C/2024 S1 and THE KREUTZ FAMILY OF SUNGRAZING COMETS

Closest to Sun: 2024 October 28 at 0.008AU (1.2 million kms or <2 solar radii)
Closest to Earth: 2024  October 24 at 0.87AU
Maximum magnitude 2.

The discovery of C/2024 S1 ATLAS during late September 2024 roused a huge amount of excitement in the astronomical community
for reasons of which I will explore in more detail below.

Throughout the 19^th century, a series of spectacularly bright comets arrived, all coming within a couple of radii of the Sun,
particularly in 1843, 1880, 1882 and 1887. German astronomer H. Kreutz proposed that the comets were related.
Fragments of a single comet that broke apart centuries and millennia ago, in a series of cascading fragmentation events.

Recently, Zednek Sekanina wrote a series of papers on the existence of a Super-Kreutz System of Sungrazing Comets.
He proposed a contact binary model as the progenitor that split in the year -371, with an approximate orbital period of 750 years.
That model was strengthened when, on January of 2019, NASA’s New Horizons mission flew past the Kuiper belt object Arrokoth,
displaying a 36km long contact binary composed of 2 planetesimals 21 and 15 kms across.
Sekanina’s papers can be accessed at https://arxiv.org/.

Orbit
Kreutz comets strongly favour viewing from the southern hemisphere due to their high southerly orbital tilt.  
They spend virtually all their time below the ecliptic and only a fraction of time above the ecliptic when rounding the Sun.
Ancient cultures in the southern hemisphere no doubt witnessed these comets but alas, historical records appear to be lacking until the 1800’s.
Favourable arrival times occur between October to March. Arrival during the months of October to December result in a morning sky appearance,
whilst an arrival between January to March results in an evening sky appearance.
Kreutz sungrazers appearing between May and July may come and go unseen,
as they approach and recede directly behind the Sun as seen from Earth’s perspective.

Let us look at Sekanina’s proposed hierarchy over the past 2,400 years.

Gret comet of -371 (The Progenitor)
Greek philosopher Aristotle made an account of this comet during the winter. “its light stretched across a third of the sky in a great band”.
Ancient Greek historian Diodorus Siculus quoted “there 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. The most important description was made by Ephorus of Cyme who reported that the comet split into 2 pieces.

Great comet of 363
Fast forward 734 years later. Roman historian Ammianus Marcellinus wrote “in broad daylight, comets were seen”. Note the plural form.
A pair of sungrazers arriving at perihelion within a day is a possible conclusion.
Note that over time, the components will gradually separate further apart, next arriving in 1106 and 1138.

Great comet of 1106
Sekanina proposes that this is the return of the first lobe, which we now call Population I (the largest Kreutz fragment).
This was well observed and recorded. Belgian historian Sigebertus Gemblacensis writes that on February 2,
a star appeared during the daytime about a cubit (1 degree) from the Sun.
It was later described by the Japanese as a gigantic comet appearing in the southwest, with a tail stretching 100 degrees across the entire sky.
It went out of sight after more than 30 days. It will next return in 1843.

Comet of 1138
This broom star was recorded by the Chinese on September 3, reported to have been seen in the east and going out of sight by September 29.
This was an unfavourable appearance and Sekanina proposes this is the return of the second lobe, which we now call Population II.
arriving during an unfavourable period of early August.

Great March comet of 1843
The return of the largest Kreutz fragment (Population I) was one of the most splendid ever seen.
It was spotted in broad daylight near the limb of the Sun on February 27.
By the time it reappeared into evening skies, it sported an extremely long tail of high intensity,
over 50 degrees long, equivalent to 2 astronomical units.

Great comet of 1882
The return of the second fragment witnessed in 1138 (Population II) and presumably the next largest.
First reported by W.Finlay at the royal observatory in Cape town South Africa on February 8, it brightened rapidly and was visible in broad daylight
by February 17, reaching possibly magnitude -13, brighter than the full moon. After its perihelion passage, the comet moved into the morning sky
and was a very prominent object sporting a brilliant 20-degree tail. On 30 September, several observers noticed that the comet's nucleus was elongated
and breaking into pieces and by the middle of the month no less than five fragments were spotted.

Great comet of 1965 (Ikeya-Seki)
Was the most brilliant comet of the 20^th century. Its orbit was remarkably like the comet of 1882, thus is a member of the Population II group,
but smaller than the 1882 comet. It was discovered on September 18, 1965, independently by amateur Japanese astronomers K. Ikeya and T. Seki.
It became visible in daylight on October 20, some estimates putting it at magnitude -10.
After rounding the Sun, it was a magnificent object in the morning sky, sporting a 30-degree tail. It too fragmented into multiple nuclei.

