The Dangerous Lives of Sungrazer Comets

Great March Comet of 1843

Do comets have family histories? Some do. We’ll discover the genealogical history of a special family of comets, the so-called Kreutz sungrazers. They are hands down among the most spectacular celestial apparitions in human history. They are glorious because they are risk takers: they come close – exceedingly close – to the sun’s broiling surface and therefore release copious amounts of gas and dust after perihelion, visible in their incredibly long and beautiful tails [1]. If the earth is well situated at that moment, commentaries from awestruck humans will fill the history books.

But hazards lurk. As the centuries wear on, the risk increases that they will inch ever closer to the sun’s surface and confront the indignity of fragmentation [2]. This is so because with each hair-raising solar passage, their cometary bodies, however tightly or loosely they may be bound, will experience differential forces of the sun’s gravity, tidal forces, tending to rip them apart. Even the daredevils that survive pay a price. Volatilization from intense solar heat may shrink them like a snowball in an oven. Being large is an advantage, but if a sungrazer gets too close to the the sun, the consequences are usually grave: it will break into fragments, some of which may or may not survive as comets, or it will vanish whole and forgotten in the sun’s cauldron.

Following the 19th century work of German astronomer Heinrich Kreutz (1854-1907), scientists have stubbornly sought to crack the mystery of these celestial showstoppers. Where did they originate? Why do they happen in groups? Will there be future sungrazers? There is still much uncertainty, but a clearer picture has recently emerged. Brian Marsden, formerly of the Harvard-Smithsonian Center for Astrophysics, a modern pioneer in this field, divided the sungrazers into two groups that he determined must have split from a truly giant, ancient progenitor comet. JPL comet specialists Zdenek Sekanina and Paul Chodas have continued that work over the last quarter-century with a virtual torrent of publications, variously revised on the basis of each new-found piece of evidence (as is the process of science), including a trove of orbital data on dwarf sungrazers detected from the Solar and Heliospheric Observatory (SOHO) coronagraphs and other solar satellites.

They accept most members of Marsden’s groups, but now Sekanina, in a recent breakthrough paper, has offered a revised theory about the Kreutz family tree, whose remaining elements “fit together as nicely as the final pair of pieces in a puzzle.” [3] It is reminiscent of the history of biology, determining the phylogeny of each species, first from basic morphology and later, with DNA analysis, sorting and shuffling things around, so that now we know that hippopotamuses are the closest living relatives to whales!

The Great September Comet of 1882 seen from South Africa

The great progenitor, Aristotle’s Comet: 372 BC

This ancient relative, the great grandfather comet, appeared in the fourth century BC. Aristotle saw it when he was 12. It must have been a colossal object. Its break-up over various perihelion passages through the centuries spawned more than a half dozen large and spectacularly famous comets and many long-gone littler ones.

What was this progenitor comet like? Imagine you are the young Aristotle. It is a “clear and frosty” winter evening of the year, 372 BC. It is during “the archonship of Asteius” around the time of the terrible “earthquake in Achaea and the tidal wave” that destroyed the city of Helike [4]. You look to the west, just after sunset. You see a “great comet” of astonishing brightness. In your Meteorlogica, composed 40 years after the event, you write, Its light stretched across the third of the sky in a great band, as it were, and so was called a path. It receded as far as Orion’s belt, and there disappeared [5].

Later writers, quoting earlier sources now lost, have given vivid descriptions of this comet. Diodorus Siculus in the first century bc wrote: [T]here was seen in the heavens during the course of many nights a great blazing torch which was named from its shape a flaming beam… This torch had such brilliancy… and its light such strength that it cast shadows on the earth similar to those cast by the moon [6].

            It was long suspected – probably even before Kreutz – that this great comet was the sungrazers’ progenitor. Marsden speculated that it was so; Sekanina’s new theory strongly supports it [7]. In short, it was real, and huge.

