A significant fraction of SOHO’s comet discoveries belong to a vast, yet poorly understood family: the Meyer-group. This blog post will be focusing on this family of comets, more specifically, its brightest members. The comets featured here were chosen based on my own personal assessment of their appearance (brightness) in SOHO/LASCO images, and thus do not represent a choice based on any published work. Moreover, note that this list is not exhaustive, but simply includes five objects that clearly “stick out” from the ordinary Meyer-group comet population.
Background: What are Meyer-group Comets?
The Meyer-group is a populous family of sunskirting comets, first recognized by amateur astronomer Maik Meyer (Germany) in 2002. To date, the group contains about 300 known members, making it the most abundant known family of comets after the Kreutz-group. However, contrary to the Kreutz-group, Meyer-group comets have solely been observed in SOHO/LASCO data. There are no recorded ground based observations of any Meyer-group members. In addition, none have ever been recovered in STEREO/SECCHI images, including the brightest members. This is likely due to their intrinsically faint nature, and the photometric properties of the STEREO/SECCHI instruments. Consequently, the lack of observations (e.g. short observation arc) make it difficult to constrain any orbits with high certainty. In fact, data lacks in order to establish (or estimate) reliable orbital periods. It is thought however that Meyer-group comets are likely intermediate or long periodic comets (Battams and Knight, 2015, and references therein). Interestingly, Rainer Kracht suggested the possibility of a ~10 year orbital period, although this was never confirmed.Figure 1: Composite SOHO/LASCO C2 image extracts showing nine different Meyer-group comets. Notice their stellar and/or elongated morphology. The orientation of the elongation depends on the time of the year (i.e. viewing geometry). Image credit: ESA/NASA SOHO/LASCO C2.
In regards to their appearance, Meyer-group comets all share a very common morphology. Nearly all Meyer-group comets are very condensed in appearance, although generally slightly elongated as seen in SOHO/LASCO C2. Their condensed appearance suggests that they are relatively “inactive” in nature (Battams and Knight, 2015). A composite image of various “typical” Meyer-group members is shown in Figure 1. Indeed, other than their varying brightness, their morphologies are strikingly similar. For comparison, Kreutz-group comets are generally somewhat fuzzy, (although highly condensed members have been observed, e.g. SOHO-3847). Marsden and Kracht comets also tend display “stellar” morphologies.Figure 2: Animation of SOHO/LASCO C2 image extracts showing Meyer-group comets SOHO-3850, SOHO-3851 and SOHO-3852. SOHO-3850 is the brightest member, while SOHO-3851 is the first to leave the FOV. SOHO-3852 is the trailing fragment of SOHO-3852. This one was discovered via an archival search for non-kreutz comets. This is the tightest cluster of Meyer-group comets ever recorded. The images were taken on 2019-11-04 and 05. Image credit: ESANASA SOHO/LASCO C2.
Unlike the Kreutz, Marsden, Kracht-I and Kracht-II groups, that appear to be dynamically evolving, the Meyer-group is most likely a highly evolved comet family (Lamy et al., 2013; Battams and Knight, 2015). Indeed, Battams and Knight (2015) suggest that the precursor of the Meyer-group may potential have first fragmented as long as ~10.000 years ago. Moreover, unlike the latter comet groups, Meyer comets rarely arrive in tight clusters. However, there have been exceptions: on November 4-5th, 2019, a group of three Meyer-group comets were seen over a span of only eight hours (Figure 2). Note that, in average, one Meyer-group comet is observed per month. This is the “tightest” known Meyer-group cluster discovered to date.
July 3rd, 1996: C/1996 N3 (SOHO)
Also known as SOHO-931, this bright Meyer-group comet was discovered by Rainer Kracht (Germany) in 2005 during search for overlooked comets in the SOHO/LASCO archives. This is the earliest recorded Meyer-group comet, and perhaps one of the brighter members of this family. In fact, Karl Battams described the comet as “one of the brightest Meyer-group comets that he has seen […] it didn’t fade noticeably until well past perihelion“. Despite its brightness, it initially went overlooked due to the low image cadence. At the earliest stages of the SOHO mission, SOHO/LASCO images were taken at a much lower rate than currently. Hence, one could expect many undiscovered Meyer-group comets to be lurking in ancient SOHO/LASCO data, but the low amount of images and their stellar appearance would make these impossible to distinguish from cosmic ray hits. C/1996 N3 (SOHO) is a special case as it was sufficiently bright to be detected in several C2 (Figure 3), and C3 (Figure 4), images. Figure 4: Animation of SOHO/LASCO C2 image extracts showing C/1996 N3 (SOHO). Due to the low and irregular cadence of images this comet went initially unnoticed. The object was relatively bright when entering the C2-FOV, unlike most Meyer-group comets that aren’t detectable in that region of the FOV at that part of the year. Image credit: ESA/NASA SOHO/LASCO C2.
