ALAN'S COMETS

TALLY ENTRIES 651-660

651. COMET P/GRISELDIS P/(493)          Perihelion: 2019 February 15.99, q = 2.570 AU

We have known for some time that the difference, in physical terms, between "comets" and "asteroids" is rather nebulous (and there is admittedly a pun in saying this). Indeed, that awareness began almost a century ago, when in 1920 the German astronomer Walter Baade (who, two to three decades later after relocating to southern California, would become one of the pioneers in the study of stellar populations) discovered an apparently asteroidal object with an orbital period of just under 14 years, a perihelion distance close to 2 AU, and an aphelion distance almost as far out as Saturn's orbit. Despite careful scrutiny over several subsequent returns, this object -- now known as (944) Hidalgo -- has never exhibited any clear signs of cometary activity, although there is nevertheless a moderately strong consensus among cometary astronomers that it is an extinct cometary nucleus. I managed to observe Hidalgo during its returns in 1991 and 2005, and I have just now picked it up on an additional return; it will pass through perihelion in late October 2018 and should reach 14th magnitude.

During the decades that have elapsed since Hidalgo's discovery many additional apparent asteroids in distinct cometary orbits have been discovered, and the consensus is that many, if not almost all, of these objects are also extinct (or perhaps dormant) cometary nuclei. Indeed, the late planetary scientist Eugene Shoemaker once estimated that, for every active Jupiter-family comet, there may be as many as eighteen to twenty extinct Jupiter-family comets. It is conceivable that some of these objects might still exhibit weak cometary activity on occasion, and I do attempt to observe any which become bright enough for visual observations (partially in the hope that I might be able to add them to my tally retroactively). The classic case is (3552) Don Quixote, originally discovered in 1983 and which has a perihelion distance of approximately 1.2 AU and an orbital period of slightly under nine years. During its 2009 return (during which I managed to observe it on two occasions as a stellar 15th-magnitude object) the infrared-sensitive Spitzer Space Telescope detected a weak coma and tail; when this was announced some years later I retroactively added Don Quixote to my tally (no. 562). During its return earlier this year Don Quixote once again exhibited weak cometary activity.

Over the past decade or so another type of object has demonstrated the "nebulous" boundary between "comet" and "asteroid." There are the objects once called "main belt comets" but now more generally known as "active asteroids." As these terms suggest, these are objects that travel in low-eccentricity (and low-inclination) orbits like those of asteroids in the main asteroid belt between Mars and Jupiter, but which have on one or more occasions exhibited some type of cometary activity. Some of these objects -- for example, the object now designated as 133P/Elst-Pizarro -- have exhibited such activity on several occasions and it appears to be genuinely due to water-ice sublimation just as in more "traditional" comets. However, other mechanisms seem to be responsible for the activity in other objects. Some of these have nevertheless been formally designated as short-period comets; the activity in 311P/PANSTARRS has been apparently due to dust clouds ejected by a very rapid sunlight-driven rotation, while the activity in 354P/LINEAR was apparently due to debris created by an impact from a smaller asteroid.

In December 2010 the large main-belt asteroid (596) Scheila exhibited a comet-like coma and tail that were very likely caused by an impacting smaller asteroid. I have been observing Scheila on a semi-regular basis ever since that occasion, and it has never exhibited any repeat of that activity; nevertheless, after the official designation of "Comet" 354P/LINEAR last year I retroactively added Scheila to my tally (no. 623), as I explain in its entry. Following Scheila's aphelion in late 2014 I have continued to follow it from time to time (no. 624), and it is presently near opposition and close to magnitude 13.5.

Another large main-belt asteroid has now been seen to exhibit a one-time burst of cometary activity. This is (493) Griseldis, originally discovered from Heidelberg Observatory in Germany in September 1902 by the German astronomer Max Wolf, and which is about 46 km (29 miles) in diameter and which has an orbital period of 5.5 years. (Wolf was one of the top astronomers of his era and pioneered the then-emerging art of astrophotography; during a career that spanned four decades in the late 19th and early 20th Centuries he discovered almost 250 asteroids -- a record for the time -- and three comets, two of which are short-period comets and one of which, 43P/Wolf-Harrington, I've observed on five returns. He has the distinction of recovering (in September 1909) Comet 1P/Halley during its 1910 return.) The name "Griseldis" is a feminine personification of the quality of virtue, and appears in some medieval and classical novels and operas.