After one orbit: the daylight comets of A.D. 363

 More than 700 years pass and Ammianus Marcellinus, the foremost Roman historian of the 4th century, records that in A.D. 363 'in broad daylight comets were seen'.  It's rare enough to see a bright comet near the Sun in broad daylight, unprecedented to see more than one at the same time.  The arrival time is consistent with the time of the expected return of Aristotle's comet, now fragmented like the aging Roman Empire would soon be. Dr. Sekanina points out that Ammianus' description fits a scenario in which a contact-binary progenitor broke up at large heliocentric distance, possibly near aphelion, into its two lobes, which Sekanina calls Lobe I and Lobe II, which subsequently split into secondary fragments, including the greatest pieces labeled as Fragment I and Fragment II.  Near or after the end of October, A.D. 363, all these pieces arrived, only days apart from each other, in the solar neighborhood, where they fragmented again, accounting for later-seen populations of sungrazers.

Why a contact binary? There’s been “gradually accumulating evidence based on spaceborne close-up imaging of cometary nuclei and Kuiper Belt objects,” Sekanina says, that “the contact binary is a fairly common figure among these objects.” Two prominent examples are comet 67P/Churyumov-Gerasimenko) and 486958 Arrokoth, a trans-Neptunian object located in the Kuiper belt [8]. Thus, the theory goes, the comet split near its aphelion into its two lobes (with perhaps a small connecting neck) that were first witnessed as separate objects in 363. Under current theory, this is the first great division of the Kreutz family tree which gave rise to all the later offspring.

After two orbits: The Great Comet of 1106

After the A.D. 363 apparition, we wait for the next full revolution of the comet. After 742 years have elapsed, we are in the early 12th century, the so-called High Middle Ages, and just after the First Crusade to Jerusalem involving knights, peasants, and serfs. It is early February, 1106, and a truly remarkable comet lights up the sky around the world. Recent detailed orbital computations have shown that it must have been the arrival of X/1106 C1, the largest mass of Fragment I. By all contemporary accounts it was a mighty one to behold. It was first seen from Belgium in broad daylight. Observers in Japan estimated its tail to be 100° in length and white in color. The Welsh ‘Chronicle of the Princes’ for 1106 reported this:  In that year there was seen a star wonderful to behold, throwing out behind it a beam of light of the thickness of a pillar in size and of exceeding brightness, foreboding what would come to pass in the future.

A Muslim chronicle said: a star appeared in the heavens with locks of hair like a rainbow. It went from the West to the middle of the heavens and it was seen close to the Sun before its appearance during the night. It continued appearing a number of nights, then disappeared.” [9]. A Chinese manuscript described it as appearing “like a broken-up star,” possibly hinting at its earlier-noted fragmentation.

Until recently, the existence of Fragment II was a mystery; in any case, no second spectacular sungrazer was known on a par with the Great Comet of 1106. We will get to that in a minute, but for the moment we’ll continue to follow the descendants of Fragment I.

After three orbits: The Great March Comet of 1843, and kin

The primary piece of Fragment I returned to perihelion in 1106, when in all probability it split apart. The entire (or the largest) mass of the 1106 sungrazer now arrives 737 years later. The world is now quite different. Spring, 1843; England’s Victorian Age is dawning; Napoleon III will soon make himself Emperor of the French. Up in the sky, after an insanely tight loop around the sun, this great mass blossoms into one of the most splendid comets ever seen, the finest of the century: the Great March Comet of 1843 (C/1843 D1). Its tail is 8° to 10° in broad daylight, estimated to extend 200 million miles through space. By mid-March it’s almost 50°. A German astronomer on the 21st traces it for 64°. The comet was spectacular by every account, the first or second brightest comet in history [10].

