Caoimhín's Content

Today, we are finally going to deal with the comets that are visible at the moment. Depending on where you are and whether or not you use a telescope, these comets have technically been visible for a while. Now that they are coming up to their best and visible to the naked eye, we are going to take a closer look. I will refer to C/2023 A3 (Tsuchinshan-Atlas) as C/2023 or comet A3, and C/2024 S1 (ATLAS) as C/2024 or comet S1. There are many C/2023 and a few C/2024 comets, hence the A3 and S1 identifiers. However, I am only going to be discussing these two comets, but I feel it is important to note all the same.

We will begin by looing into the West, at sunset on the 12th of October. As the sky darkens, comet C/2023 A3 (Tsuchinshan-Atlas) should come into view. From here in Ireland, this early in the month, it is still quite low and close to sunset. It is just about visible, and zooming in a little bit should reveal that it is a comet, but as it very close to the sunset, you do need to be careful. Never point your telescope at anything that’s too close to the Sun. From the countryside it will be visible earlier, but the sunset sky would still be quite bright especially if you’re in the city.

C/2023 indicates that this comet was discovered in 2023, and (Tsuchinshan-ATLAS) refers to its discoverers. Tsuchinshan refers to the Purple Mountain Observatory on Hainan Island in China, who discovered this comet in January. Just a few weeks later, the comet was independently discovered by ATLAS in February. ATLAS refers to the Asteroid Terrestrial-impact Last Alert System, which sounds a lot scarier than it is, and isn’t just one telescope. ATLAS is really 4 telescopes, two in Hawai’i, one in Chile and one in South Africa. The South African telescope discovered this comet. When two independent dicoveries occur, it is only fair to name the comet after both discoverers. ATLAS has discovered a lot of comets, as has the Purple Mountain Observatory. We can find another comet that ATLAS discovered this year, but it’s not visible at the same time as C/2023 A3 from Ireland on the 12th of October, unfortunately. In fact, 2024 S1 (ATLAS) is difficult to see from Ireland at all, so we will deal with it first, because there is only so much I will be able to say and it isn’t really relevant to Ireland.

If we could look at the midday sky with no atmosphere in the way, we would see the other comet from Ireland. With no atmosphere in the way, we would certainly see C/2023’s tail as early as the 12th of October, but C/2024 would still be below the horizon. By moving forward in time, C/2024 S1 will come into view, rising above the horizon, looping around the Sun, and then dropping down out of view again. This comet is expected to be quite bright as it passes through perihelion, but unfortunately from our perspective here in Ireland, we’re not going to be able to see that, occur. It is very hard to see comets as they pass close to the Sun, but solar telescopes can help, especially if they block the bright disc of the Sun. Telescopes sometimes obscure the disc of the Sun in order to make the corona visible, simulating the effect of a total solar eclipse. This can help with observations of things close to the Sun as well. If we could see it there during the day, we would see the angle at which this comet is coming up to orbit the Sun, it’s going to put it mostly below our horizon. For us in Ireland, this comet, C/2024 S1 ATLAS, it’s not going to be really visible. It will be tough in general in the Northern Hemisphere especially on its approach to the Sun. From the Northern Hemisphere, it appears that C/2024 is coming in under the Sun, putting it closer to the horizon. When the comet comes out on the other side of its orbit, it will seem to be above the Sun and higher in the sky. This would bring us into the morning in November, and it will still be really close to the sunrise. We might catch it a little bit in the morning, but the angle will still be quite steep. We will have a much better chance of seeing it as it’s leaving the solar system, but if you’re closer to the equator you’ll have an even better chance, even as it coming in towards its perihelion. You’ll also have a better chance of seeing it on it’s approach if you’re in the Southern Hemisphere, but far in the South you’ll have the same issue while it leaves as the far North has during approach, it will be very low in the sky.

C/2024 S1 (ATLAS) is one of the comets that the Asteroid Terrestrial-impact Last Alert System of telescopes has spotted in 2024. Given that it was discovered so recently, its exact orbital characteristics are not accurately known, nor is its period. C/2023 was spotted in 2023, so we have had longer to calculate its behavior, and it is though to be a non-periodic comet, or be about to become one. I mentioned before that a lot of comets have long periods, so depending on how long your life is and depending on how young you are when you see them, some comets are once in a lifetime. Comet A3 might be a once-in-its-lifetime comet. This comet may have been through the inner solar system before, tens of thousands to hundreds of thousands of years ago. However, there is a good chance that thanks to orbital perturbations, influence from the gravity of other planets, as this comet leaves the solar system this time, it might get ejected, it might get thrown out of our solar system altogether. Whether this comet has ever been passed us before, this this time it looks like it isn’t coming back. Thankfully, it is visible to the naked eye, so we have at least this one last chance to see it.

A little later in October, C2023 A3 may be a little fainter, as it is further from the Sun, but it will also be in the sky later, when the sky gets darker. This can combine to make it easier to see with the naked eye. From a dark sky, not only is the bright central dot, the nucleus surrounded by a coma, visible, even the tail may be visible to the naked eye. A tail is the classic signifier of a comet. Any telescope should give an incredible view. In fact, large Telescopes with a small field of view may actually cut off the end of the tail. Comets are so close to us and their tails can be so long, lower power magnification can be better. Even a rather small binoculars should help you see the tail, and hopefully the whole extent of it. It is close to the Sun on the 12th, but from at least the 14th it should be visible with the naked eye. It should still visible as a dot as long as everything’s nice and dark through till the 27th, as it’s tail will shrink and fade as it moves away from the Sun. I have an odd feeling that by Halloween it’s going to be too faint to see with the naked eye, but time will tell.

