What makes the interstellar comet 3I/Atlas so weird? Why are we finding so many interstellar visitors recently? How are they made, and where do they come from? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

Imagine you live in a small town. Maybe it's easy for you to imagine it because you do. You've spent your whole life there. You know all the people and all the people know you. Everything is the same. Years go by. Decades. Nothing ever changes. And then one day, a car drives through. It's got a strange license plate. It's not even from the next state over. It's not even from across the country. It's from across the world. It's from a completely different country. And it doesn't stop. It doesn't slow down. It blasts by so quickly that by the time anybody notices, it's already on the outskirts of town. Although, to be fair, except for the corner store, the whole town can be considered outskirts. In 2017, our first out-of-towner came for a brief visit. The first known interstellar object detected within the solar system, our good friend Oumuamua. Now, Oumuamua was pretty weird. It was really long and cigar-shaped, or possibly pancake-shaped. Remember, we don't actually have a picture of it.

It was so small and moving so quickly, and by the time we saw it, it was already on its way out. So it was just a little dot of light that would flicker and vary as it rotated, and from that we tried to figure out its shape, and so there's some unknowns here. And Oumuamua was weird. It was unlike any other solar system object. I mean, there were close analogs. There were things that kinda sorta looked like Oumuamua in our own solar system, but nothing exactly like it. And just like a stranger blown through town, Oumuamua made us wonder, is this what all interstellar objects look like? Or was Oumuamua special? And if it was unique, if it was special, then what were the odds that the first time we detected an interstellar object, it turned out to be a weirdo? All very valid questions with no satisfying answer. We only had that one example, and last time I checked, that's not nearly enough data to draw solid conclusions from. Then in 2019, we spotted another visitor. It was a comet named Borisov.

And despite the fact that it was moving way, way too quickly, it was otherwise an unremarkable chunk of rock and ice. It looked like and acted like a normal comet. Two visitors. One largely unlike anything we've ever seen in the solar system, and the other generally unremarkable. What gives? What is the universe trying to tell us with all these messengers? Well, last year we had a new guest. A third out-of-towner to come blasting its way through 3i Atlas, or just Atlas for short, when we're outside of a formal setting. It was first spotted on July 1st, 2025, but then as soon as the discovery was made, astronomers went back and looked through archival images and found that it had actually been spotted a few days prior, just no one had noticed. And the instrument to do the discovery was the Asteroid Terrestrial Impact Last Alert System, which is supposed to be monitoring the skies for doomsday asteroids, and okay, it is doing that, but it also spotted this new object. 3i Atlas made its closest approach to the Sun on October 29th, 2025, passing just inside the orbit of Mars.

And Atlas is weird. So Muamuu was weird. Borisov, not so weird. And now Atlas back on the weird column. So we've got two for weird, one for not weird. What's going on? You could reasonably say that Atlas was unlike any other comet in the entire solar system. And you'd be right. What gives? You start listing the properties of 3i Atlas and you realize it is distinctly different than any other comet we've ever seen. Here's just a small list. It's small. It's only a few hundred meters wide. It's smaller than a football field. It has a gas plume that contains much more nickel than we've ever seen in other comets. Usually, comets are low in heavy elements, which, because these heavy elements tend to sink towards the center of a solar system during formation, and comets tend to form on the outskirts. Atlas doesn't actually have a lot of water, which comets, being dirty snowballs, tend to have a lot of water, and Atlas didn't. As Atlas got closer to the sun, it got brighter because it got warmer and it was reflecting more sunlight, but it did so much more quickly than typical comets.

It contributed to Patreon. Let me tell you, most comments, they just swing by, they listen to one episode, and they're gone. They're not here for the long haul, unlike you. But this one, Atlas, subscribe to Patreon. That's patreon.com slash pmsutter, where you can support this show. What else is weird about Atlas? Well, for a long time, it didn't have a coma or a tail. It eventually got both of them, but it just took a while longer than we expected. And finally, on its way out, it started accelerating, as in, instead of being drawn towards the sun, it looked like it was being pushed away from the sun, which is kind of weird. And I'm going to be honest, even though this is a small list of weird things about 3i Atlas, that adds up to a lot. Just looking at 3i Atlas in isolation... It is an odd duck. It feels like it doesn't belong. And we have way more data about 3i Atlas than we did for Amuamua or for Borisov. So we got a really, really good look at this thing. And every time we looked, it showed us something new, something we didn't expect comets to do.

