As of January 2022, the STEREO spacecraft have discovered a combined total of 131 comets (mainly via the Sungrazer [citizen science] project). Like for SOHO, most STEREO comets are doomed sungrazers that never make it back into dark skies. Rarely however, STEREO-A and B have found comets that do not follow sungrazing orbits, some of which were later recovered from Earth. Such STEREO discoveries are often made possible due to the spacecraft’s differing locations (i.e. view points) from Earth and SOHO, allowing some comets to be obvious as seen from STEREO-A and/or B while being difficult to spot elsewhere (at least, initially). C/2014 C2 (STEREO) was one such comet.
C/2014 C2 (STEREO) was discovered on February 7th, 2014, by amateur astronomer Alan Watson (Melbourne, Australia) in near-real time STEREO/SECCHI HI1-A images. Alan is among the pioneering STEREO comet hunters. He is credited for the first STEREO comet discoveries, both in STEREO-A and B. To date, he has found more than 30 STEREO comets, along with several SOHO comets. According to the STEREO@SunGrazer.group.io forum and Alan’s personal blog, he found the comet shortly after a hiatus from comet hunting.
Alan described the comet as bright and moving rapidly. He was quickly able to identify the comet in HI1-A images taken days prior the discovery frames. After coming across Alan’s report, I was able to locate the object in COR2-A images of February 1st. A closer inspection revealed the comet entering COR2-A on January 31st. It was also briefly visible in COR1-A (the only non-group STEREO comet to have been so). At that time, the object was not detectable from Earth (low solar elongation), and was not well located to be observed by SOHO or STEREO-B. Notice how the comet rapidly fades in HI1-A. It was last seen by STEREO on February 4th, after it exited the HI1-A FOV. It probably reached a peak apparent magnitude of +4 or +5 as seen from STEREO-A. Below is an animation I put together of its transit in STEREO/SECCHI (other than COR1).
As stated at the beginning of this post, C/2014 C2 was unlike most STEREO-discovered comets. Most STEREO comets tend to be Kreutz-group sungrazers, none of which have survived perihelion. C/2014 C2, on the other hand, had a perihelion distance of q= 0.51 AU, thus, was not a sun-approaching comet. Moreover, seen that perihelion didn’t take place until February 18th, the comet was even further from the Sun when visible in STEREO. In fact, C/2014 C2 was just a very fortuitously placed [small] comet. It passed directly midway between the Sun and STEREO-A, even transiting the solar disk!
The consequence of the above alignment was an intense enhancement of the comet’s brightness, due to forward scattering of the Sun’s radiation onto the comet. Hence, forward scattering, rather than the comet’s intrinsic variations in brightness, were the main governing factor of its apparent brightness in STEREO-A. This explains the comet’s rapid drop in brightness once it entered HI1-A (as it increased in elongation from the Sun). The comet’s location midway between the Sun and STEREO-A is also likely to explain its rather rapid motion from its view point. Other SOHO/STEREO comets to have displayed similar traits in STEREO/SECCHI include P/2003 T12 (SOHO) and 414P/STEREO.
Acknowledgements: I wish to thank Jean-François Soulier, Michael Jäger and Taras Prystavski for their permission to use their images in this blog post.
From December 23rd to the 25th, the SOHO/LASCO cameras were treated to a moderately-bright Kreutz-group sungazing comet, possibly of the II-subgroup. The comet reached its maximum brightest on Christmas Eve (~+5 mag), and fully disintegrated on the morning (UT) of Christmas day, probably several hours before perihelion. A faint tail was observed in SOHO/LASCO C3, as well as in C2. The comet was also visible in STEREO/SECCHI COR2-A, and might be possible to recover in high-resolution HI1-A images from Dec 22-23rd, once those images become available.
On December 23rd, two Kreutz-group comets were reported in SOHO/LASCO C3 by prolific SOHO comet hunter Worachate Boonplod. The first one (omitted here) was quite faint, condensed, and vanished within the next day. The second comet, however, brightened nicely. By December 24th it had developed a faint tail and was easily visible in C3 images (Figs. 1 and 4). That same day, before passing behind SOHO/LASCO’s occulting pylon, it reached a maximum apparent brightness of about +5 mag (estimation based on a quick comparison with field stars using SIMBAD: Fig. 2).
The comet entered SOHO/LASCO C2 on December 25th, where it was seen rapidly fading (Figs. 3 and 4). As it first entered the C2 FOV (~03:30 UT), the comet’s head was quite prominent, although short-lasting. By ~07:00 UT the comet had become headless, indicating that the nucleus had probably disintegrated. After 10:00 UT the comet and its remains were no longer detectable in SOHO/LASCO. Fates such as this one are sadly those of nearly all Kreutz-group comets.
Below (Fig. 4) is an animation that I put together showing the comet’s end-of-journey as seen in SOHO/LASCO. The dark diagonal bar in the SOHO/LASCO C3 (blue) images is the occulting pylon.
As far as STEREO observations go, the comet was already apparent in the low resolution real-time COR2-A images of December 24th – 25th (Fig. 5). It could also potentially be detectable in high-resolution HI1-A images from December 22nd – 23rd (I estimate), once those are made available. It might be possible to recover it in COR1-A images too. These images should be available within the next few days.
Based on the comet’s trajectory in SOHO/LASCO, it imay be that this sungrazer is a member of the Kreutz-II subgroup. This would imply that it is probably more directly related to sungrazers such as C/1882 R1 (Great comet) and C/1965 S1 (Ikeya-Seki), compared to members such as C/1843 D1 (Great comet) or C/2011 W3 (Lovejoy). Kreutz-II comets tend to be somewhat rarer than those of the Kreutz-I subgroup.
