Studying a distant visitor: What we know about Interstellar Object 3I/ATLAS

There are millions of objects in our Solar System. Among these, only three have been found to have come from beyond. Known as interstellar objects, these are bodies that travel extraordinarily long distances between stars and pass briefly through systems like ours. 

The most recently discovered is 3I/ATLAS, also known as C/2025 N1 (ATLAS) and previously as A11pl3Z. This comet was spotted by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station in Chile in July 2025. Despite the alarming name of that alert system, 3I/ATLAS poses no threat of impact. Instead, it will pass through our Solar System and continue on its long journey through the galaxy. 

Dr. Bryce Bolin is a research scientist at Eureka Scientific Incorporated and the lead author on a study describing the discovery and the physical characteristics of this interstellar comet. Bryce is one of only a few astronomers in the world who have studied all three known interstellar objects, including 1I/ʻOumuamua (discovered in October 2017) and 2I/Borisov (discovered in August 2019). He joined Planetary Radio host Sarah Al-Ahmed in August 2025 to discuss 3I/ATLAS and how studying these fleeting visitors helps us learn about planetary systems beyond our own. 

The transcript below has been edited for length and clarity. Listen to the full episode here.

Sarah Al-Ahmed: This is one of those stories that just lights the imagination on fire. Interstellar objects provide one of our only opportunities to be able to really observe material coming into our Solar System from somewhere else entirely, and I think people are right to be really excited about this. 

Bryce Bolin: It's just mind-blowing. You have all these theories about how many of these things exist out there and what their properties could be like, but the difference between theory and what we actually observe can be quite large. So for 'Oumuamua, we think it's most likely a comet because it has large non-gravitational perturbations like a comet. Comets “defy the laws of gravity” — their orbits can’t be explained by gravity alone because they emit gases, and this has a non-negligible momentum on their trajectory. 

Comets outnumber asteroids by quite a lot in our star system, and in extrasolar star systems, we think they should as well. You'd think that what's going to be ejected into space and seen somewhere else in the galaxy is going to be a comet, but we didn't know. 

And so when 'Oumuamua was defying the laws of gravity, that kind of gave us a sigh of relief. The other two interstellar objects, 2I/Borisov and 3I/ATLAS, are clearly comets. So it seems that so far all the interstellar objects are comet-like material, which checks with our expectations of planet formation and what we think the material around other stars should be like. 

Sarah Al-Ahmed: What kind of things have you been studying during the limited amount of time that we've known about the subject?

Bryce Bolin: For 3I/ATLAS, our first look at it was using broadband colors, getting optical wavelength colors. We also observed it with the Keck Telescope to get near-infrared colors and spectra. We can see possibly the emission of cometary gases like cyanogen, diatomic carbon, and triatomic carbon, and this can be indicative of the devolatilization of the comet. These molecules first originate as ices in the comet, and then the heat from the Sun causes the ices to sublimate. They undergo complex reactions while they're in the coma of the comet, and one of the products can be cyanogen, as well as the triatomic carbon. 

We're also going to get some James Webb observations of these objects. I'm not formally a part of that team, but James Webb will be able to observe the comet in wavelengths well beyond the visible and near-infrared, enabling us to detect the emission of cometary gases directly. 

Sarah Al-Ahmed: Some things I've seen online suggest that this object could be much older than other things in our own Solar System. What do we know about where this object came from and its potential origins?

Bryce Bolin: My understanding is that this is actually pretty difficult to find out. It turns out that the galaxy is quite complicated. It's not like you have all the stars in the galaxy orbiting one central thing like a star, a supermassive black hole. They do in a way, but the gravitational potential is quite variable in the galaxy because it's huge and has irregularities — molecular clouds, spiral arms, stellar clusters. And so the way that this affects the orbits of stars and interstellar objects that they float around in the galaxy is quite difficult to understand. Over millions of years, a object could encounter spiral arms and all sorts of things that can really throw off its trajectory in the galaxy. And so I think it's really hard to say anything about an interstellar object’s trajectory beyond maybe one or two galactic orbits — which, for our Sun, takes about a million years. For something billions of years old, like I’ve seen some papers state, its trajectory and origins are pretty hard to pin down.