How are we supposed to understand this? How do we frame this? How do we wrap it? What do we learn from Atlas? Well, here's the thing about comets, folks. There's a saying. Comets are a lot like cats. They have tails, and they do what they want. And just like cats, not all of them even have tails. Take any comet you want, even something famous like Halley's Comet. And once you start writing down enough properties, you can spot all the ways that your chosen comet is completely unlike any other comet in the solar system. Comets are a really broad catch-all category of kinda mostly icy, somewhat rocky debris left over in the outskirts of a solar system that sometimes make their way inwards. That's really broad. That simple definition hides a lot of diversity. And because it's such a broad class, anything that gets lumped in that class has a really, really good chance, once you start really observing it and really nailing down the details, of it looking really weird. I mean, check this out.

We can play the same game with planets. Planets is a bucket. It's a category of labeling things. But Earth is the only planet with liquid water on its surface. Saturn is the only one with giant rings. Yeah, there's still planets, but as soon as you start drilling down on the details of any one planet, it looks unique. It looks unlike any other planet. You can do it with people. You are completely and totally unlike any other human being that has ever lived. But you're still human, I'm assuming. So at some level, we expect 3I Atlas to be unique because all comets are unique. And of course, an interstellar comet is going to get a lot more attention and observation and scrutiny because it's a special comet. And the more we observe it, the more we're able to highlight its differences relative to other known comets. And those differences are going to get a lot more media attention because, once again, it's special. No other comet gets this kind of observation time and journal articles and press releases.

I bet if we... 3I Atlas looks totally different, looks totally weird. It looks like it is on the surface a comet, but then it's doing a bunch of uncommet things. So at first glance, it's hard to pin it down to describe what exactly it is. But I bet if we played the exact same game with any other comet, even Halley's Comet, you would find a list of things that make Halley's Comet completely and totally unlike any other comet that we know. Okay, 3I Atlas is different than any other comet. So is it just a comet? Are we blowing the whole thing out of proportion? Or does it have a fundamental weirdness to it? Well, Atlas is on its way out. And once a stranger leaves town, it's going to be the subject of town gossip for weeks to come. And your first instinct is to automatically highlight the differences between the stranger and your community. And you might just conclude that the stranger is weird. And you might just conclude that all strangers are weird. You might just conclude that the world is full of weirdos except for everyone in your town.

Is that the right conclusion that we have from 3i Atlas? That all interstellar comets are weird? Well, kind of, sort of. Interstellar comets are weird. Yeah, they're still comets. They still fit in that bucket of comet family. Just like the stranger coming to town is still a person, just a weird one. And comets are like cats, each one is going to be a little bit different. But we expect interstellar comets to be even more different than average. Why? Because they're different. A stranger that comes to town is going to be different than the people in town. They might look different, or speak a different language, or prefer different kinds of cheese. So at one level, they're going to be different because every person is a unique and individual snowflake. And at another level, they're going to be different because... Because they are. There's regular different, and then there's different different. You dig? They share enough similarities with the comets in our own solar system that we can say, yeah, it's a comet.

But on the other hand, interstellar comets are a different breed than local comets. They are a little different different. They do exist in their own kind of subcategory. That's what we're starting to learn. Or at least they can. Borisov was pretty boring, as far as we could tell. But Amumu was weird, and Atlas is definitely weird. So it's starting to look like interstellar comets might be a subcategory. Like the same species of comet, but a different breed. Why is that? Well, one, they came from somewhere else. They were literally born in a foreign solar system. Our own solar system has a particular mix of elements of chemistry in the pre-solar nebula. We had a particular pathway with the mass of our sun and when it turned on and the kind of radiation and light it emitted when it was young. And the balance of elements as they swirled around our solar system. And if you go out to a different solar system, you have a different chemistry. Different temperature. Different parent star. Different amounts of dust.

Different formation pathways. And so on and so on and so on. It is different. It has a different upbringing. Atlas is not from around here. It doesn't share our exact same chemistry. And for another thing... Why we now are beginning to learn that maybe interstellar comets really are different, different, is they got kicked out. When you make a solar system, it's a messy thing. It's not easy. You start with this pristine gas cloud that's hydrogen, helium, some dust, heavy elements, and then it starts to collapse. It starts to spin. You form a star. You form some planets. There's still a lot of material left over. most of that material gets ejected. You have a little random rock just passes within the vicinity of a giant planet like Jupiter. Sometimes it can go plunging into Jupiter and add to Jupiter's mass, and sometimes the gravitational interaction just flicks it away. Gone. Bye-bye. Not every comet. In fact, the vast majority of comets. Bits of debris left over. Stuff that never got to form a planet, but was still bigger than a speck of dust.