Although brighter than the Kreutz-group average, the new SOHO comet is much smaller, and fainter, than any of the abovementioned great comets. To visualize this, Fig. 7 compares a C2 image extract of the new SOHO comet and of C/2011 W3 (Lovejoy). By appearance (e.g. brightness and tail length), both comets are clearly of two distinct categories. In fact, the small SOHO/STEREO-discovered sungrazers are sometimes referred to as “dwarf” comets. C/2011 W3 reached a maximum brightness of mag -3 or -4, which itself is much fainter than C/1965 S1 and C/1882 R1, that reached mag -10 and -15, respectively! On the other end of the scale, the faintest known Kreutz-group comets are those barely detectable in SOHO/LASCO C2 images (the C2 limiting magnitude is around +10 mag, from memory).
Sungrazer Comet Section Facebook group. Sungrazer project website report page. TheComet Watch website, more specifically. The SIMBAD astronomical database. Personal analysis of SOHO/LASCO and STEREO/SECCHI images, made available by the STEREO Science Center and the Naval Research Lab.
On November 9th, SOHO observed a close pair of Kreutz-group comets make their way towards perihelion. Although each individual fragment was very “ordinary” in appearance (as far as SOHO comets go), obvious pairs such as these are not an everyday occurrence! These fragments probably represent a larger comet that split within the last few years. The brightest fragment was also observed by STEREO-A. Discovery credit goes to Worachate Boonplod (Thailand), Zesheng Yang (China) and Robert Pickard (USA).
Fig. 1: Animation of SOHO/LASCO C2 image extracts (original and noise-subtracted frames) showing the faint pair. Both fragments are similar in appearance (condensed, with no tail), despite being of unequal brightness. Image credit: ESA/NASA SOHO/LASCO C2.
On the early hours of November 9th (UT), Worachate Boonplod reported a potential Kreutz-group comet in SOHO’s LASCO C3 images (Fig. 2). Due to its very faint appearance in these images, he was not quite sure if it was real. However, soon after his report, additional C3 images were released, fortunately confirming his sighting. In fact, at the time of discovery, the comet was only hours away from entering the SOHO/LASCO C2 FOV. C2 images are of much higher resolution than C3, so it’s always exciting when a C3-detectable comet enters these images (the majority of Kreutz-group comets are too faint to appear in C3).
Eventually, Worachate’s comet entered the C2 FOV, at around 08:45 UT. To everyone’s surprise, the C2 images revealed a close-accompanying fragment, that was too faint to appear in C3 (but still obvious in C2). This second comet was reported by Zesheng Yang and Robert Pickard within only 13 seconds of each other (Fig. 2)! Unfortunately, like all of SOHO’s Kreutz-group comets, both fragments rapidly disintegrated and vanished before reaching perihelion.
Unlike SOHO/LASCO C2, STEREO/SECCHI HI1-A images show only a single comet, rather than two close fragments. To explain this, I wonder if (i) the fainter fragment was too faint to appear in these images, or (ii) the pair was visually too close to be resolved from STEREO-A’s perspective. We will probably never know.
Despite such nice pairs not being an everyday occurrence, cases like this one are actually not uncommon within the Kreutz-group. In fact, Kreutz-group comets often travel in larger “clusters” (mostly as groups of several comets dispersed over a few days, as seen in SOHO/STEREO). This is because the Kreutz-group is a dynamically evolving comet family, often fragmenting throughout the course of their orbits. In fact, I wrote about a similar case a few years ago. Below (Fig. 4) are some other similar examples of the past.
Copyright (c) 2021 Trygve Prestgard. All rights reserved
Thanks to the abundance of data collected by the SOHO/LASCO and STEREO/SECCHI imagers, sungrazers are observed on a daily-weekly basis. Although most sungrazers are difficult to spot and vanish within hours of detection, we are occasionally treated to some that are easily visible, often displaying long and obvious tails (Fig. 1). Sometimes, these bright comets may even be accompanied by surrounding smaller fragments. In fact, a few of such occurrences have coincidentally taken place on Halloween! Hence, in honor of today’s holiday, this blog post will be describing some of the “Halloween sungrazers” that we have observed over the years!
2009: A Moderately Bright Kreutz and a CME!
On October 30th, 2009, SOHO comet hunter Bo Zhou (China) discovered a previously unknown Kreutz-group comet in SOHO/LASCO’s C3 images. The object, C/2009 U13 (SOHO), rapidly brightened, reaching mag ~+6 to +5 when it entered SOHO’s LASCO C2 FOV on October 31st (Fig. 2). In C2 the object displayed a condensed head with a long (and fainter) tail (debris trail). Unfortunately, as is the fate of all of SOHO’s Kreutz-group comets, C/2009 U13 rapidly disintegrated. The comet had already faded out of view before passing behind the occulting disk.
Unlike most Kreutz comets however, C/2009 U13 was sufficiently bright to appear in STEREO/SECCHI’s COR2 images (Fig. 3 and 4). In these images the comet appeared headless, which one could perhaps describe as a “curved streak”. The lack of an apparent head in STEREO (as well as its fainter appearance) is most likely due to the difference in the bandwidths observed by the LASCO and COR2 instruments, in addition to the COR2 images being polarized . This is also the reason why COR2 detects fewer comets than LASCO, despite the higher resolution of the former.
In addition to the relatively bright Kreutz, the comet’s demise was coincidentally accompanied by a nice coronal mass ejection (CME)! The CME is easily noticeable in both the LASCO C2 and COR2-A animations (Fig. 2 and 4).
2011: A Flurry of Kreutz-group Comets!