On March 17, 2015, astronomers Dave Tholen, Scott Sheppard, and Chad Trujillo detected a short tail on Griseldis in images taken with the 8.2-meter Subaru Telescope located on Mauna Kea in Hawaii. Additional images obtained with one of the twin 6.5-meter Magellan Telescopes (based at Las Campanas Observatory in Chile) four nights later also show this feature, albeit weaker, but it has not been detected on any subsequent images. At the time these images were taken Griseldis was a little over 1 1/2 years past its perihelion passage (and thus slightly closer to aphelion), and in light of all the available evidence Tholen and his colleagues have concluded that the activity Griseldis exhibited is very probably the result of an impact event, just as was the case with Scheila.

Based upon the precedent I established with Scheila, I have concluded that I am justified in including Griseldis as a comet for tally purposes. It was at opposition in early July 2017 and I made one unsuccessful attempt for it in late June of that year; while it was probably too faint for me to detect then, its location within rich Milky Way star fields didn't help. During its present period of visibility I made my first attempt for it on the evening of September 8 and successully detected it as a stellar object of magnitude 14 1/2 that exhibited distinct motion over an interval of an hour.

At present Griseldis is located in south-central Pisces some three degrees west of the star Delta Piscium and is traveling almost due westward at just under 15 arcminutes per day. It is at opposition at the very beginning of October when it should be a few tenths of a magnitude brighter than it is now, and I should be able to follow it until sometime in November (when it will be located some 1 1/2 degrees north of the star Omega Piscium), at which time it should fade beyond my range. Following its perihelion passage next February it is in conjunction with the sun during the latter part of May; meanwhile, it is possible it may be just bright enough for me to detect around the time of the next opposition in early February 2020, at which time it will be located in southern Lynx. I do not expect Griseldis to exhibit any repeat of its previous cometary activity during this time, although I suppose the possibility exists that I could be pleasantly surprised.

(493) Griseldis (center, with downward-pointing "tail") on March 17, 2015, as imaged with the 8.2-meter Subaru Telescope at Mauna Kea Observatory in Hawaii. Image courtesy Dave Tholen, Scott Sheppard, and Chad Trujillo.

INITIAL OBSERVATION: 2018 September 9.24 UT, m1 = 14.5, 0.0' coma, DC = 9 (41 cm reflector, 229x)

 

652. COMET ATLAS C/2018 L2     Perihelion: 2018 December 2.58, q = 1.712 AU

Another ATLAS-discovered comet enters my tally. This one was discovered by the Mauna Loa telescope on June 6, 2018, and was reported as being around magnitude 15 1/2 to 16 -- rather bright for a survey discovery these days. Although it was located fairly far south at a declination of -42 degrees, it was otherwise accessible without much difficulty from my observing site, but my visual attempt for it a few days after its discovery was unsuccessful. I did, however, successfully image it with the Las Cumbres Observatory telescope at Siding Spring while it was still listed on the Minor Planet Center's Possible Comet Confirmation Page.

The comet is traveling in a moderately high-inclination (67 degrees) direct orbit and, having already passed through opposition in early May, has remained in the evening sky ever since its discovery while steadily climbing northward. Due in significant part to our summer monsoon as well as to the health issues I described in an earlier entry, I did not attempt it again until early September, by which time its declination had reached -8 degrees. I possibly saw "something" during my first attempt, but it was located in a very poor star field and I couldn't convince myself that I was actually seeing anything. Finally, on the evening of September 10 -- when it was located in a better star field -- I clearly saw the comet as a diffuse, slightly condensed object just a bit fainter than 13th magnitude.

This more recent Comet ATLAS is presently located fairly low in my southwestern sky after dusk, in northern Libra some five degrees northwest of the star Beta Librae, and is traveling towards the east-northeast at slightly less than half a degree per day. Despite the fact that it is still 2 1/2 months away from perihelion passage and is continuing its northward climb, I will probably not be able to obtain many observations of it; it was closest to Earth (1.95 AU) in mid-June and is now moving over to the far side of the sun, and accordingly will be sinking lower and lower into the western sky. At present its elongation is 50 degrees, with this decreasing to 40 degrees in early October and reaching a minimum of 32 degrees in mid-November. By that time it will be in near-conjunction with the sun (although well to the north of it), where it will remain well into 2019, with the actual moment of conjunction coming in mid-January, when it will be 45 degrees north of the sun and located in Vulpecula. The elongation reaches a maximum of only 47 degrees in mid-February (while passing directly across the central portions of the Veil Nebula in Cygnus during the first week of that month) before shrinking again, reaching a minimum of 35 degrees in mid-May -- before which time it passes some four degrees north of the Andromeda Galaxy M31 shortly after mid-April -- and then it finally begins to increase again. I suspect the comet will be long past the point of visual detectability by then; while it might reach perhaps 12th magnitude around the time of perihelion, it will likely fade beyond visual range within one to two months after the beginning of 2019.