The Great March comet of 1843

Thirty-seven years later, the somewhat less brilliant Great Southern Comet of 1880, seen only in the southern hemisphere, sported a 50º tail. Those two were discovered only after perihelion, but the ultra-spectacular Great September Comet of 1882, found two weeks before perihelion, was fully visible in daylight next to the sun. In Spain, residents stopped in the streets to admire it. It caused a sensation everywhere." After perihelion, “the comet began to become visible again in the morning hours before sunrise as one of the most brilliant celestial objects of all time [11]." Then, in 1887, the Great Southern Comet appeared, also dazzling but quick to come and go and not seen in the northern hemisphere. These famed comets rising from the South had highly eccentric, needle-shaped orbits that wrapped ever so tightly around the sun before exiting fast on nearly the same path. And herein lay a small historical conundrum. Hoping to score a similar success like Edmond Halley with his discovery of a 76-year repeater, 19th century astronomers were long obsessed with the idea that the sungrazers were returns of a single object. Maybe C/1843 D1 was the return of the comet of 1668 (175 years), or of the sungrazer of 1705 (138 years). Or that C/1880 C1 was the return of C/1843 D1 (37 years); or – downright ridiculously – that C/1882 R1was the return of C/1880 C1 (2 years)! JPL comet scientist Zdenek Sekanina summed up this early sungrazer science as “rather pitiful” [12]. Clearly, a broader vision was required. Danish mathematician Thomas Clausen had once suggested the notion of cometary systems, an idea later pursued Martin Hoek, director of the Utrecht Observatory. He noticed that “the paths of two or three comets had a common point of intersection far out in space, indicating with much likelihood a community of origin,” perhaps arising from the ancient disruption of a common parent mass. His insight raised the question of how we may define these cometary ‘communities’ and discover their origins.

Population genetics (of comets)

There are a handful of mathematical markers that serve to distinguish one comet orbit from another: the time of perihelion, T; the perihelion distance, q  (in astronomical units, au), and three angular coordinates (in degrees): the longitude of the ascending node, Ω; longitude of the perihelion ω; and angle of inclination i [13].

From similarities in the orbital elements of earlier-seen comets, Halley, for example, was able to conclude that the comets of 1531, 1607, and 1682 were one and the same comet whose return in 1758 he famously predicted [14]. We can apply the same Halley look-back method to match the markers for each of the four great comets mentioned above to see how they resemble each other. 

You can see how similar their numbers are. Notice the hazardously tight q for C/1887: this daredevil ventured too close to the sun (only 27,000 km above the solar surface) and literally lost its head hours after perihelion, remembered now as the “headless wonder” [15]. These tightly grouped comets are called ‘Population I’ comets.

The fate of Fragment II of Aristotle’s Comet

Dr. Sekanina has persuasively identified the long missing largest piece of Fragment II as the so-called Chinese Comet of 1138, passing perihelion about three decades after X/1106 C1 [16]. It fathered two brilliant offspring: C/1882 R1 and C/1965 S1. Other down-line descendants of Lobe II became C/1970 K1 and (with greater uncertainty) C/2011 W3. You’ll recognize the names of these spectacular 19th through 21st century showpieces:

The Great September Comet of 1882 (C/1882 R1 A). This comet, which some say was the brightest of all the sungrazers in the last 200 years, is believed to be the largest surviving mass of Fragment II of Aristotle’s Comet. [17]

Comet Ikeya-Seki (C/1965 S1-B), perhaps the greatest comet of the 20th century.

Comet White-Ortiz-Bolelli (C/1970 K1). [18]

Comet Lovejoy (C/2011 W3), a breathtaking recent comet.

Here is the updated table of the key orbital elements of the comet populations descended directly or secondarily from Lobe II of the ancient progenitor, with the same markers (with q in au, and the angular coordinates in degrees) that we used in our first table:

These numbers are also of a kind, with a few variants justifying different population assignments. C/1882 R1 is the defining member of the other main group of sungrazers called Population II. But note the differences between the top and bottom pairs of comets. C/1970 K1 passed significantly farther from the sun (its q is larger) and it had a smaller longitude of the ascending node (Ω). It does not fit Marsden’s original grouping scheme. Not knowing what else to do with it, Marsden assigned it its own subgroup IIa, which (with certain associated dwarf sungrazers mentioned below) is the principal representative of Sekanina’s Population IIa. Also unique, C/2011 W3 passed much closer to the sun and had an even smaller nodal longitude. Sekanina assigns it (with its associated dwarf sungrazers) to Population III. Indeed, each new sungrazer that comes our way seems to slip through the net of Marsden’s original two-group scheme.