Similarly, to 2009, Halloween day of 2011 was greeted by a brighter-than-average Kreutz-group comet (SOHO-2159). However, unlike 2009, this comet was accompanied by a flurry of four companions (Fig. 5)! The bright comet was first reported by Szymon Liwo (Poland) on October 29th, 2011, in SOHO/LASCO C3 images. The comet rapidly brightened, reaching mag +5 to +4 on October 30th. Over the course of October 30th and October 31st, SOHO-2159 entered the C2 FOV, although rapidly fading. Eventually it passed behind the solar occulter, never to reemerge.
On October 30th, Zhijian Xu (China) reported another comet in C3 images. This object, SOHO-2160, followed a significantly different trajectory from SOHO-2159, typical of Kreutz comets of the II subgroup. Both comets eventually entered the C2 FOV that same day.
Again on October 30th, Michal Kusiak (Poland) reported two faint Kreutz comets in C2 images (SOHO-2161 and SOHO-2162), both leading SOHO-2159. SOHO-2160 and SOHO-2161 entered the C2 FOV almost simultaneously (Fig. 5). Lastly, Masanori Uchina (Japan) and Zhijian Xu reported another Kreutz in C3 (SOHO-2163). This one entered C2 several hours after SOHO-2159, on October 31st. These fainter comets were all probably between mag +8 and +10, which is most typical of SOHO’s Kreutz-group comets. A close-up of all five fragments can be seen in Fig. 6.
Among these five comets, only three are easily recoverable in STEREO/SECCHI HI1-A: SOHO-2159, SOHO-2161 and SOHO-2163 (in order of apparition in Fig. 5). SOHO-2159 was also sufficiently bright to appear in COR2 images (Fig. 8). SOHO-2162 might be visible too, just ahead of SOHO-2159 (Fig. 8). I was not able to recover SOHO-2160.
2012: A Moderately bright Kreutz-group comet!
Again, similarly to 2009, Halloween of 2012 was greeted with a nice Kreutz-group comet! Despite the comet’s official designation, C/2012 U3 (STEREO), it was discovered on October 30th, 2012, in SOHO/LASCO C3 images. At the time, the SOHO/LASCO dataflow had been interupted due to the passage of Hurricane Sandy, leaving a temporary data gap of about one day. When the dataflow returned, SOHO comet hunters were surprised to see a bright Kreutz-group comet in real time C3 images. Consequently, the comet was quickly spotted by several observers, including Szymon Liwo, Krzysztof Kida (Poland) and Zhijian Xu, who are credited with its discovery.
The comet entered the C2 FOV in the late hours of October 30th (UT time) (Fig. 9). In these images the comet displayed a somewhat similar appearance to C/2009 U13 (SOHO), although with a more noticeable tail. The comet rapidly faded, vanishing completely on October 31st. C/2012 U3 was also easily recovered in SECCHI data from STEREO-A, both in HI1-A and COR2-A images (see Fig. 10). In COR2-A the comet displayed a “headless” appearance, again typical of Kreutz-group comets in COR2. The comet’s peak magnitude was about mag +5 or +6.
2015: Another Flurry of Kreutz-group Comets!
As far as SOHO comets go, Halloween of 2015 was marked by the apparition of six Kreutz-group comet fragments in real time images! The first one to be discovered, SOHO-3033, was found by Zhijian Xu, and Worachate Boonplod (Thailand), in real time C3 images of October 30th, 2015. This comet brightened nicely, reaching mag +6 by the time it entered the C2 FOV on October 31st. The comet displayed a condensed head, and a long faint tail (Fig. 10 and 11). Zhijian Xu reported a second (fainter) Kreutz in C3 images of October 30th, that trailed SOHO-3033 by less than a day (SOHO-3035, Fig. 11 and 12).
As SOHO-3033 entered C2, a close fragment was found moving alongside its tail (now designated SOHO-3036). Later that day, Bo Zhou and myself came across even fainter Kreutz-group fragments: SOHO-3038 and SOHO-3037 respectively). Both comets entered the C2 FOV within two hours of each other. SOHO-3036, SOHO-3037 and SOHO-3038 were all too faint to be recovered in C3 images. Similarly to SOHO-3033, SOHO-3035 was found to be accompanied by a fainter fragment too: SOHO-3041. It is rare for C2 to observe this many Kreutz-group comets in one day. Most are most likely direct fragments of one another.
What about 2021?
Unfortunately, no sungrazers have been endorsed in today’s SOHO/LASCO or STEREO/SECCHI images, so far. The last definite comet was reported by Rafal Biros, in real time C2 images from October 30th. Hopefully the next sungrazer will show up soon!
 Rainer Kracht (2007): LASCO C3 and SECCHI COR2A, personal website.
Comet designations were extracted from the BAA comet section. The magnitude estimations are my own.
On July 15th, at around 20:00 – 21:00 UT, the SOHO and STEREO-A spacecraft were treated to a spectacular Coronal Mass Ejection! Based on STEREO-A’s observations, the event occured on the backside of the Sun, as seen from Earth. This post is simply to report on the beautiful images that SOHO and STEREO caught of the eruption.
Coincidentally, the event took place almost exactly 21 years after the Bastille day event (July 14th, 2000). In my opinion, this is among the most impressive CMEs observed by SOHO and STEREO during this solar cycle! Such events should further increase in frequency as the Sun continues to increases in activity (the peak of the current solar cycle should take place around 2025).
View from SOHO/LASCO
As seen from SOHO/LASCO (hence Earth’s perspective), the event appeared as a halo CME which included a prominent ejection to the SE of the Sun. This ejection could be tracked for days in SOHO/LASCO C3, as it slowly dissipated into space.