Most of that gets kicked out. Only a small fraction of the population of comets-to-be become actual comets. And maybe, maybe comets that kicked out come from a different population than the comets that get to stay. Maybe they're smaller or they're lighter. Or they have more metals or something. Maybe they were formed in the early, like the inner regions of a solar system and then due to gravitational instabilities got kicked out. And then what we associate with comets in our own solar system, they formed on the outskirts. Maybe certain kinds of comets, depending on where they are formed, how they're formed, what they're made of, have a tendency to get kicked out. And it makes them a different breed. And lastly, interstellar comets are different because they have different lives. Solar system comets just hang out in the Oort cloud for billions of years until a little gravitational tug sends them flying into the inner solar system. Billions of years receiving a steady supply of incredibly feeble but not zero sunlight from our sun.

But interstellar comets have a different life. They're wanderers. They're vagabonds. Hobos even. How long do they spend in the void? Maybe it's billions of years. Maybe it's only a few hundred million years before they encounter another solar system. As for the radiation they receive, well, it's essentially nothing once they get kicked out. Even though the comets in our own solar system seem to be like a light year away from the sun, that's not nothing and that's constant over billions of years. These interstellar comets, well, they could be... 10 light years away from the nearest star for their entire lives until they encounter something like the Sun. So not only are they born somewhere else, not only might they be a different kind of comet that's more susceptible to getting kicked out, but they also spend billions of years doing things that solar system comets do not do. They have different kinds of radiation environments. Different kinds of exposure. Different histories of temperatures.

They accumulate different kinds of interstellar dust. The whole package. If you think I'm just getting into the nerdy weeds here, well, that's where all the magic of science happens, folks. It's the kind of stuff that makes the universe so rich and varied and interesting. Details matter. And I know I said my last point was the last one, but I lied. The real last point is that we have to remember that we don't get to observe interstellar comets when they're, you know, interstellar. We only get to see them when they get close to us, which means we only get to see them when they get close to the sun. The same is true for all comets. We have no direct observations of comets in the Oort cloud. We only suspect it's there because the comets have to come from somewhere. We only get to learn of comets when they come into the inner solar system, when they are being heated by the sun. That's when we get to study them. And the way a solar system comet approaches the sun, encounters the sun, is different than the way an interstellar comet does.

And that's because they're fast. Really fast. It changes how interstellar comets relate to the Sun. 3i Atlas was traveling at about 33 kilometers per second, which is about 74,000 miles per hour. That's twice as fast as a typical solar system comet. Which means it comes screaming towards the Sun much faster, spends less time looping around the Sun, and leaves much faster. And we know what happens to comets when they get close to the sun. They start to warm up. This changes their surfaces, creates a coma and tail. They start outgassing. It even sometimes destroys them, evaporates them. If they're held together too loosely, their parts just bake apart. Remember, a comet, when it breaks apart near the sun, this is the first time it's been near our sun in over 4 billion years. Three-eye atlas and other interstellar comets have different relationships with the sun. Not just during their lives, but in these moments when they happen to pass through an inner solar system like our own. It spends less time doing all the things that we normally associate with comets doing.

This can explain why it took so long to get a coma and a tail. There's less leisure time. It's just buzzing by. It's got no time to heat up long enough to form a coma and a tail. No, it's out of here. And that non-gravitational acceleration, by the time it finally did warm up enough, it was already on its way out. And so it started outgassing, which is the polite term for comet farts. It started outgassing on its way out, but it was already so far out that we couldn't see the gas being expelled itself, just the effects of the gas pushing on the comet. like a sort of mini comet rocket. So you put it all together. Interstellar comets, the evidence is slim, but it's pointing in this direction, that interstellar comets are comets. They were born and raised in another solar system, and they got kicked out, but they are just comets. And the reason they tend to look so weird, again, I know we're talking two out of three here, Not the greatest statistical power, but it's what we got. The evidence is leaning towards these are just comets that were born somewhere else, had a tendency to get kicked out, spent all of their lives wandering interstellar space, and then spent only a brief amount of time coming close to a star like the sun.

And that when you add up all of these, when you stack all of these different histories and formation mechanisms and chemistries and relationships with the sun on top of each other, it's enough to make a plausible case that the reason that Three High Atlas looks wildly different than any other comet is because one, comets are weird and unique and individual snowflakes in and of themselves. So already you expect a lot of variation when you go from comet to comet to comet. You expect a huge range of masses, sizes, shapes, metal content, all of that. Water content. You expect already a wide range. And then you layer on top of that, that this really is a stranger of a different breed. And you can explain why 3i Atlas is weird. But why now? Our first interstellar visitor that we detected was in 2017. Found another one in 2019. Here we are in 2025 with our third. That's slow. That's one every two or three years or longer. Okay, but it's faster than it was before because prior to 2017, there were zero known interstellar objects observed in the solar system.