Comet Ikeya-Seki

Game Changers

            If you read through the papers Sekanina authored himself or with Paul Chodas, and more recently with Rainer Kracht, you will see the evolution of their theory since Brian Marsden’s pioneering work, shaped by empirical evidence and better mathematical techniques as the years went by. These events in particular were game changers in the study of the Kreutz sungrazers:

(1)   The discovery from spacecraft that many comets are contact binaries, suggesting the possibility, or even likelihood, that the progenitor comet was a contact binary and its superfragments were its original lobes.

(2)   The delayed post-perihelion fragmentation of Comet Lovejoy. The comet underwent severe morphological changes several days after perihelion, including loss of its nuclear condensation (a euphemism for ‘its head’) [19]. Intensive study of this comet yielded evidence – adding to the emerging view – that sungrazing comets can and do fragment non-tidally anywhere in their orbits [20], and clusters of tidally fragmented comets appear to arrive within 80 to 90 years of each other [21].

(3)   The realization that cometary fragmentation is normal and consistent with the historical record with C/1882 R1, and with the disintegrations of Comet Shoemaker-Levy 9 (D/1993 F2) in 1993 and Comet Schwassmann-Wachmann (73P) in 2006 (both of which I witnessed telescopically), in which “cascading fragmentation” can create ghostly “strings of pearls” as the fragments pass across the sky [22]. Comet West (C/1975 V1) also broke into four parts a month after perihelion.

(4)   New analysis of the great population of dwarf sungrazers detected by spacecraft, most especially the SOHO spacecraft, revealing up to nine separate groups of sungrazers, many not seen from Earth – including and enlarging upon the already known representatives of Populations I, II, IIa, and III – further bolstering the case of continuous and ongoing fragmentation of these objects, and the likely return of future sungrazers in the near and far future [23].  

(5)   From these studies, it’s plain that there are more astonishing sungrazing comets to come, as evidenced for example by the recent arrival of C/2024 S1 (Atlas). This Population II sungrazer, arriving only 13 years after Lovejoy, may be a fragment of the parent comet of the Great September Comet of 1882 and Comet Ikeya-Seki (C/1965 S1 [24].

Comet Lovejoy

Simulation

Below is a computer simulation of the innermost orbits of three great Kreutz comets at perihelion, Great March Comet C/1843 D1 (bright red), Ikeya-Seki C/1965 S1 (dull red), and Lovejoy C/2011 W3 (brown). I created it with Maple mathematical software using NASA/JPL Solar System Dynamics (SSD) database orbital elements [25]:

Appendix

 Chart of Notable Sungrazing Comets

(From JPL Small-body Database)

ACKNOWLEDGEMENT I wish to acknowledge and warmly thank Dr. Zednek Sekanina, Senior Research Scientist at Jet Propulsion Laboratory/CalTech, for his review of this article, his many constructive insights on the history of his work, and his welcome edits to the above account.

REFERENCES

Clerke, Agnes M. (1902) 2003. A Popular History of Astronomy During the Nineteenth Century. Decorah, IA, Sattre Press.

Kreutz, Heinrich. (1888). Untersuchungen über das Cometensystem 1843 I, 1880 I und 1882 II: I Theil: Der grosse Septembercomet 1882 II. Kiel. C. Schaidt. Marsden, B.G. “The Sungrazing Comet Group. II.” Astron J.  98, no. 6 (Dec. 1989): 2306-2321.

___________. “The Sungrazing Comet Group.” Astron J.  72, no. 9 (November 1967): 1170-1183.

Sargent, David. 2009. The Greatest Comets in History: Broom Stars and Celestial Scimitars. New York: Springer.

Sekanina, Zdenek. “Comet ATLAS (C/2024 S1) — Second Ground-Based Discovery of a Kreutz Sungrazer in Thirteen Years.” arXiv:2411.12941 [astro-ph.EP]. (November 2024). https://doi.org/10.48550/arXiv.2411.12941.