View from STEREO/SECCHI
The STEREO-A spacecraft is located about 46° behind the Earth, and thus saw the CME from a significantly different perspective. Based on these images, it was obvious that the ejection was headed in the opposite direction from Earth. Hence, it will not enter the FOV of the heliospheric imagers.
Copyright (c) 2021 Trygve Prestgard. All rights reserved.
From May 7th to May 10th, the coronagraphs aboard the SOHO and STEREO-A spacecraft witnessed the rise and fall of a bright Kreutz-group comet. In fact, the comet reached +1 mag on May 10th, making it among the brightest sungrazers to have been discovered by SOHO, more specifically through the Sungrazer Project. Author of the discovery is Szymon Liwo, an amateur astronomer and long-time SOHO comet hunter from Świdnica, Poland.
On May 7th at 12:23 UT, amateur astronomer Szymon Liwo reported a previously unknown Kreutz-group comet entering the SOHO/LASCO C3 FOV. As it is rare that Kreutz comets are bright enough to be detectable near the edge of the SOHO/LASCO images, it was obvious that the object had the potential of becoming an impressive comet. Fortunately it lived up to expectations. On May 10th the comet reached mag +1, making it among the brightest sungrazing comets to have been discovered by SOHO. In fact, the comet’s brightness caused it to saturate the SOHO/LASCO detectors (see Fig. 1). Unfortunately, soon after entering the SOHO/LASCO C2 FOV, the comet started to rapidly fade. Eventually, it fully disintegrated. This is the fate of most sungrazers, especially those discovered by SOHO and STEREO. In fact, no SOHO-discovered Kreutz-group comet has ever survived perihelion.
On May 10th, SOHO comet hunter Worachate Boonplod (Thailand) reported a faint object closely trailing the Szymon’s bright comet (Fig. 2). Due to its apparent proximity, it is possible that Worachate’s object may have been a fragment that recently split off from the comet. It is not unusual that Kreutz-group comets travel in close pairs, or in clusters (e.g. Fig. 3). This is unlike the Meyer-group, which is likely to be a much more evolved comet family.
On May 10th Szymon’s comet entered the FOV of STEREO-A’s SECCHI COR2 telescope. As can be seen in Fig. 4, the comet follows a significantly different trajectory in these images relative to SOHO’s observations (Fig. 1). This is due to the differing perpectives between both spacecraft. Indeed, STEREO-A is located 52° behind SOHO, meaning that it observed the sky as SOHO would in February. In Fig. 4, notice how the tail (debris trail) grows in length, as the comet progressively fades. This is due to the object rapidly disintegrating. Similarly to LASCO, the comet was not observed to re-emerge from behind the COR2-A solar occulter. Worachate’s faint fragment was too faint to be recovered in STEREO/SECCHI.
Szymon Liwo has been an active member of the Sungrazer project for 10 years. As a result of years scrutinizing the SOHO/LASCO images, he has found a total of 110 sungrazing comets. In fact, among Szymon’s numerous discoveries are several objects similar to his recent Kreutz-group comet. Indeed, Szymon is also the [co-]discoverer of the following objects: SOHO-2143, SOHO-2505, SOHO-3069 and SOHO-3478 (Fig. 5). He also co-discovered one SWAN comet, C/2015 F5 (SWAN-Xingming). Moreover, Szymon is uncle of Rafal Biros, the current record holder for youngest SOHO comet discoverer.
Note: coincidentally, this bright comet appeared almost exactly a decade after SOHO-2062, a similarly-bright Kreutz-group comet. More information about that comet can be found here. Szymon’s comet has yet to receive an official designation.
Acknowledgements: I wish to thank Szymon Liwo for the information that he provided in order to complete this article.
References: The information provided in this article is based on my personal analysis, the Sungrazer Comet Section facebook group, the Sungrazer Project website, and personal communication with Szymon Liwo. Additional sources have been incorporated into the text.
On February 26th, amateur astronomer Michael Mattiazzo (Australia) discovered a previously unknown comet using publicly available data from the SOHO/SWAN instrument! At the time of discovery, the object (C/2021 D1) was already near perihelion, and close to maximum brightness (mag +11). Due to the comet’s location (poor solar elongation), despite its brightness, it had escaped previous detection by ground-based minor planet surveys (e.g. ATLAS, CSS, Pan-STARRS). Fortunately, comet SWAN is moving east towards increasingly favorable skies. Sadly, however, it is slowly fading (currently ~ +12 mag), and is expected to have reached mag +15 by June of this year.The comet currently displays a somewhat condensed appearance, although a faint tail has been reported.
Michael Mattiazzo discovered C/2021 D1 (SWAN) on February 26th, using near-real-time Comet Tracker images from the SOHO/SWAN website. More specifically, he found the comet in data from February 19th – 23rd, where it was situated only tens of degrees NE of the Sun, in the Pegasus constellation. Located aboard the SOHO (Solar and Heliospheric Observatory) spacecraft, the SWAN (Solar Wind Anisotropies) cameras image the sky in the Lyman-α band, a wavelength that happens to be particularly sensitive to comets. In fact, although SWAN mainly focuses on the solar wind, it has also been used to study comets (e.g. Combi et al., 2000). In order to make this possible, the instrument must first block the overwhelming glare of the Sun and Earth. As a consequence of its build-up, SWAN is capable of imaging the sky at low solar elongation, a blind spot for most ground-based minor planet surveys. However, due to the very poor resolution of its images (of ~1°), only comets brighter than ~12 mag are possible to detect using this data. Despite this, SWAN has enabled the discovery of fourteen comets (not including two co-discoveries), as well as one recovery! Most SWAN comets nowadays are found at low solar elongation, by amateur astronomers regularly studying the data. This was the case of C/2021 D1 (SWAN). Due to the faint nature of this comet in SWAN data (Fig. 2), it could not be confirmed using these images alone. Ground-based observations were hence necessary to confirm the comet.
