Ask A Spaceman - Ep. 269: Why Are Interstellar Objects So Weird?
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The universe is sending us cosmic visitors from other star systems, and they're not what we expected. In 2017, Oumuamua arrived—cigar-shaped, tumbling, and unlike anything in our solar system. Then in 2019, comet Borisov showed up: normal, icy, and unremarkable. Now, in 2025, 3i Atlas joins the lineup—small, metal-rich, low in water, slow to form a coma, and accelerating away from the sun as if pushed by its own outgassing. At first glance, these interstellar objects seem wildly inconsistent. But the real insight isn’t that they’re weird—it’s that they’re not supposed to be normal. Comets are already a wildly diverse group, like cats with different personalities. Interstellar comets, however, are a distinct breed: born in alien solar systems with different chemistry, kicked out by gravitational chaos, and spending billions of years in interstellar darkness before briefly blazing through our sun’s neighborhood. Their extreme speed and short visit mean they don’t have time to behave like typical comets. The real reason they seem so strange? We’re only seeing them at their most dramatic moment—when they’re near the sun. And with the upcoming Vera C. Rubin Observatory, we’re about to see hundreds of these visitors a year. Suddenly, three oddballs aren’t anomalies—they’re the beginning of a statistical revolution. One day, we might even intercept one. The universe isn’t sending us freaks.
Interstellar comets like 3i Atlas appear weird not because they’re alien, but because they’re comets from another solar system with different chemistry, formation, and history.
We only observe interstellar objects when they’re near the sun—meaning their brief, high-speed flybys prevent them from behaving like normal comets.
The Vera C. Rubin Observatory will likely detect one interstellar object per month, transforming our understanding from isolated oddities to a statistical population.
Comets are inherently diverse—like cats—so expecting them all to act the same is a mistake; interstellar comets are just a more extreme version of that diversity.
3i Atlas’s delayed coma, unusual metal content, and non-gravitational acceleration are explainable by its short time near the sun and interstellar origin.
…and 3 more takeaways available in PodZeus
The Stranger in Town: A Metaphor for Interstellar Visitors
The episode opens with a vivid metaphor: a small town where a stranger from another country drives through at high speed, leaving only a fleeting impression. This sets the stage for the arrival of interstellar objects—Oumuamua, Borisov, and 3i Atlas—as cosmic strangers that appear briefly and leave us wondering about their origins.
Oumuamua: The First Weirdo
Oumuamua, the first confirmed interstellar object, was detected in 2017. It was elongated, tumbling, and unlike any known solar system object. Its shape was inferred from light flickers, but no image exists. It sparked questions: was it special, or are all interstellar objects this strange?
Borisov: The Normal One
In 2019, comet Borisov arrived—fast-moving but otherwise unremarkable. It looked and behaved like a typical comet: icy, dusty, with a coma and tail. This contrasted sharply with Oumuamua, raising the question: are interstellar objects a mixed bag?
3i Atlas: The Third Visitor, and the Most Weird
“It's not just a comet. It's a comet that was 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.”
Why Are They All So Weird?
The real answer: comets are already a wildly diverse group. Like cats, each is unique. Interstellar comets are even more different because they formed in alien solar systems with different chemistry, formation pathways, and histories. They’re not just strangers—they’re a different breed.
“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.”
“Once Rubin comes online, we expect that number to skyrocket. Some estimates suggest that we'll be finding a new interstellar object every single month.”
“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.”
Host
Pamela Sutter
person
3i Atlas
other
Oumuamua
other
Comet Borisov
other
Vera C. Rubin Observatory
other
Comet Interceptor
other
Asteroid Terrestrial Impact Last Alert System
other
John M.
person
Tim E.
person
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