Figure 4 is an animation of the comet in C2. Notice how the comet was already obvious when entering the C2 FOV. At that time of the year (June-July), most Meyer-group comets would be too faint to have been detected in this portion of the FOV. Moreover, the comet was sufficiently bright to appear in at least twelve C3 frames, despite the low image cadence. Below (Figure 5) is a C3 image extract showing the comet. Figure 5: SOHO/LASCO C3 image extract showing C/1996 N3 (SOHO). Although a relatively bright object, the image compression makes this comet fairly difficult to spot. Image credit: ESA/NASA SOHO/LASCO C3.
June 10th, 1997: C/1997 L2 (SOHO)
Also known as SOHO-11, this is the first Meyer-group comet to have been discovered. It was found by Shane Stezelberger (USA), a former member of the SOHO/LASCO team, in June of 1997. This comet was among those studied by Maik Meyer which allowed him to discover the Meyer comet group in 2002. Note that, in addition to being the first Meyer-group comet, it is also only the second “non-Kreutz” comet discovered by the SOHO/LASCO team (the first being SOHO-8, a non-group comet). Figure 3: Composite image extract showing C/1997 L2 (SOHO) in SOHO/LASCO C2. This is the last image of the comet prior to it leaving the C2 FOV. Moreover, if not taking into account the image compression, this image is also the highest resolution frame showing the comet. Notice its typically elongated, typical of Meyer-group comets. Image credit: ESA/NASA SOHO/LASCO C2.
Similarly to C/1996 N3 (SOHO), the comet appeared during the early stages of the SOHO mission, thus a period of low SOHO/LASCO image cadence. Despite this however, C/1997 L2 was visible in several SOHO/LASCO C2 images, as well as being obvious in numerous C3 frames (Figure 5). Moreover, as seen in Figure 4, the elongated nature of C/1997 L2 (SOHO) is clearly visible. The official report lists this object to have reached a magnitude of around +5. However, by comparing C/1997 L2 with the other Meyer-group comets listed here, I estimate that it may have reached maximum magnitude of about +6.5 mag. The differences are significant, and it may partially be due to the method I apply.* Despite this discrepancy, it is obvious that C/1997 L2 (SOHO) was among the brighter subset of Meyer-group comets.Figure 4: Animation of SOHO/LASCO C3 image extracts showing C/1997 L2 (SOHO). The comet may initially be difficult to spot because of the high image compression as well as variable image exposure(?). It’s apparent in the left portion of this figure, progressively moving away from the Sun and fading. Image credit: ESA/NASA SOHO/LASCO C3.
February 27th, 2011: SOHO-2030
Amateur astronomer Masanori Uchina (Japan) was the first to report this bright Meyer-group comet. He found it in real time (half resolution B/W) SOHO/LASCO C2 images of 2011-02-27, but quickly recovered it in C3 images, where it was simultaneously visible. Due to a data gap of several hours, the comet was already obvious and near its maximum brigthness by the time was discovered. Personally, I was unable to recover any traces of the comet prior to the gap. It would have been observable in C3, to the lower-left of the Sun (similarly to SOHO-2247, see below). The brightest moments of SOHO-2030 are shown in Figure 5 and 6.Figure 5: Animation of SOHO/LASCO C2 image extracts showing the transit of SOHO-2030 throught the C2 FOV. These are the highest resolution images of the comet. Image credit: ESA/NASA SOHO/LASCO C2.
In my opinion, SOHO-2030 is easily one of the brightest Meyer-group comets observed by SOHO/LASCO. It could be detected long after it had passed perihelion (more than a day). As Karl Battams stated “Masanori’s Meyer comet was very impressive! It has to be one of the brightest we’ve seen. A very nice find!”. By crudely comparing the object to various field stars*, I estimate that the comet reached a maximum magnitude of about +6.5. Figure 6: Animation of SOHO/LASCO C2 image extracts showing SOHO-2030 arcing around the Sun (perihelion and post-perihelion). These frames show the comet at its brightness. Notice how it slightly fades at the end of the animation. Image credit: ESA/NASA SOHO/LASCO C2.
