this particular comet, however, the usage of three different names was probably unavoidable. It was discovered by the Pan-STARRS program in Hawaii on January 23, 2016, however once enough positions had been obtained so that a reasonable preliminary orbit could be calculated, astronomers determined that it had already been discovered, not once, but twice before: the Mount Lemmon survey in Arizona had discovered it on November 2, 2015, whereupon it had been assigned the designation 2015 VL62, and an amateur astronomer, Bill Yeung, had discovered it (via a remotely operated telescope in New Mexico) on December 18, 2015, whereupon it had been assigned the designation 2015 YY6. Neither of these two earlier discoveries had been followed well enough for a linkage to be made or for a reliable orbit to be computed until after the Pan-STARRS discovery.
I've discussed the Mount Lemmon and Pan-STARRS programs in earlier entries in either "Countdown" or "Counting Comets." Bill Yeung (full name William Kwong Yu Yeung) was born in Hong Kong but currently resides in Canada, and has operated remotely controlled telescopes at private observatories in Alberta, Arizona, and New Mexico. He has discovered over 2000 asteroids that have received permanent numbers -- which, including the various surveys, currently puts him in 14th place on that overall list. In September 2002 he discovered a faint object provisionally designated as J002E3 that was at first thought to be an asteroid, but which has apparently turned out to be the S-IVB third stage of the Saturn V rocket that launched the Apollo 12 mission. Yeung has one other comet discovery, also made in 2002: a short-period comet, now designated 172P/Yeung, that has an orbital period of 6.8 years. This particular comet remains very faint and I have never attempted it; it most recently returned to perihelion in March 2017.
Except for its unusually long name, Comet C/2015 VL62 is otherwise one of the many faint, nondescript, large-q long-period comets that show up in my tally quite regularly. It is a first-time visitor from the Oort Cloud, and is traveling in an orbit with an inclination of 166 degrees, i.e., the orbit is inclined only 14 degrees with respect to the ecliptic, but is retrograde. After being in conjunction with the sun in late March 2017 it began emerging into the morning sky, and I made my first attempt in late May and a handful of additional attempts during late June; I occasionally thought I might be seeing an extremely faint "something" but I could never convincingly see anything definite to my satisfaction. On my first attempt of the current dark run, in the early morning of July 18, I suspected a very faint, small, slightly diffuse object a bit fainter than 14th magnitude, and over the course of the next half hour was able to verify my suspect as being the comet via its motion.
The comet is currently located in eastern Pegasus five degrees east of the star Alpha Pegasi (Markab), the southwestern star of the "Great Square" of Pegasus. It is traveling slightly southward of due west, presently at a little over half a degree per day, and passes two degrees south of the above star on July 26. Its apparent motion continues to accelerate as it passes half a degree south of the star Xi Pegasi on August 1 and a similar distance north of the star Zeta Pegasi two days later, by which time its speed will have increased to 50 arcminutes per day. The comet is at opposition on August 17 and is nearest Earth (1.74 AU) four days later; at that time it will be still be located in Pegasus (albeit in the southwestern corner of that constellation) but crosses into northwestern Aquarius on August 26; it will then be traveling at its peak speed of one degree per day and should be near its peak brightness, perhaps half a magnitude or so brighter than it is now.
At the time of its perihelion passage the comet will still be in northwestern Aquarius, some four degrees southwest of the star Alpha Equulei. As it pulls away from both the Earth and the sun it should begin fading and slowing down, and its motion turns more and more southerly; it crosses into southeastern Aquila on September 5 and remains within that constellation before finally crossing into northeastern Sagittarius on October 8. Sometime during this period it will likely have become too faint for visual observations. The planetary perturbations it has encountered during its passage through the inner solar system have placed it into a slightly hyperbolic orbit, and it will eventually leave the solar system altogether.
INITIAL OBSERVATION: 2017 July 18.32 UT, m1 = 14.3, 0.4' coma, DC = 6-7 (41 cm reflector, 229x)
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