We went millennia of astronomical observations without once spotting an interstellar visitor. And then in less than 10 years, we've got three of them. And the reason is... It's like, you know, like you're living, you spend 40 years living in your small town, and then in a week, three strangers come by. And you might be asking, what's going on? Where are all the strangers coming from? Why now? Why not before? Well, you're not going to spot the strangers if you don't open up your windows. And we finally opened up the windows. The thing is, we have new instruments. We have new telescopes. Things like the Atlas Telescope, which are designed to find small, dim, fast-moving objects, which is exactly what an interstellar comet is. We have instruments like Pan-STARRS. This is a telescope in Hawaii that scans the entire sky over and over again, looking for stuff that moves. It's like a cosmic motion sensor. The thing is, our astronomy is starting to get good enough. that we can find all the little bits.

We think these interstellar visitors have been coming since forever. We just didn't have the capabilities to detect them. And it's about to change a lot. You're about to start hearing about a lot more interstellar visitors, and that's because of the Verace Rubin Observatory. which I need to do a whole episode on, please feel free to ask me about what's coming down the line with the Rubin Observatory, which is a monster of a telescope. It's currently being finished in the Atacama Desert of Chile. It's got the mirror the size of a small apartment. Its camera is three gigapixels. Good luck getting that on your phone. Its mission is that every few nights it will photograph the entire visible sky. Let me say that again. It will photograph the entire visible sky every few nights. It's going to make a movie of the sky. And that's going to change this whole ballgame. Right now, our interstellar visitor count is three. We're really proud of those three. We'll work really, really hard to get them.

We have posters of them on our walls in departments, okay? But once Ruben comes online, we expect that number to skyrocket. Some estimates suggest that we'll be finding a new interstellar object every single month. That's how often... These objects are actually passing through the solar system. Just we don't have the eyes to see it. One a month. By the end of the 10-year mission of Ruben, we'll have hundreds of objects, interstellar objects in our catalog. We'll go from like, oh, look at that freak accident to here is the statistical distribution of galactic debris. This is huge because right now, everything I said, this like, okay, here's why Atlas is weird. One, because it's a comet and all comets are weird. And then here's why it's extra weird. All right, this is plausible guesswork based on a very, very limited amount of data. Once you have hundreds of comets, interstellar objects, then we can really say, okay, are generally interstellar comets weird. And we can start to mine these for more information.

Once we have better numbers, once we have better handles, we can start to pin down how many objects are actually in the galaxy. If these estimates hold up, if we start capturing one a month or seeing one a month, then that will give us a better estimate where something like a trillion, that's right, a trillion objects Interstellar objects are wandering the spaces between the stars at any given day. Once we start getting them at once a month, we might be able to intercept one. Right now, we're just waiting to see an interstellar comet come by the solar system. And then if we wanted to get a sample of it, we would have to design the mission, launch the mission, plan it, put it on the rocket pad, wait for the weather to clear it. Oh, okay, it's already gone. Actually, he didn't even get out of the fourth meeting to decide on the mission patch design before the comet is gone. But if we start getting more... One a month. We can start planning for it. And there's a mission in the works. It's an idea, but it's in the works.

It's called the Comet Interceptor. It does exactly what it says. It's very impatient engineering. Instead of waiting for a comet to show up and then trying to build a rocket fast enough to catch it, which is impossible, we're going to park a spacecraft in space and just let it wait. And then, boom, a month later, two months later, three months later, the Rubin Observatory spots one. If it's a good candidate, off it goes. That's the idea, at least. It's a pretty far-fetched idea, but, you know, I like far-fetched ideas. With a lot more numbers, we'll be able to learn a lot more about what our own solar system is made of, how it formed, what makes our solar system different than other solar systems. Right now, we only have remote observations. We have pictures of our telescopes of what other solar systems are like. If we can start capturing these visitors more... getting more detailed images, maybe even getting a piece of them. We'll have direct experience with these strangers from another world.

And maybe with enough time, those strangers won't be so strange after all. Thank you to John M. and Tim E. for the questions that led to today's episode. Keep those questions coming. That's askaspaceman at gmail.com or the website just askaspaceman.com. It is your questions that keep this show running. I can't thank you enough. Speaking of gratitude, why don't you head on over to Patreon? That's patreon.com slash pmsutter. It is how you can keep this show going by supporting it directly. I truly am grateful for every single contribution that keeps this show running. I'd like to thank my top Patreon contributors this month. This is not nearly the entire list, just the top ones. We've got... Keep those questions coming, drop a review on your favorite podcasting platform, or just keep enjoying the show. I'll see you next time for more Complete Knowledge of Time and Space. Thank you.

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