March 1st, 2012: SOHO-2247
SOHO-2247 was first reported by amteur astronomer Krzysztof Kida (Poland) in real time SOHO/LASCO C3 images of 2012-02-29. At the time the comet was discovered, the object was a couple of degrees south-east of the Sun, overlapping the Kreutz-group comet track (Figure 7). Due to its brightness, it was detected much earlier in its path than most Meyer-group comets would. Consequently, having been discovered in a region where Meyer-group comets are uncommonly observed, it was initially assumed to be a Non-group comet. It was only as the comet kept approaching the Sun that it become clear that it was a Meyer-group member. Sergei Schmalz (Germany) was the first to suggest its classification.Figure 7: Composite image of SOHO/LASCO C3 image extracts showing one of the earliest images in which SOHO-2247 was detectable. The comet was about twleve hours from entering the C2 FOV. It is rare that a Meyer-group comet is bright enough to be detected so early prior to perihelion (in C3). The faint “streak” near the box is a Kreutz-group comet (SOHO-2246). Image credit: ESA/NASA SOHO/LASCO C3.
As the object kept approaching the Sun, it gradually brightened, becoming obvious by the time it had reached the C2 FOV. Again, by crudely comparing the object to various field stars, I estimate that the comet reached a maximum magnitude of about +7.5, thus somewhat fainter than SOHO-2030. Despite this, it is definitely among the brighter members, in my opnion. As Sergei wrote on the SOHOHunters yahoo forum: “Well, it’s evident now – yes, it is a Meyer-group comet. And a beautiful one!“. Coincidentally, a coronal mass ejection took place close to the time the comet reached perihelion. The eruption can be seen in both in Figure 8 and 9, along with the comet. Figure 8: Animation showing SOHO-2247 briefly transiting the SOHO/LASCO C2 FOV on March 1st, 2012. These are the highest resolution images of the comet. Notice the coronal mass ejection (CME) to the west of the comet. Image credit: ESA/NASA SOHO/LASCO C2.
Figure 9: Animation of SOHO/LASCO C3 image extracts showing SOHO-2247 arcing around the Sun (perihelion and post-perihelion). Notice the obvious coronal mass ejection that coincidentally took place during the comet’s passage. Image credit: ESA/NASA SOHO/LASCO C2.
June 17th, 2020: Currently Undesignated
Amateur astronomer Worachate Boonplod (Thailand) discovered this bright Meyer-group comet in SOHO/LASCO C2 images of 2020-06-16. Unlike most Meyer-group comets, this one was already (easily) detectable when enterering the C2 FOV. Most Meyer-group comets aren’t detectable until hours just before they leave C2, just around perihelion. The comet entered the FOV only hours after Kreutz-group comets SOHO-3999 and SOHO-4000, the latter still visible in the animation below (Figure 10). As of the time of this blog post, this is the most recent (real time) Meyer-group comet discovery.Figure 10: Animation of SOHO/LASCO C2 image extracts showing the Meyer-group comet from 2020-06-16/17 transiting the FOV. Notice how the comet was already apparent as it entered the FOV. Most Meyer-group comets are only detected hours before leaving the FOV (which is close to the time they reach their brightest). Notice also the Kreutz-group comet SOHO-3999, apparent early in the animation. Image credit; ESA/NASA SOHO/LASCO C2.
Figure 11: Image comparison between C/1997 L2 (SOHO) and Worachate Boonplod’s recent Meyer-group comet. Notice how both comets are of comparable brightness, if not the latter being slightly brighter (taking into account the data compression in the former case). Image credit: ESA/NASA SOHO/LASCO C2.
Similarly to SOHO-2030, I estimate this comet may have reached a maximum magnitude of about +6.5 based on field star comparisons. In fact, the brightness is comparable, if not slightly brighter, than C/1997 L2 (SOHO) (Figure 11). Consequently, this makes it one of the brightest Meyer-group comets observed by SOHO. As Karl Battams stated in a response on Twitter “[…] that must be one of the brightest Meyer’s we’ve ever seen!”. The STEREO-A spacecraft was favourably placed to observe the comet in its HI1-A imager. Unfortunately, I was unable to recover it. It must have faded too much by the time it reached the HI1-A FOV.Figure 12: Animation of SOHO/LASCO C3 image extracts showing the Meyer-group comet from 2020-06-16/17. These images were taken just as the comet exited the C2 FOV (Figure 10). Note how the comet gradually fades. At the time of these images, the comet had just passed perihelion. ESA/NASA SOHO/LASCO C3.
I wish to thank amateur astronomer Peter Berrett (Australia) for providing the coordinates of SOHO-3786 and SOHO-3852 (Figure 1 and 2).
*The magnitude estimations I provide in this work are based on a rough comparison between the comets and visual magnitudes (Johnson V filter) of field stars (provided by the SIMBAD database). Perhaps it is possible that this band is not the most adapted when making such comparisons?