INITIAL OBSERVATION: 2018 September 11.11 UT, m1 = 13.3, 1.2' coma, DC = 2-3 (41 cm reflector, 70x)

 

653. COMET 46P/WIRTANEN          Perihelion: 2018 December 12.94, q = 1.055 AU

In one of my tally entries from a year ago I commented that, ever since I relocated to 16 Springs Canyon in 1995, I have managed to add at least one comet to my tally during the month of September every year. I have now continued that remarkable string for the 24th consecutive year, and indeed did so in rather dramatic fashion by adding three comets to my tally over a span of only four nights during this month of September 2018. And there is still a bit of September left to go, although with the upcoming full moon -- the Harvest Moon, in fact, with its long series of bright moonlit evenings -- and no strong candidates for tally additions at this time, I doubt if I'll add any more comets before month's end.

The third comet of this "trio" is an old friend that I am now seeing for the sixth time. I observed it during its 2008 return (no. 416) as a part of "Countdown" and I discuss its observational history, and my own history with it, in its "Countdown" entry. As I predicted there, the intervening return in 2013 was very unfavorable and I did not look for it, but it is now back for the best return it will ever make. It was recovered on June 18, 2018 by a team led by Lori Feaga from the University of Maryland while utilizing the Discovery Channel Telescope based at the Lowell Observatory in Arizona. (An earlier apparent recovery on May 8 reported by members of the same team turned out to be a false alarm, as the "comet" turned out to be a faint main-belt asteroid that, coincidentally, was located close to the P/Wirtanen's expected position and that exhibited its expected motion. Once this was recognized, the comet -- fainter than expected -- was belatedly recognized on the same images.) I made my first attempt for it in early September but didn't see anything convincing, however on the morning of September 12 I successfully observed the comet as a small, diffuse object of 14th magnitude. It has brightened rapidly since then, by over half a magnitude when I observed it again six mornings later.

On December 16 Comet P/Wirtanen passes only 0.077 AU from Earth -- the eighth-closest approach to our planet of all the comets on my tally. (Curiously, this encounter takes place exactly one year, to the very day, after the seventh-closest approach to Earth of the comets on my tally.) Because of this close approach and the accompanying very favorable viewing circumstances, the comet is the target of a special NASA-coordinated observing campaign during this return. I am involved with this campaign and will be contributing both visual observations as well as images taken with the Las Cumbres Observatory network -- indeed, I have already obtained one such image, taken about two weeks ago -- and possibly some DSLR photographs. Although I am not quite to the point where I am working with students yet, it is conceivable that I might have some working with me by the time of the close approach to Earth.

At present Comet 46P/Wirtanen is located in southern Cetus three degrees southwest of the star Upsilon Ceti (and some seven degrees south of the nearby solar-type star Tau Ceti). It is traveling towards the southeast at a present rate of 20 arcminutes per day, crossing into northwestern Fornax within a few days and being at opposition during the third week of October, when its declination will be -31 degrees. After reaching a peak southerly declination of -33 degrees in early November the comet begins a rapid climb to the north-northeast, back into Cetus and then into Eridanus and (during the second week of December) Taurus. At the time of its closest approach to Earth it will be located four degrees southeast of the Pleiades star cluster (M45) and will be traveling (still towards the north-northeast) at a little over four degrees per day. Based upon the brightness I've observed during previous returns, it may be 11th to 12th magnitude during October, brightening from 10th to 9th and possibly 8th magnitude during November, and could well be a naked-eye object of 4th or 5th magnitude (but with a very large, diffuse coma) around the time of its closest approach.