Following the comet’s discovery, attempts at constraining a rough orbit were made. This was particularly challenging, not only because of the short observation arc, but also due to the very low resolution of the SWAN Comet Tracker images. Consequently, preliminary estimations varied widely. Indeed, some proposed solutions suggested that it might have been a sunskirting comet, or that it was related to the Great Comet of 1686 (C/1686 R1). Other solutions suggested that it was headed for a close approach with Earth in early-March (which sadly did not happen). The uncertainty in the comet’s trajectory and location made it initially difficult to locate the comet from Earth. Astrophotographer Nicolas Lefaudeux (France) took a wide-field image of the region on February 27th, however early attempts at locating the comet in his image failed.
On February 28th, after multiple attempts since the comet’s discovery, astrophotographer Michael Jäger (Austria) became the first to recover C/2021 D1 (Fig. 3). Jäger measured the comet to be ~10.5 mag in his confirmation image (Fig. 3). He further described the comet as displaying a condensed appearance, with an apparent diameter of 3.5′, and even showing a very faint tail. The comet was also independently recovered by Krisztián Sárneczky (Hungary). Following Jäger’s announcement, other astrophotographers (e.g. Luca Buzzi and Nick James) were also able image the comet that same day (Fig. 4). Based on these new observations, astronomer Alan Hale (U.S.A.) was able to locate the comet in Lefaudeux’s image.
Since the comet’s recovery, with the increasing observations and growing observation arc, the comet has been found to be periodic (P= 76.9 years), with a perihelion on that occurred on T= February 27th at q= 0.89 AU. Fortunately, the comet is slowly moving towards more favorable skies. However, it is also slowly fading. By June the comet will likely have faded below mag +15, assuming no outbursts occur.
Acknowledgements: I wish to thank Michael Jäger and Nick James for granting me permission to use their images of C/2021 D1 (SWAN).
References: The Comet_mailing_list.io group, Gideon van Buitenen’s website, the facebook ICQ Comet observations page, and the SOHO/SWAN website. Other sources are referred to in the text.
Copyright (c) 2021 Trygve Prestgard. All rights reserved.
In the past, many interesting and/or bright comets have coincidently appeared in SOHO’s LASCO FOV around the late-December holidays. For instance, December 2011 was marked by the spectacular passage of comet C/2011 W3 (Lovejoy); 2010 by SOHO’s 2000th comet discovery and a sungrazer comet “storm”, while SOHO’s first “bright” (mag +1) sungrazer was observed only a couple of days before Christmas day, 1996. In fact, currently, the bright comet C/2020 S3 (Erasmus) is slowly transiting SOHO/LASCO c3 (Figure 1). In this blog post I will be discussing the various late-December “holiday comets” that appeared throughout the 2000s.
2000 – Comet C/2000 W1 (Utsunomiya-Jones)
This small comet was discovered independently by Syugu Utsunomiya (Japan) and Albert Jones (New Zealand) in mid-November, 2000. At the time of discovery, the comet was already +8.5 mag, although decreasing in solar elongation. In fact, it was only about five weeks from perihelion, which took place on December 26th at only q= 0.32 AU from the Sun. A couple of days later, the comet entered the SOHO/LASCO C3 FOV. As can be seen in the animation below (Figure 2), the comet was quite obvious in SOHO/LASCO, shining at around mag +6, although slowly fading. It left the FOV on January 4th, 2001. Unfortunately, the comet eventually disintegrated on its journey outbound. Some ground-based images of this comet (before and after perihelion) can be found here.
2003 – Several Kreutz-group Sungrazing Comets
From December 19th to 27th, 2003, a total of twelve Kreutz-group sungrazing comets were recorded transiting the SOHO/LASCO FOV. In fact, there may have been more, seen that this period was marked by numerous data gaps. As can be seen in the animation below (Figure 3), two of the observed sungrazers were relatively bright, displaying long and obvious tails (SOHO-714 and SOHO-721). Three other bright (although significantly fainter) members were also included in the animation, these generally being fairly condensed or globular, with short tails (SOHO-713, 715, and 719). An even fainter member, SOHO-720, can also be seen (see details below). These objects were discovered by members of the Sungrazer citizen science project, more specifically by Michael Boschat, Rainer Kracht, Rob Matson, John Sachs and Xing-Ming Zhou. All the comets disintegrated soon after discovery, as most SOHO-discovered Kreutz-group comets tend to do.
As mentioned above, in addition to the more obvious comets, a much fainter member is also visible in Figure 3. Unlike the other five, this one resembled a small, faint, diffuse cloud of debris, indicating that it had probably disintegrated prior to entering the SOHO/LASCO C2 FOV. This object trails SOHO-719. Below (Figure 4) are one of the frames best showing both comets together (contrast enhanced).
Due to data gaps, the extent of the tail of SOHO-721 could not be seen in SOHO/LASCO C2 (Figure 3). Its long tail was however clearly visible in C3, despite the lower resolution, as can be seen in the animation below (Figure 5). Jumps in the animation are also due to data gaps.
2004 – Comet C/2004 V13 (SWAN) and C/2004 Y4 (SOHO)
Also known as, SOHO-884, C/2004 V13 (SWAN) was a relatively bright sunskirting comet that transited the SOHO/LASCO C3 FOV from December 16th to 21st, 2004. The object was initially reported by Michael Mattiazzo (Australia) in SOHO/SWAN data from November 2004, however it was very faint in those images and hence difficult to confirm. Moreover, the object was at low solar elongation, hence further adding to the above difficulty. Fortunately, Michael noted that it was on a path towards the Sun and predicted that it might enter the SOHO/LASCO FOV in mid-December.