__________. “New Model for the Kreutz Sungrazer System: Contact-Binary Parent and Upgraded Classification of Discrete Fragment Populations.” arXiv:2109.01297 [astro-ph.EP]. (September 2021). https://doi.org/10.48550/arXiv.2109.01297.

__________. “Statistical Investigation and Modeling of Sungrazing Comets Discovered with the Solar and Heliospheric Observatory.” Astrophysical Journal, no. 566 (February 2002): 577-598. https://doi.org/10.1086/324335.

Sekanina, Zdenek, and Paul W. Chodas. “Comet C/2011 W3 (Lovejoy): Orbit determination, outbursts, disintegration of nucleus, dust-tail morphology, and relationship to new cluster of bright sungrazers.” arXiv:1205.5839v1 [astro-ph.EP]. (May 2012). https://doi.org/10.48550/arXiv.1205.5839.

_________. “Fragmentation hierarchy of bright sungrazing comets and the birth and orbital evolution of the Kreutz system. II. The case for cascading fragmentation.” Astrophysical Journal 607, no. 1 (July 2007): 663-657. https://doi.org/10.1086/517490.

_________. “Fragmentation hierarchy of bright sungrazing comets and the birth and orbital evolution of the Kreutz system. I. Two-superfragment model.” Astrophysical Journal 607, no. 1 (May 2004): 620-639. https://doi.org/10.1086/383466.

_________. “Fragmentation Origin of Major Sungrazing Comets C/1970 K1, C/1880 C1, and C/1843 D1.” Astrophysical Journal no. 581 (December 2002): 1389-1398.

Sekanina, Zdenek, and Rainer Kracht. “The Great Comet of 1106, A Chinese Comet Of 1138, and Daylight Comets in Late 363 as Key Objects in Computer Simulated History of Kreutz Sungrazer System.” arXiv:2206.10827v2 [astro-ph.EP]. (July 2022). https://doi.org/10.48550/arXiv.2206.10827.  NOTES

[1] Kreutz sungrazers travel on similar orbits with typical periods of about seven to ten centuries. They are usually seen between August and April, and best observed from the southern hemisphere. Those appearing between May and July approach from behind the sun from our vantage point and may arrive and sail off unseen.

[2] Marsden (1989) 2313. “[R]epeated fragmentation of the members of the Kreutz group as they keep returning to perihelion at random times results in… a tendency for q generally to decrease, suggesting that all members will eventually hit the Sun." The letter q is the standard symbol for an object’s perihelion distance expressed in astronomical units. A perihelion distance q of .004638 au is about one solar radius.

[3] Sekanina (2021), 14.

[4] According to the Greek geographer Pausanias, a great earthquake in 373 BC destroyed the Achaean city of Helike. The quake caused the city to subside, and a massive tsunami then killed all its inhabitants.

[5] Quoted from Seargent, 67, with some minor textual emendation by Dr. Sekanina consistent with his sources on this text.

[6] Ibid.

[7] Sekanina (2021), 14-15.

[8] Sekanina (2021), 6.

[9] The accounts in this paragraph were taken from Seargent, 91-94.

[10] Unless otherwise noted, the historical accounts in this section are from Clerke, 362.

[11] Kreutz, 108, Untersuchungen über die Identität des Cometen 1882 II mit dem grossen Cometen von 1106.

[12] Sekanina, from private correspondence with the author.

[13] There are a few things to remember when orienting the orbit of the comet in space relative to our solar system. The plane of a comet’s orbit intersects the ecliptic plane along a line, the endpoints of which are called the nodes. Where the comet is ascending at that point, the node is called the ascending node. The number of degrees along the ecliptic plane from the vernal equinox ϒ (the standard reference point in the solar system) to the ascending node is called the longitude of the ascending node and is denoted by Ω. Now, the number of degrees from the ascending node (now measured counterclockwise for along the plane of comet’s orbit) to the comet’s perihelion point is called the longitude of the perihelion, denoted by ω. The tilt of the comet's orbital plane and ecliptic plane is called the comet’s inclination, denoted by i. The shape of an orbit is determined by its eccentricity, denoted by e. The values of q and e determine the semimajor axis a as well as the orbital period P. All of the sungrazers have near-parabolic or parabolic orbits, so eccentricity isn’t particularly useful measure for comparison among them.

