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Today, we are going to take a look at a couple of comets, including the interstellar comet 3I ATLAS. However, we’re going to start with a comet that was discovered much earlier this year, Comet C/2025 A6 Lemmon.

From the first part of the name, C/2025, we know this was a comet that was discovered in 2025. A6 means it was the sixth comet discovered in the first half of January. B would mean the second half, C is the first half of February and so on, but skipping I as it could be confused with 1. Lemmon comes from Mount Lemmon Observatory, which is the observatory that discovered this comet. It’s an observatory that looks for near Earth objects like comets and asteroids, as part of the Catalina Star Survey, operated by part of the University of Arizona in the USA. The Mount Lemmon Observatory discovered this comet way back at the start of the year, just as the comet was coming in towards the inner solar system. Moving from mid to late October, the comet is still getting brighter and still getting closer and closer to the Sun. Even though this comet was discovered way back at the start of the year, it’s still just coming up to its closest approach to the Earth, which should happen on the 21st of October. It won’t come too close, still 0.6 AU away, which may seem small, but is almost 90 million kilometers.

Pushing forward into November, it will reach its closest position to the Sun. When the comet is at its closest position to the Sun, it will be a little bit harder to see due to its position, but it is usually at its brightest. Based on the magnitudes suggested by Stellarium, we’re not going to get the comet visible to the naked eye. Even at the lowest magnitude that we’re able to get it, which is the brightest that we’ll be able to see it, it’s still not bright enough. While it will be above the horizon just above the Sun, it’s certainly not visible to the naked eye in the city, and with a magnitude of just 10, it’s not going to be visible to the naked eye anywhere. It already has a diffuse tail, and there is a good chance that it will generate more of a tail as it comes into its closest position to the Sun.

At the moment, it’s just passing under the handle of the Plough or the Big Dipper and it should be visible with binoculars. It’s currently closer to magnitude 11, which is pretty faint. Through binoculars, you should be able to see it as slightly fuzzy, not quite as sharp as a star, even if the tail isn’t visible. It also won’t stay in the exact place like a stars, but it does move slowly. Moving through time second by second, it’s not going to move drastically, but if we skip through time it drifts compared to the background stars. You can see that happening just over the course of one night, because it is moving pretty quickly compared to distant planets. Comets do move pretty fast, even if it isn’t noticeable over the course of just a minute or two, a couple of photographs in a row would show the comet moving differently to the background stars. That motion is the main way these things get discovered, that’s how we find out about these comets, especially while their to far from the Sun for a tail to form.

That’s C/2025 A6 Lemmon. The C stands for comet, and it would be P for a periodic comet. The comet likely does have some period, on the order of thousands of years, but we haven’t seen this comet go through multiple periods yet, so confirming that it is a periodic comet is a little bit tricky. Something might happen before it comes back around again, it might even blow up when it gets too close to the Sun. That can happen, although it isn’t particularly likely this time. The comet isn’t getting particularly close to the Sun as it passes by, and close approaching “sungrazers” are usually the ones that fall apart. Besides, C and P, there are other designations such as I for interstellar. There was a new interstellar comet discovered this year, Comet 3I/ATLAS. Its full name is C/2025 N1 ATLAS, discovered in the first half of July. You might remember interstellar objects from a previous video where we talked about 1I ‘Oumuamua, the interstellar object that was discovered back in 2017.

1I/’Oumuamua wasn’t really considered to be a comet because it didn’t behave like one. It didn’t produce the tail or the coma that you would expect from a comet. Comets are icy, watery bodies that produce tails when they get close enough to the Sun for the ices to thaw. 1I/’Oumuamua didn’t produce that kind of tail, so it wasn’t considered to be a comet, just an interstellar object. 3I/ATLAS however, it is still an interstellar object, but it is also considered to be a comet because it does seem to be producing a tail. We’ve only discovered three of interstellar objects to far, this one, ‘Oumuamua, and also 2I/Borisov. C/2019 Q4 or 2I/Borisov discovered in August 2019, perhaps didn’t get as much attention, but 2I/Borisov was also an interstellar comet. As such 2I/Borisov was really the first interstellar comet discovered. 3I/ATLAS is a good bit fainter, over 14th magnitude, a much lower magnitude making it even harder to see than Comet Lemmon. As usual, its magnitude picks up as it gets closer to the Sun, getting up to magnitude 13.97. Unfortunately, at those very close positions to the Sun around perihelion, it’s going to be very difficult to see from the Earth. It gets up to its highest magnitude for a few days around its closest approach to the Sun, which still isn’t particularly close to it.

Taking a look at the sky during midday, with the atmosphere turned off, lets us keep track of the comet even when it’s right next to the Sun in the sky. This lets us see it making its closest approach to the Sun later this month and then continuing out through the solar system. Going through the solar system is really the important thing, this is an interstellar object. Based on its trajectory, it’s just flying through the solar system, coming in one side and going out the other side. 3I/ATLAS really isn’t part of our solar system at all, it’s not gravitationally bound by our Sun. Moving forward to December and it will come it’s closest to the Earth. It will still be over one AU away, 1.8 AU or so. It will be back down to around magnitude 14, so very difficult to see. It’s not getting particularly close to the Earth, but interestingly it did get particularly close to Mars, just about 0.2 AU.

If we take a look at all of this from above, it will be clear how far away things really are. The Solar System Observer is a viewpoint from above the solar system which is one of the options offered by Stellarium. Currently, the comet is passing through the inner solar system. It came in from almost the opposite side of the Sun to the Earth, not getting particularly close to the Earth at all. As it comes past the Sun it gets a little bit closer to us as we orbit. Comet 3I/ATLAS was visible, briefly, from Mars. Both the Trace Gas Orbiter and the Perseverance Rover were able to catch a glimpse of it. Unfortunately, Mars isn’t very bright compared to the Sun, Venus and the Earth, so it doesn’t show as easily in Stellarium. However, a few months ago it can be seen coming through the asteroid belt. Several of the big asteroids, like Vesta, Pallas and Juno, were reasonably close to its path. Either way, this comet is not getting anywhere close to the Earth, so it’s certainly nothing to be worried about. Also, it is acting like a comet, it’s producing a tail and doing everything we’d expect a comet to do. As such, it’s even less likely to be anything strange. In fact, it’s less of an unfamiliar object than 1I/’Oumuamua, which of course has already comfortably passed through the solar system quite a bit ago. Even though 3I/ATLAS is getting a lot of the media attention because it is an interstellar object, the comet that we’re more likely to be able to see through something like binoculars, is C/2025 A6 Lemmon.

They are two of the big comets that are in the sky at the moment. 3I/ATLAS, is going to be incredibly tough to see, but it is interesting to see an interstellar object. If you have a big enough telescope, it’s worth trying to take a look. Comet C/2025 Lemmon is just back in the direction of the Plough or the Big Dipper at the moment. In mid to late October it is just under the Handle of Plough, near Cor Caroli. It is up all night, but occasionally such things will get blocked out by the ground or by things close to the ground, even if they’re circumpolar, even if they’re not going to actually go under the horizon. I hope that you get a chance to see at least C/2025 A6 Lemmon if not 3I/ATLAS, and of course 3I ATLAS is just a comet passing through our solar system, nothing to be worried about, even if it is interstellar.

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