Sarah Al-Ahmed: Do we have a general idea of the way that this thing might be passing through our Solar System?

Bryce Bolin: 3I/ATLAS is actually going to get quite close to the orbit of Mars, which could be an interesting opportunity for spacecraft at Mars to try and observe this thing. I'm a fan of it. I think they should give it a try. 

3I/ATLAS will be observable until the end of August, early September-ish, and then it goes into solar conjunction, meaning the Sun will block the view of 3I/ATLAS from Earth. But when Earth gets around the other side of the Sun, we'll be able to see it again.

Sarah Al-Ahmed: Since there's so much material coming off 3I/ATLAS, it's got to be hard to figure out exactly what size its nucleus is. Do we have any size estimates?

Bryce Bolin: There are two ways of doing that. At the present brightness of the comet, it's about 18th or 17th magnitude, and located about three or four astronomical units from the Sun. We can get an estimate of the nucleus by comparing its brightness at that distance to other interstellar objects we’ve observed. At a similar distance from the Sun, 2I//Borisov had a brightness that was a couple of magnitudes fainter. We think its diameter is somewhere in the order of a few hundred meters. So when you have two orders of magnitude of difference, that implies a size of about a kilometer for a 3I/ATLAS. 

The other way of doing this is with high-resolution imaging. David Jewitt at the University of California, Los Angeles, observed this object with the Hubble Space Telescope. The great thing about Hubble is that you have observations from space that aren't affected by the smearing of the Earth's atmosphere. This lets you resolve the nucleus as well as the tail and coma, which is harder to do from Earth. You can basically measure the light from the nucleus with much less contamination from the dust. 

It's like looking at a fuzzy cat. Have you ever seen a big fluffy cat after it goes to the groomer and it gets a buzz cut?

Sarah Al-Ahmed: Yes.

Bryce Bolin: So it's kind of a similar thing with comets. When I was a kid, we had a Persian cat, and my mom had allergies, so he would have to get a haircut. This big fluffy cat, and after the buzz cut, it’s a lot smaller. And so using the Hubble Space Telescope versus ground-based observation is like the difference between buzzing the cat’s fur versus just giving the cat a little bit of a trim. You can get a much better estimate of the size of the cat. You need a higher resolution to shave most of the fur off and get a much accurate estimate of the size.

Sarah Al-Ahmed: There's a lot of complexity to the way that comet tails behave. What's going on with the tail or tails on this object?

Bryce Bolin: The 3I/ATLAS is quite interesting in the sense that the tail is going in a strange direction. Comet dust, when it gets ejected into space, is usually blown in the direction opposite to the Sun due to solar radiation pressure. But curiously, this is not what we see for 3I/ATLAS. Instead, its tail is going in the direction of the Sun.

The interpretation is that you have large dust particles that are being ejected into space, and they could be hundreds of microns across. They're too heavy due to their size to be blown in the opposite direction from solar radiation pressure. So I think what's happening is that the Sun-facing side of the comet is heating up, and that is where the particles are coming off in the direction of the Sun. And the solar radiation pressure isn't strong enough to blow them around and go in the opposite direction again. 

Based on this information, we think dust is leaving the comet at a very slow speed, about a meter per second. This suggests that the comet may be weakly active right now.

Sarah Al-Ahmed: These objects are so cool and can teach us so much, and potentially someday we'll be able to rendezvous with one and learn even more.

Bryce Bolin: So that's the dream. There's a mission called the Comet Interceptor, which has that as a goal. I don't know exactly when they're going to launch, but I'm thinking it's going to accelerate because of the discovery of 3I. 

Sarah Al-Ahmed: Well, sometime when I bring you back on to talk about the next interstellar object you're studying, we'll see whether or not that's true. We know that these objects must exist and that they must come through our Solar System at some point, but we're only now beginning to have the technologies to discover them. And I think we're about to potentially discover a lot more because of tools like the Vera Rubin Observatory.

Bryce Bolin: It could be very soon. It could be two months from now. It could be a year from now, it could be five years from now. You never know, but I think we can expect 4I — the fourth interstellar object — to be in the not-too-distant future.