If you’ve been observing Comet C/2023 A3 (Tsuchinshan-ATLAS) over the past year or so, you may be aware of a new paper that suggests it could already be ‘doomed’.
The night sky never gets truly dark in summer for those of us at mid-northern latitudes, meaning most of the galaxies, nebulae and star clusters are hidden from view.
Yet there’s currently plenty to see in the summer sky: Jupiter and Mars shining together in the east before dawn, noctilucent clouds painting the sky with streamers, billows and whirls of electric blue light…
But many amateur astronomers and skywatchers around the world are patiently – or impatiently! – waiting for summer to end and autumn to begin.
Because that’s when we might have a bright comet in our sky.
The hype around Comet C/2023 A3
Soon after it was discovered in January of last year, studies of the orbit of Comet C/2023 A3 (Tsuchinshan-ATLAS) suggested it could become very bright in the evening sky in October 2024.
It may even become as bright as zero magnitude, the reports said.
That would make it the brightest comet to grace our northern sky for many years.
Since then the comet has been observed and photographed regularly, as it moved slowly through Virgo and into neighbouring Leo.
It has developed a short, curved tail, and although it hadn’t brightened as much or as quickly as expected or predicted, the general optimism was it was still going to be a naked-eye object in October’s evening sky, after rounding the Sun.
Is Comet C/2023 A3 doomed?
The optimism surrounding Comet C/2023 A3 was somewhat was dampened – if not drowned – at the start of July by scientific paper entitled ‘Inevitable Endgame of Comet Tsuchinshan-ATLAS (C/2023 A3)‘ predicting the future for A3 is looking very bleak indeed.
The paper, written by respected comet expert Zdenek Sekanina, suggests the aforementioned failure of A3 to brighten as predicted means the comet might be in the process of disintegrating.
It might even have disintegrated already.
Inevitably, for the next couple of days there was much wailing and gnashing of teeth in the comet community.
When the paper’s contents were picked up by astronomy and science websites, word began to spread across social media that Comet A3 was ‘doomed’ and that the proverbial big show in October had been cancelled, with no hope of refunds for the skywatchers who had bought early tickets.
The Comet A3 paper
Here’s the abstract from the beginning of the paper:
Hopes are being widely expressed that C/2023 A3 could become a naked-eye object about the time of its perihelion passage in late 2024. However, based on its past and current performance, the comet is expected to disintegrate before reaching perihelion. Independent lines of evidence point to its forth-coming inevitable collapse.
The first issue, which was recently called attention to by I. Ferrin, is this Oort cloud comet’s failure to brighten at a heliocentric distance exceeding 2 AU, about 160 days pre-perihelion, accompanied by a sharp drop in the production of dust (Af_). Apparent over a longer period of time, but largely ignored, has been the barycentric original semimajor axis inching toward negative numbers and the mean residual increasing after the light-curve anomaly, suggesting a fragmented nucleus whose motion is being affected by a nongravitational acceleration; and an unusually narrow, teardrop dust tail with its peculiar orientation, implying copious emission of large grains far from the Sun but no microscopic dust recently.
This evidence suggests that the comet has entered an advanced phase of fragmentation, in which increasing numbers of dry, fractured refractory solids stay assembled in dark, porous blobs of exotic shape, becoming undetectable as they gradually disperse in space.
You can read the paper here: https://arxiv.org/abs/2407.06166
As you can see, that was very gloomy reading indeed. But some are arguing that reports of the death of Comet A3 have been exaggerated.
Comet C/2023 A3: a new hope?
In the aftermath of the paper’s publication, other comet experts raised their hands and said “Hang on, that doesn’t seem right…”
They argued that the comet’s stalled brightening could be explained by other things, such as the angle at which we’re seeing it from Earth at the moment.
They also pointed out that images of the comet show no sign of any disintegration, no clumps of material falling away from it or moving down its tail etc., which we usually see during the disintegration of a comet.
Indeed, images of A3 by comet photographer Gerald Rhemann at the start of July, under the famously dark skies of Farm Tivoli in Namibia, show A3 has developed a short but noticeable ion tail, suggesting the comet nucleus is healthy and not falling apart.
And the below image was captured by José J. Chambo, another prolific comet photographer, from Namibia at the end of June 2024.
As you can imagine there is now a lot of discussion in the comet-observing community, with some groaning “I told you so!! It’s another Kohoutek/ISON! This always happens!”
Other grizzled veterans, who have been here many times before, are sighing “Calm down everyone, it’s looking ok…”
Some are even suggesting the comet is still on track to become a naked-eye object in October, with a peak brightness of zero or even negative magnitude.
So, what’s the truth? Is Comet A3 falling apart like a clown car, as other promising comets have done in the past, and all we’ll see of it after it rounds the Sun will be a faint, dusty trail?
Or is it still doing just fine, out there between Mars and Jupiter, as it closes in on the Sun?
The answer, as ever, is “We’ll just have to wait and see”!
Predicting the behaviour of comets is famously and notoriously hard.
Comets we thought were going to be bright have fizzled out and left us shaking our fists at the sky in frustration.
C/2021 S1 (ISON) is a notable example of a comet that promised so much, only to disintegrate before it reached its predicted peak.
Other comets predicted to be nothing special have been lovely to see and photograph.
So, while the initial predictions suggesting A3 might become a Great Comet, perhaps even the brightest since Hale-Bopp graced our skies back in 1997 have now largely been discounted, there’s still a chance that it might be a very nice sight in the evening sky in mid-October, after its closest encounter with the Sun on 27 September 2024.
And in the absence of any clear images actually showing A3 disintegrating, let’s just cross our fingers that it will give us all an early Autumn treat, instead of a pre-Halloween horror show…
If you’ve been observing or photographing Comet C/2023 A3, we’d love to hear from you. Do you think it’s all over, or is the best yet to come? Get in touch via contactus@skyatnightmagazine.com.