Following it passage by Earth the comet continues its rapid north-northeast climb, crossing into Perseus, Auriga (passing one degree southeast of the bright star Capella on December 23), and Lynx, where it will be located when it goes through opposition again just before the end of December. Having slowed down to one degree per day (and continuing to decrease), it reaches its farthest north point, declination +59.5 degrees, during the second week of January 2019, at which time it crosses into western Ursa Major. The comet then begins dropping towards the south-southeast, being at opposition yet again in mid-February (by which time its motion will have decreased back to 20 arcminutes per day) and then crossing into Leo Minor in mid-March and into north-central Leo around mid-May. While it may still be as bright as 7th or 8th magnitude at the beginning of January, I expect that it will fade fairly rapidly, perhaps to 11th magnitude during February, and I will probably lose it by sometime in March.

This is quite probably the last return during which I will observe Comet P/Wirtanen, although it is certainly going out with a "bang!" With its current orbital period of 5.4 years the next return (perihelion April 2024) is very unfavorable, while the return after that (perihelion October 2027) is reasonably good. The comet has another decent return in 2040 (perihelion early October), during which time I would be 82 years old. After that a pair of close approaches to Jupiter will conspire to keep it unfavorably placed for quite some time, and not until 2066 is there even a moderately decent return, however by that time its perihelion distance will have increased to 2.0 AU and its orbital period will have increased to 7.3 years. Even the 2027 return is after the "retirement" timeframe I have discussed in previous entries, although I suppose time will tell.

Meanwhile, I am continuing my recovery from the health issues I experienced a few months ago, although I still have quite a ways to go. I am happy to say that I am more-or-less back to my regular observing activity, and I have started working out again on a limited basis. My biggest priority right now is fundraising so that Earthrise can carry out the activities and fulfill the vision that I have described elsewhere on this web site. Perhaps the observations I will be making of Comet P/Wirtanen during this unique return and my involvement in the observing campaign will help towards that end.

INITIAL OBSERVATION: 2018 September 12.35 UT, m1 = 13.9, 0.6' coma, DC = 2 (41 cm reflector, 229x)

         

654. COMET 78P/GEHRELS 2          Perihelion: 2019 April 2.70, q = 2.014 AU

At the beginning of the previous entry -- my third tally addition during the month of September 2018 -- I commented that I probably would not be adding any more comets to my tally that month. It turns out that September wasn't quite finished with me after all . . . And as I commented at the end of this particular comet's "Countdown" entry for its 2012 return (no. 492), its current return is rather unfavorable, and I never expected to look for it, let alone observe it. However, once the Harvest Moon had cleared out from the evening sky the last few nights of September were very clear -- a situation which changed shortly after the beginning of October when moisture from the remnants of Hurricane Rosa flowed into the area, bringing clouds and rain -- and after seeing some recent CCD images and brightness measurements that suggested the comet might be worth attempting, I decided to try for it on the evening of the 29th -- and was rather surprised when I saw a very dim and diffuse but nevertheless distinct 14th-magnitude object at the correct position. I successfully observed the comet again on the following night.

On this time around P/Gehrels 2 was recovered on May 15, 2018 by Japanese amateur astronomer Toshihiko Ikemura, and independently a day later by the Catalina Sky Survey in Arizona. It went through opposition just after mid-August and at present is located in western Aquarius four degrees southwest of the star Beta Aquarii, where it will go through its stationary point during the first week of October and resume direct (eastward) motion. After traveling through northeastern Capricornus the comet crosses back into Aquarius during late November, and continues traveling east-northeastward through that constellation -- passing some six degrees south of the "water jar" in late December -- before entering Pisces during the second week of January 2019, where it remains until it disappears into the dusk a month and a half or so later. I don't expect much change in brightness -- at most, a slight brightening of a few tenths of a magnitude -- during this period, and to be truthful I doubt if I'll be getting more than a handful of observations of it.

In its "Countdown" entry I noted that P/Gehrels 2's return in 2026 is also quite unfavorable, although it might potentially be bright enough for visual observations in the morning sky after its perihelion passage in late June of that year. As I have commented in several previous entries, however, that is after my currently-planned "retirement" from visual comet observing, so, I suppose we'll see . . . In any event, my days of observing this comet are numbered regardless of whether or not I see it in 2026; its "Countdown" entry mentions the close approach to Jupiter in 2029 and the subsequent much larger orbit, and another close approach to Jupiter in 2046 increases the perihelion distance even further to 5.1 AU and the orbital period to approximately 40 years.

INITIAL OBSERVATION: 2018 September 30.13 UT, m1 = 14.1, 1.0' coma, DC = 2 (41 cm reflector, 229x)

 

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