That is assuming a view from the countryside of course, not all of us are so lucky to be under dark skies free from light pollution. If you are looking at a sky with some light pollution, roughly what I’d see from Cork City, you should still be able to see the nucleus with the naked eye, even if the tail is a little too faint. If you from a bigger city I apologize, it may not be visible if their is too much light pollution. The tail may be very faint, but even with quite small binoculars it should become visible from a city like Cork. Even with a telescope just a couple of inches across, 6 or so centimeters, you should get a view of the comet, even from the city. It should be about 4th magnitude even a little later in October, which is certainly easy to see. If we push all the way through to Halloween, it seems to be predicted to be nearly 7th magnitude. Nearly 7th magnitude is faint, even under a dark sky. I guess if I say that Uranus isn’t visible to the naked eye, I should be saying that this comet will be invisible to the naked eye by Halloween if it becomes that faint. It would certainly be tough to pick it out from the fainter stars, but maybe if you’re in the countryside.

It is a good thing that comet A3 has already passed perihelion. Comets can get very close to the Sun, and those that get especially close are often called sungrazers. Some sungrazers are part of a specific group, as comets that get too close to the Sun can be ripped apart by the Sun’s gravity. This may have lead to a particular group of comets that get very close to the Sun called sungrazers, that are believed to be the remains of a comet that already got too close to the Sun and broke apart. The Kreutz Sungrazers include some bright and historical comets, and may all just be fragments of an older comet that broke up, all still following roughly the same orbit, each fragment now making their own close approaches along the path that destroyed their parent body. C/2024 S1 may be such a comet. C/2023 A3, at its closest approach to the Sun, was still 75 million kilometers from it, half an AU away from the Sun. This puts it, of course, half way between us and the Sun. C/2024 S1 may pass just a million or so kilometers from the Sun, roughly 0.008 AU. C/ 2024 S1 however, it hasn’t yet passed perihelion, but it is going to get very close to the Sun, so there is the possibility that it could get ripped apart by the Sun. This may have already happened, as comets can disintegrate as they approach the Sun, before they reach it. On the other hand, we might get to see it on its way in and it might be destroyed by time it’s leaving again.

Before finishing up with these comets, it’s important to take a different perspective into account. From the Earth it can be hard to really see how these comets are moving around the Sun, after all we are moving as well. We are going to head back out to the Solar System Observer, that distant point where we can look down on the solar system. I know I seem to be making a habit of this in recent pieces, but of course it’s a very interesting perspective and not one we can actually see from. No telescope has been sent way above the solar system to look down on it from above.

Looking down on the solar system for early to mid October, with the positions of the inner planets marked, we can get a better idea of where the comets are and where they are going. Comet A3 is roughly between Venus and the Earth and it’s on its way out of the solar system. It came in from almost the opposite side of the Earth, and moving in roughly the opposite direction. It orbited the Sun, and came about as close to the Sun as Mercury, so pretty close. It then continued to move, almost as far away from the Sun as Venus gets and within half an AU of the Earth. It will then continue away from us, as we move away from it in our orbit, and out of the solar system. This comet, A3, seems to be making an orbit on a hyperbolic trajectory. A parabolic trajectory is a normal, often very tight, or highly eccentric, ellipse, which will bring a comet around again and again. A hyperbolic trajectory, that’s one that indicates that the comet might actually get thrown out of our solar system all together and that is a possibility with this comet. A hyperbolic trajectory can be seem like a quite wide ellipse, but it would not loop back around, eventually continuing away from the Sun indefinitely.

Moving on to C/2024 S1, this comet seems to be making a pretty close approach to the Earth, if we are just looking at it from above. It is predicted to have an incredibly close pass to the Sun. As I mentioned, that kind of very very close approach is dangerous for a comet, it’s dangerous to get too close to the Sun. The orbit of this comet is incredibly elliptical, indicating a long period, and similar sungrazers have periods of around 600 years or more. However, these other sungrazers do seem to have orbits that terminate at an aphelion and allow them to return, albeit from very far away. As the comet moves away from the Sun, if it survives its perihelion, it seems that it will get pretty close to the Earth. On the 7th of December, it looks almost in line with the Earth from the perspective of the Solar System Observer. However, it is would actually be far below the plane of the ecliptic underneath the Earth.

Sticking with this date far forward in December and coming back to the Earth we can double check the distances involved. We will return to the default location back in Ireland, but I’m not too worried about whether or not this is actually going to be visible to us here in Ireland, as it likely won’t be. Getting rid of small details like the ground and the atmosphere, we would be able to see the far below us and far below the plane of the ecliptic, almost as far from where we are as we are from the Sun. With things like this, comets in particular, because their orbits can be so so highly inclined, it’s really hard to figure out where they’re going to be or what they’re going to look like without taking that kind of a three-dimensional perspective.

Both of these comets were discovered, at least partly, by ATLAS, which means ATLAS was doing its job. The Asteroid Terrestrial-impact Last Alert System is looking for things that are going to come close to the Earth and giving us a warning before they get too close. That’s the job of the ATLAS group of telescope, and they spotted these comets months before they came particularly close to the Earth. Even though the C/2024 S1 might not be visible, C/2023 A3, will be, and it should be visible just a bit after sunset for a few weeks at least.

I hope you enjoyed this piece about some of the upcoming comets and how strangely they move through the sky and space. If you did enjoy it then please do like it and if you’d like further support my content you can subscribe to this website and my YouTube Channel. Hopefully you’ll get to see at least one of these comets and hopefully I’ll see you back here next time.