In 1995, NASA then launched the Solar and Heliospheric Observatory SOHO, primarily designed to observe the Sun.
However, it ended up becoming the most prolific comet hunting machine ever created.
To date, a total of over 5,000 comets have been discovered, the majority of these were found to be members of the Kreutz sungrazing family!

Most of these are intrinsically faint and burn up as they approach the Sun.
These SOHO dwarf objects are the end products of the fragmentation process.
What was interesting about the data was the notable INCREASE in numbers of them during the 2000’s indicating a possible return of a big one.

Great Comet C/2011 W3 Lovejoy
Queensland amateur astronomer Terry Lovejoy discovered this comet on 2011 November 27.
Initial orbital calculations indicated that it was a member of the Kreutz group and this was the first pre-perihelion ground based detection
of a Kreutz sungrazer since Ikeya-Seki in 1965. Perihelion was a mere 0.0055AU or 140,000kms from the solar surface on 2011 December 16.0UT.
Very few people expected it to survive, especially since the intrinsic magnitude at discovery was a mere magnitude 14.
This is much fainter than the norm.
Clearly, the Kreutz sungrazers are in a class of their own because these objects have already experienced intense heat at previous apparitions.
The solar corona has a temperature of 2 million degrees. You can imagine that on its previous encounter some 700 years ago, it was baked into a pulp.
On its next return, it displays minimal activity until very near the Sun, only emitting copious amounts of dust at perihelion.
The orbital period of comet Lovejoy is about 600-700yrs, typical of Kreutz members.
The comet appeared quite impressive in SOHO LASCO C3 images from 2011 Dec 14 at 3:00UT. It dived into the sun on Dec 16.0UT
and spectacularly appeared on the other side, shining at magnitude -4 but stripped of its tail.
Sure enough, a new tail started to rapidly develop as the head receded from the Sun as per below image on 2011 Dec 17
By the morning of 2011 Dec 22 (21.7UT) the comet had moved sufficiently far enough from the Sun to be visible in morning twilight.
From a backyard in suburban Adelaide, I observed a bright tail, 11 degrees long with the unaided eye, through moderate light pollution.
The surface brightness of the tail was brighter than any portion of the milky way.
Clearly this was the best comet that I had seen, except for comet McNaught in 2007.
The head however was considerably fainter, giving the impression of a "headless comet".
This approximates the description given to the great comet of 1887. By December 24^th, the tail had grown to a visual length of 20 degrees,
but the brightness appeared to have faded slightly. 2 mornings later in a darker site, I estimated the naked eye tail length as 30 degrees,
and photographically out to 40 degrees. Clearly the tail was growing rapidly but becoming fainter with each passing night.

Terry’s discovery threw a spanner in the works for Kreutz hierarchy, as it didn’t fit with either population I or II and was in a class of its own.
It was eventually designated as a Population III object.

C/2024 S1 ATLAS
Was discovered by the ATLAS survey on 2024 September 27. After only a couple of days observations, the speed and apparent motion
of the comet was indicative of a Kreutz member! Its orbit was virtually identical to the Great Comet C/1965 S1 Ikeya-Seki,
with an arrival time 58 years and 1 week later. Perihelion was expected on 2024 October 28, a mere 0.008AU from the Sun (2 solar radii).
C/2024 S1 ATLAS was probably one of the tailing fragments from the breakup of the 1138 comet.
After discovery, warning signs were apparent as the comet failed to brighten.
Then on October 17 and again on October 19, it experienced multiple outbursts and rapid changes in morphology such as coma size and ion tail length.

Outburst of C/2024 S1 ATLAS on 2024 October 16 at 08:59UT previously noted by A. Pearce.
The nucleus brightened from magnitude 14.0 to 12.8 V in 24hrs.

C/2024 S1 ATLAS on 2024 October 19 at 08:50UT.
The comet was 9 days to perihelion. I made a visual estimate of 8.1 using 15x70mm binoculars on 2024 October 19 at 18:15UT.
The tail was not visible in bright moonlight.

It then subsequently experienced a fade from magnitude 8.0 to 10.3 in 2 days.
Things were looking catastrophic with the nucleus appearing elongated.

By the time it entered the SOHO LASCO C3 field of view on October 26 at 14UT, it was obvious that the comet was not going to survive.

As you can see from this episode, the lifespan of the Kreutz sungrazers may be relatively short, astronomically speaking,
and the Sun will end up consuming them all.
The 1843 comet appears to be the primary nucleus that should survive the longest and will next entertain the masses sometime around 2580.