On December 16th, 2004, former SOHO comet hunter John Sachs (USA) reported a bright non-group comet entering SOHO/LASCO C3 FOV. The object was spotted independently by Heiner Otterstedt (Germany) who posted his report only 1,5 minutes after John’s. Moments later, former SOHO comet hunter Sebastian Hönig suggested that John’s comet was the one reported by Michael in SWAN data. By combining the newly obtained SOHO observation with those from SWAN, he estimated (based on his own rough calculations) that the object would reach perihelion on December 21st, at only q= 0.23 AU from the Sun. These two parameters are comparable to those of the final solution published by the MPC. Figure 6 shows that comet entering the SOHO/LASCO C3 FOV. Notice the obvious tail dynamics, resulting from its interaction with the solar wind.
Unfortunately, despite its bright appearance in SOHO/LASCO, the comet soon rapidly faded and morphed into a diffuse appearance by the time it recovered by ground-based instruments (less than two weeks after exiting the C3 FOV) (Figure 7). It is hence clear that C/2004 V13 did not survive its solar encounter.
Coincidentally, soon after the passage of comet SWAN in SOHO/LASCO, another obvious SOHO non-group comet was spotted! Designated C/2004 Y4 (SOHO), it was significantly fainter than C/2004 V13, but relatively bright as far as SOHO non-group comets go (around mag +6). It was discovered in Heiner Otterstedt in C3 images from Dec 24th, 2004, and was detectable in SOHO/LASCO until December 28th. As described by Heiner on his website: “It took a long clockwise arc around the edge of C3, becoming moderately bright and then faded out to the lower right of the Sun” (Figure 8). Overall, C/2004 Y4 displayed a condensed appearance, with a somewhat elongated morphology at its brightest. No obvious tail was observed. The comet was not recovered from Earth.
2008 – Comet 210P/Christensen
Officially discovered by Eric Christensen of the Catalina Sky Survey (CSS) in May of 2003, this small comet (P/2003 K2) was unfortunately difficult to observe due to poor solar elongation. As a consequence, it was difficult to establish a highly reliable orbit at the time. Note that Michael Mattiazzo (see above) reported this object individually in SOHO/SWAN images taken the month prior, but its sighting could not be confirmed in these images alone. Fortunately, the comet was recovered by Alan Watson (Australia) in STEREO/SECCHI HI1-B images of December 2008 (Figure 9), as it was again heading towards perihelion. In those images the comet had already reached mag +10.
Interestingly, the comet passed almost perfectly between the Sun and the Earth during its 2008 return, making an easy target for SOHO. Indeed, the resulting forward scattering made it easily observable in SOHO/LASCO (between mag +5 and 6), as can be seen in the C3 animation and the C2 image extract below (Figures 10 and 11). A faint tail could also be seen. In addition, a few Kreutz-group sungrazers coincided with the appearance of this comet in SOHO/LASCO.
2009 – Sungrazing Comet C/2009 Y4 (STEREO)
Also known as STEREO-23, this bright Kreutz-group sungrazer was discovered by amateur astronomer Alan Watson (Australia) via the Sungrazer citizen science project (see above). He first spotted the comet in near-real time STEREO/SECCHI HI1-A images of December 31st, 2009, where the comet was hardly detectable above the background noise level. However, the comet rapidly brightened over the next few days, reaching mag +1 on January 3rd. The days following its discovery the comet was recovered in SOHO/LASCO and STEREO-B SECCHI imagery. The animation below (Figure 12) shows STEREO-23 as seen in STEREO/SECCHI HI1-B images. Notice how the comet develops a long (a couple of degrees) tail, that remained visible even after it left the FOV. C/2009 Y4 (STEREO) is one of the brightest sungrazers discovered by STEREO.
SOHO comet hunter Michal Kusiak (Poland) was the first to recover the comet in SOHO/LASCO. As it entered the SOHO/LASCO C3 FOV the comet was hardly detectable (around mag +10), but it quickly brightened to the point where it saturated the SOHO/LASCO C2 and C3 detectors on January 2nd and 3rd (Figure 13). Soon after reaching peak brightness in the early morning of January 3rd (UT), the comet started to rapidly disintigrate. It did not survive its solar encounter and was not recovered after it passed behind the solar occulter (Figure 14).
Amateur astronomer Jiangao Ruan (China) discovered a small leading fragment in SOHO/LASCO C2 images (SOHO-1782), that was later also recovered in STEREO/SECCHI HI1-A . It was only visible in three or four SOHO/LASCO images (five are generally required in order to confirm a SOHO comet), hence had it not been for STEREO/SECCHI, this comet would have remained unconfirmed (probably an X/comet). It was too faint to appear in STEREO-B and SOHO/LASCO C3 images.
Comet C/2020 P4 (SOHO) was a unique non-group comet discovered in SOHO/LASCO data on August 6th, 2020. Not only was it brighter than most of SOHO’s non-group comets, but it could be resolved into three (if not four) individual fragments! This is unlike any previously observed SOHO/STEREO non-group comet. My guess is that C/2020 P4 was an intrinsically faint comet that underwent a series of fragmentation events soon before perihelion.This blog post is mainly focused on the comet’s discovery and appearance in SOHO/LASCO.