[14] Edmond Halley first inquired if comets appearing at different times in history are in fact the same comet orbiting the sun. His work on the subject appeared in 1705 under the title, A Synopsis of the Astronomy of Comets.

[15] C/1887’s admission to this group was uncertain for Marsden but confirmed by Sekanina. Sekanina (2002), 585, and citations within.

[16] Sekanina & Kracht (2022).

[17] Sekanina & Chodas (2007), 660; Sekanina & Chodas (2004), 662.

[18] This comet defined Marsden’s Group IIA, not neatly fitting his Groups I or II. See Marsden (1989). Marsden found that some sungrazing comets discovered from the spaceborne SOLWIND and solar Maximum Missions (SMM) coronographs also seemed to be sortable into Group I or II. See the NASA/JPL Solar System Dynamics (SSD) database, Small-Body Database Lookup. See also https://cometography.com/lcomets/1970k1.html for some notes on its discovery.

[19] Sekanina & Chodas (2012). The comet survived perihelion but began to fragment 2 days later.

[20] According to Sekanina, when a cometary nucleus splits into two parts, either fragment leaves the point of breakup with an orbital momentum that differs from the parent’s orbital momentum. This difference is manifested by a separation velocity that determines the fragment’s heliocentric orbit that deviates from the parent’s orbit. Sekanina (2021), 5.

[21] Sekanina and Chodas (2004), 657. “[T]he course of evolution of the Kreutz system at the upper end of the mass spectrum may be better ascertained from the distribution of the sungrazers’ arrival times than from the sources of subgroups. If so, the fragment hierarchy should be determined primarily by the cascading nature of the fragmentation process,” according to Sekanina and Chodas (2004). Hence the updated tables reveal clusters showing relatively similar times of arrival, consistent with the trailing nature of the fragmentation process depicted by those authors.

[22] Sekanina & Chodas (2007), 660 et seq. “Because of the alignment of the fragment condensations in a train embedded in an elongated sheath of diffuse material, they were sometimes likened by the observers — just as the condensations of D/Shoemaker-Levy 9 more than 100 years later. . . — to a ‘‘string of pearls’’ or ‘‘beads on a thread of worsted.’’ Ibid.

[23] Sekanina & Chodas (2007), 658-657. Sekanina has reorganized his thinking to accommodate the fact that there are many more sungrazers than were originally known. New discoveries have reshaped his thinking in a way best summarized by this quote from the paper of his a few years ago: “Given the thousands of members known nowadays, I deem it more appropriate to call their total the Kreutz system and to divide the ensemble into fragment populations. Accordingly… I consistently replace the term used by Marsden with the new one, even when I refer to his own work: his Subgroups I and II are now Populations I and II, respectively.” Sekanina (2021), 2.

[24] Sekanina says, “The new sungrazer might be closely related to comet du Toit (C/1945 X1), but most exciting is the possibility that it is a fragment of the parent comet of the Great September Comet of 1882 (C/1882 R1) and comet Ikeya-Seki (C/1965 S1).” Sekanina (2024), 1 & 7.

[25] The image is approximately to scale and accounts for comet positions drawn from their orbital elements as of their perihelion dates, ignoring the solar system barycenter offset. The solar system barycenter is the point about which the sun, planets, asteroids, and comets actually revolve. It represnts the combination of the mass of the sun + planets, whose aggregate mass is thus greater than the mass of the sun alone, and hence the barycentric orbital periods are slightly shorter than periods calculated without it (as in the Appendix chart). The position of perihelion relative to the sun itself is in fact continually offset by where the planets are, but is influenced mainly by the position of Jupiter and to a lesser extent Saturn at any given time. The effect is slight (it still lies within the sphere of the sun).