Caoimhín's Content

Today, we are going to be taking a look for and at a new comet. The comet in question was discovered quite recently, just last month in March. The comet is called C/2025 F2 (SWAN). That’s C/ for “comet” and 2025 because it was discovered in 2025, that part is nice and easy of course. The F2 relates to when it was discovered, it’s not the first comet that was discovered in 2025, that would be A1, if there is an A1. The letter indicates the half-month, so if no comet is discovered in the second half of January, then there is no B that year. F is the second half of March, and 2 for the second comet discovered in that division. The (SWAN) at the end is for the discovering telescope or observatory.

The other letters and numbers this year haven’t gotten a lot of attention, as is the case most years. A lot of comets, and a lot of other objects, they’re not necessarily going to be impressive or interesting or easily visible for an amatuer astronomer, and so they don’t get talked about as much. A comet is just a lump of ice and rock way out in space, it’s not guaranteed to look nice from the Earth or be easily observable for most people, a lot of comets are discovered by large observatories with very powerful telescopes. In the case of F2 (SWAN), SWAN is an instrument on the SOHO satellite. SOHO is the Solar Orbiter and Heliosphere Observer satellite and SWAN is the Solar Wind ANisotropies instrument. It takes images of the Sun’s corona, but by studying these images people sometimes spot comets, and that’s what’s happened here. It is a very fancy imager on an incredibly expensive satellite, it’s a specialized piece of equipment. The kinds of comet that a specialized piece of equipment like that can discover, they’re not guaranteed to be visible from the Earth with a normal telescope or binoculars. A big deal doesn’t get made about a lot of comets that are discovered, it’s only exceptional comets that look nice to us in our sky. This one, Comet 2025 F2, is apparently going to be a good one.

First off, I need to add this comet to Stellarium. I use Stellarium to simulate the sky and produce the videos attached to these articles. This is a new comet, so of course Stellarium doesn’t have it yet and it doesn’t automatically get updated. Updates for Stellarium happen and they normally fix software issues or add little features. New stars or asteroids may be added, but for some of the more transient objects, you need to add them yourself. You add them with the Solar System Editor plugin. That is in the configuration menu under the plugins tab, just the Angle Measure Tool. You may remember from last time, the Angle Measure tool is a plugin that needs to be loaded at startup. Since I activated the Angle Measure tool for the last piece, it has been loading at startup ever since and remains available. I already have the Solar System Editor set to at startup, I have this selected because it is useful to be able to add comets and things.

Once the solar system editor is set to load, you may need to restart your software, and then you press the button to configure it. This brings up the Solar System editor itself. This allows you to import objects. It allows you to export as well, but more importantly it allows you to import details on minor objects into Stellarium. With the options on the first tab, you need to have the details in a file, either a file that you’ve downloaded or written yourself. If you know the right syntax to write the file, then you can write down what, what characteristics you want this object to have. That allows you to drop in things like fantasy objects, new planets, fake moons, really almost whatever you want. Otherwise, it is useful for adding many things at once, and also how you export pre-existing minor planet data. There is also a list of the minor solar system objects that are already on the system. They cover a big range, including some big ones like Vesta. This list shows a lot of new-ish comets, including some from 2025. The first in the list is A3, but they only get as recent as E, the first half of March. It doesn’t go as far as F, and we want F2. That means I’m going to have to import it, and the second tab lets me import it individually. There are various lists of objects that you can download rather than import, to get the orbital elements of various minor object in the sky. However, in this case it is better to search for the one particular comet on its own. It needs to be written correctly, so it is C/2025 F2, no need for the (SWAN) part in the search. With the object found on an online database, all you need to do is add the object and it gets imported just like that.

Now Stellarium will display that comet when you search for it. At the moment it is low to the northern horizon all night. It’s just above the Sun as it goes down at sunset. It’s a little bit lower than the Andromeda galaxy, so it’s not quite circumpolar for us here in Ireland, it is dipping below the horizon for the middle of the night. Before it sets it is a little west of due north, right under the Andromeda galaxy. Taking a slightly closer look and it is pretty visible immediately. It’s being shown as magnitude 7.8, and that’s definitely within the realm of binoculars in a city like Cork. It is pretty small, it’s listing a core diameter of 10 kilometers, which is big, if that were to crash into us, it wouldn’t be great, but it is still pretty small for a comet. Also, being recently discovered, this could just be a placeholder value. It doesn’t look very detailed in Stellarium, and of course there aren’t any very detailed images of it yet, but there are images of it already and from what I’ve seen it doesn’t seem to have developed a tail just yet. That’s going to happen a little bit later when it gets closer to the Sun.

Looking a little bit further into the future, moving forward in time, its position changes. It moves up above the Sun and it looks like it’s coming over the north of the Sun from the west. For us here in Ireland it ends up pretty close to the horizon, so that is going to make it a little bit harder for us to observe. HOwever, right now it is out there and it’s definitely in the realm of binoculars, binoculars should be enough to observe this comet. It is very low to the north right now, but it is coming closer to the Sun. This comet is on its way to perihelion, and as it gets closer it will get brighter. It’s going to get its closest to the Sun in early May. If we keep coming forward in time the comet comes around and ends up under the Sun. Once we get into May, it looks like this is going to be really an object more for the Southern Hemisphere. It ends up out of view for us, but if you are in the Southern Hemisphere, then it could still look quite bright. As it moves away, it will get fainter, quickly dropping to magnitude 9. This isn’t incredibly bright, and it’s definitely not as bright as it is at the moment. The comet may get up to magnitude 6-ish at the start of May, according to Stellarium that is when it’s at its brightest and its closest position to the Sun, perihelion, as well. I will come back to look at this again a little bit closer to those dates.

Coming back to just at the end of this month, the comet is in the glow of sunset, and by then it’s got a little bit of a tail developing there. Again, that’s Stellarium predicting that it will have a tail at that point, and it ends up very close to the Pleiades. Heading out to the countryside will give you a better view, of course, everything looks better in the countryside. At the end of the month it doesn’t seem quite visible to the naked eye, possibly due to the Crescent Moon. The Crescent Moon may enough to block it out, although I do think with a magnitude 6.3, it should be on the edge of being visible to the naked eye. However, it is very low in the sky, it’s getting reduced to 7.2 there by the air masses of the atmosphere because it’s so low in the sky. It looks like it might be tricky for us to see here in Ireland regardless, but it is out there.

That is the comet, C/2025 F2 (SWAN). We will take another look at it once we’re into May, or at least closer to end of April, once it’s closer to perihelion. We may take a look at it from the southern hemisphere as well. Over the next few days, it looks like it will be visible to the naked eye under dark skies, once the Moon is out of the way. It is very low to the north, very close to the Andromeda galaxy. It will sort of move from the Andromeda galaxy over towards the Pleiades or the Seven Sisters over the couple of weeks. If you’ve got a binoculars, especially if you’re in the countryside, that comet is out there and observable.

I hope that you enjoyed this piece, I hope that you now know the process of how to add a comet like this to Stellarium. If you did enjoy this piece, please do like it and if you like this kind of content you can subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.