SOHO comet hunter Worachate Boonplod (Thailand) initially reported the object as one “single” comet when he first discovered it in real time SOHO/LASCO C3 images of August 5th – 6th, 2020. Indeed, although the comet displayed an elongated morphology, there was not yet any evidence of its multiplicity (see Figure 1). However, later that same day, more recent SOHO/LASCO C3 images revealed a second condensation, thus showing that the elongated nature of the object was due to (at least) two fragments. Worachate was the first to report the presence of a second companion. As of that point, it seemed that the object consisted of only two fragments: A moderately bright object (C/2020 P4-A) followed closely by a slightly fainter condensation (C/2020 P4-B) (see Figure 1).Figure 1: Images of C/2020 P4 (SOHO) as seen at different points when present in the SOHO/LASCO C3, prior to entering the C2 FOV. Left: C/2020 P4 as seen in C3 when discovered by Worachate Boonplod. Although elongated, the resolution of the image is not high enough to distinguish more than one object. Right: One of the discovery images revealing the object’s multiplicity for the first time. The resolution is still insufficient to reveal any additional fragments. Image credit: ESA/NASA SOHO/LASCO C3.
Finally, on August 7th at ~12:00 UT, the fragments entered the SOHO/LASCO C2 FOV. To everyone’s surprise, these images revealed two additional fragments! The leading fragment (C/2020 P4-A) was accompanied by a very faint object moving alongside it (C/2020 P4-C), while the second condensation itself (C/2020 P4-B) consisted of two distinct ones. Unfortunately, the proximity of the latter two makes it impossible to confirm them as separate objects. Both P4-A and P4-B displayed faint tails as seen in C2. In fact, P4-C appears to be located within the tail of P4-A, at least from SOHO’s perspective. As mentioned previously, Worachate Boonplod discovered P4-A and P4-B, while Masanori Uchina was the first to report P4-C (see Table 1). Worachate was also the first to report the double condensation of P4-B. Below is an annotated image and an animation showing C/2020 P4 (and all its fragments) as seen in C2.
Figure 3: Animation of C/2020 P4 transiting the SOHO/LASCO C2 FOV. Notice the faint tails associated with the two brightest fragments (P4-A and B). The brightness of the tail of P4-A possibly appears enhanced by its convolution with P4-C. Image credit: ESA/NASA SOHO/LASCO C2.
P4-C and the double condensation of P4-B were only observed in C2 images. Hence, they were never seen again once the comet exited the C2 FOV in the late hours of August 7th (UT). The overall brightness of C/2020 P4 reached its peak on August 8th (when around perihelion), before slowly fading by August 9th. C/2020 P4-A and B were no longer distinguishable after Aug 9th, partially due to SOHO’s observing geometry. C/2020 P4 was last detectable in C3 images of August 11th. It was never recovered from Earth. This was hence the last time any of the fragments could be resolved. A C3 animation showing C/2020 P4 receding from the C2 FOV is shown below (Figure 4). Notice how it continues to brighten and then slowly fade, while appearing progressively more condensed.Figure 4: Animation of C/2020 P4 as seen in SOHO/LASCO C3 after having transited the C2 FOV (images from August 7th 18:00 UT – Aug 9th 12:00 UT). Notice how the elongation decreases, partially as a result of the spacecraft’s observing geometry. This makes it impossible to distinguish the two main condensations, P4-A and P4-B, after August 8th. Faint tails are detectable. Image credit: ESA/NASA SOHO/LASCO C2.
Amateur astronomer Alan Watson (Australia) recovered the comet in STEREO/SECCHI HI1-A data of August 5th, 2020. In those images, the resolution was insufficient to resolve the individual components, although it did appear somewhat elongated on August 8th and 9th. The comet appeared to brighten as it arced around the Sun, before it left the HI1-A FOV on August 9th. It was even bright enough to have been detected in real beacon HI1-A images. The comet was quite obvious and showed a several-degree long tail. This is much longer than observed by SOHO, which may be due to the observing geometry of STEREO-A and the lower limiting magnitude of the HI1-A instrument. Michael Mattiazzo was able to recover the comet in SOHO/SWAN images taken only a week before its apparition in SOHO/LASCO.Figure 5: Animation of C/2020 P4 transiting the STEREO/SECCHI HI1-A FOV (August 5th -9th). Unlike in SOHO/LASCO, the comet shows an obvious tail in these images. The resolution however is insufficient to resolve any individual fragments, however the elongation increases just before it leaves the FOV. Notice the Pleiades a couple of degrees W and NW of the comet. Image credit: ESA/NASA SOHO/LASCO C2.
Table 1: Summary Information
Internal SOHO designation
Maximum apparent magnitude
~+5-6 (fainter than A and C)
Table summarizing the various designations of the C/2020 P4 (SOHO) fragments and their discoverers. Magnitudes are from Seiichi Yoshida’s website. The resolution of the STEREO/SECCHI HI1-A images were insufficient to distinguish the fragments. C/2020 P4-B was too faint to appear in C3. Note that C/2020 P4-C likely consisted of two very close condensations of comparable brightness.
Other than being a relatively bright non-group comet, C/2020 P4 was unique as it could be resolved into three or four fragments. While it is common to observe close clusters of Kreutz-group comets in SOHO/LASCO (see Figure 6), this is rarely the case of non-group comets. The only other comparable case was that of SOHO-277 and 278 on December 20th, 2000. These were a close pair of comets discovered by German comet hunters Maik Meyer and Sebastian Hönig, respectively. Sebastian reported a possible third component that same day, but it was too faint to have been confirmed.
Other than the potential SOHO-277/278 triplet, there have also been some cases of non-group comet pairs. For example, Zhijian Xu (China) and Karl Battams (US/UK) reported a pair non-group comets in C2 images of March 17th, 2012 and March 13th, 2015, respectively (see Figure 7). Unlike C/2020 P4 however, all three groups were much fainter and rapidly disintegrated. C/2020 P4 is (or was) most likely a significantly larger comet. Figure 7: Two non-group comet pairs (SOHO-2252, –2253 and SOHO-2890, -2891) seen entering the SOHO/LASCO C2. Ulike the recent sungrazer triplet, these pairs were fainter and appear to have disentigrated only hours after these images were taken. Image credit: ESA/NASA SOHO/LASCO C2.
On August 25th, the SOHO and STEREO spacecraft observed a previously undiscovered sungrazing comet. Although thousands of “sungrazers” have been observed by the spacecraft’s coronagraphs, this one was significantly brighter than most. In fact, it reached magnitude +3 on August 27th, before fully disintegrating. The object was a Kreutz-group member, part of a vast family that includes many great comets, most notably C/1843 D1, C/1965 S1 and C/2011 W3.
Figure 1: SOHO/LASCO C2 image extract showing the comet only hours before it vanished. The apparent “tail” is actually more likely a trail of debris, a result of the comet’s disintegration. Image credit: ESA/NASA SOHO/LASCO C2.
In the early hours of August 26th (UT), amateur astronomers Junda Liu and Peiyuan Sun reported the presence of an unknown comet entering the lower left edge of the SOHO/LASCO C3 FOV (Figure 2). Based on its trajectory, it was obvious that it was a Kreutz-group comet on its way to perihelion. Both the discoverers are members of the Sungrazer Project, a citizen science project dedicated to discovering near-sun comets in SOHO/LASCO and STEREO/SECCHI data. In fact, this was the first comet discovered by Junda Liu.Figure 2: A SOHO/LASCO C3 image extract showing the comet as it appeared only hours after the discovery images. Although apparently small, it is much brighter than most Kreutz-group comets would appear at that distance from the Sun. Image credit: ESA/NASA SOHO/LASCO C3.
It is rare that a Kreutz-group comet is bright enough to be observed at the very edge of the SOHO/LASCO C3 FOV. Most sungrazers of this family are only visible much closer to the Sun, when they reach their very brightest (10-15 solar radii. Knight and Battams, 2014). These tiny comets tend to disappear after only a few hours, as they rapidly disintegrate and fade below the instrument’s threshold (Figure 3). According to Karl Battams (NRL): “It’s maybe once every 3 or 4 years we discover a Sungrazer at the edge of the C3 field of view like this!” Consequently, as the comet was already easily detectable when entering the FOV, it was evidently going to become a very bright object. Figure 3: A “typical” Kreutz-group comet as seen in SOHO/LASCO C3. The comet is designated SOHO-2572 and the images are from August 18th, 2013. Although having been present in the FOV for more than a day, this object was only detectable (for only a few hours) when at its brightest, before it rapidly faded below the instrument’s limiting magnitude. The low image resolution, along with the comet’s faint nature, made it impossible to detect any tail. Image credit: ESA/NASA SOHO/LASCO C3.
As expected, the comet brightened significantly over the course of the next couple of days (Figure 4). By August 27th, it had reached magnitude +3. Its brightness was such that it slightly saturated the SOHO/LASCO detectors. However, by 08:00 UT that same day, the object started fading dramatically, clearly indicating that the nucleus was disintegrating. This was sadly expected, as this tends to be the case of all Kreutz-group comets discovered by SOHO. Indeed, although relatively bright, the sungrazers detected by SOHO (and STEREO) are generally intrinsically faint (small). Based on the values provided by Knight et al. (2010), one can assume that the comet probably had an initial diameter of a only few tens of meters. The object was never recovered after it passed behind SOHO/LASCO’s occulting disk.Figure 4: SOHO/LASCO animation showing the comet’s inbound journey towards perihelion. Note how the comet rapidly brightened, before suddenly fading. The apparent “tail” is actually more likely to be a trail of debris, a result of the comet’s disintegration. Image credit: ESA/NASA SOHO/LASCO C2.
In addition to having been observed by SOHO, the comet was sufficiently bright to clearly appear in STEREO/SECCHI’s real time (beacon) COR2-A images (Figure 5). Unlike SOHO/LASCO, real time SECCHI data are of very low resolution. These are, however, exchanged by higher quality data a few days later. Figure 6 shows the comet as seen in a fully processed COR2-A image taken on August 26th. Note the difference in the comet’s trajectory as seen from STEREO, relative to SOHO. This is due to their different in locations in space, hence perspective.Figure 5: Animation of COR2-A real time beacon images showing the demise of the new Kreutz-group comet. Note the difference in its trajectory relative to that observed by SOHO. This is due to STEREO-A’s location ~60° behind the Earth and SOHO. The images were taken on August 27th. Image credit: NASA/SSC STEREO/SECCHI COR2-A.
Perhaps the comet’s most obvious feature was its prominent tail, which grew progressively longer as it neared the Sun. Rather than being an “ordinary” tail, it was probably a debris trail – remnants from the comet’s vanishing nucleus. The “tail” was also clearly visible in COR2-A, however its morphology differed significantly from the “streak-like” appearance observed by SOHO (Figure 1 and 4). This was [again] due to the spacecraft’s differing perspectives.Figure 6: A full-resolution STEREO/SECCHI COR2-A image extract showing the comet. Notice the tail’s (debris trail’s) curved nature, compared to the “streak-like” morphology observed by SOHO/LASCO. Image credit: NASA/SSC STEREO/SECCHI COR2-A
Although now-gone, the comet is still being studied. As Karl Battams posted on Twitter: “I might actually resurrect this subject next week, pending some additional analyses. There might have been something slightly different about this particular Kreutz compared to most other Kreutz, but I need to look at some extra data first”. In addition, I believe that it is important to study the full resolution STEREO/SECCHI COR1-A images from August 27th, once those are made available. It is possible that the comet may have been faintly detectable in those.