This year, we welcome a rare visitor to our skies—the interstellar comet 3I/ATLAS! As the third interstellar object ever discovered by humanity, it’s a once-in-a-lifetime encounter. Yet some speculate even more boldly: Could it be an alien probe traversing our solar system? Regardless of which theory you believe, you can track its journey in the Sky Tonight app. Until November 2025, the comet will remain too close to the Sun to observe. In the meantime, seize the opportunity to admire the two bright comets currently gracing the night sky—C/2025 R2 (SWAN) and C/2025 A6 (Lemmon).

What is 3I/ATLAS?

3I/ATLAS is a newly discovered interstellar object—a rare visitor from beyond our solar system. It was first spotted on July 1, 2025, by the ATLAS survey telescope in Chile. The official view (supported by NASA, ESA, and most astronomers) is clear: 3I/ATLAS is a natural comet—the third confirmed interstellar object after ‘Oumuamua and Comet 2I/Borisov. Yet not everyone is convinced, with some suggesting its unusual characteristics leave room for more exotic interpretations.

Could 3I/ATLAS Be an Interstellar Spacecraft? A Harvard Professor’s Theory

Harvard professor and theoretical physicist Avi Loeb ignited this debate by suggesting we shouldn’t rule out the boldest possibility: 3I/ATLAS might not just be a natural chunk of ice and dust, but an artificial object controlled by extraterrestrials. His arguments include the following points.

3I/ATLAS is unusually massive

When first observed, 3I/ATLAS appeared excessively bright at that distance. If its light came solely from solar reflection, the object’s diameter would need to reach 20 kilometers—far larger than expected for a random interstellar rock. Statistically, the probability of encountering such a behemoth in the first place is only about 0.0001. Loeb contends this makes the discovery suspicious.

3I/ATLAS Doesn’t Resemble a Comet

Comets typically put on a show as they approach the Sun—emitting long tails of gas and dust while leaving distinct chemical “fingerprints” in their spectra. Yet when Loeb proposed his theory, 3I/ATLAS showed none of these traits. Its spectrum lacked the typical signals of gases like carbon dioxide or methane, displaying only a reddish reflection of sunlight. This reddish glow could be explained by organic dust on its surface, but to Loeb, it hinted at something unusual. Neither an erupting comet nor an ordinary asteroid… so what exactly is it?

3I/ATLAS follows an “excessively neat” trajectory

Random interstellar visitors typically enter at odd angles, flung by gravity in all directions across the solar system. Yet 3I/ATLAS glides almost perfectly along the plane of planetary orbits—and passes precisely near Jupiter, Mars, and Venus. To Loeb, this seems too coincidental. Such a trajectory resembles a deliberately planned flyby mission for close-up exploration of the solar system, rather than a random intrusion.

Moreover, grazing large planets theoretically enables “gravity assist” maneuvers—a common technique for human spacecraft to gain acceleration without extra fuel, extending their reach. So the question arises: Is 3I/ATLAS merely following the laws of physics… or deliberately exploiting them?

3I/ATLAS Seems to Be “Dodging Detection”

During its journey through the solar system, 3I/ATLAS will pass near three planets—but carefully avoids Earth. In late October, it will pass behind the Sun, vanishing from our telescopes’ view for weeks. Loeb notes that if this object is indeed under intelligent control, this would present an ideal opportunity for covert orbital adjustments “out of Earth’s line of sight.” To him, this mirrors precisely what a technologically advanced probe aware of surveillance would do.

3I/ATLAS Could Pose a Threat to Earth

As mentioned earlier, when 3I/ATLAS disappears behind the Sun, it could indeed alter its course undetected. If its new trajectory targets Earth, this would not be a friendly visit. In this scenario, it could signify a “deliberate rendezvous” with Earth—and the potential intent might not be benign. He even suggests humanity might need to consider defenses, though any technology capable of interstellar travel could render our defenses obsolete.

While Loeb acknowledges in his blog post that this “spaceship hypothesis” is more of a thought experiment than a definitive conclusion, it has undeniably generated significant headlines and debate.

Comet 3I/ATLAS: The Official Scientific Stance

Hubble Space Telescope image of interstellar comet 3I/ATLAS captured on July 21, 2025, from a distance of 365 million kilometers. The comet displays a teardrop-shaped dust coma, while background stars appear streaked due to exposure time.

Not everyone subscribes to Loeb’s “alien probe” theory. NASA, ESA, and the broader astronomical community have unequivocally stated that the latest evidence overwhelmingly confirms 3I/ATLAS is a comet.

3I/ATLAS isn’t that big

While early calculations suggested it might be a “giant” object, Hubble imaging reveals otherwise. Current estimates place its nucleus diameter between 0.6–5.6 kilometers—perfectly normal for a comet. Its earlier brightness stemmed mainly from surrounding dust, not from being a massive rock.

3I/ATLAS behaves exactly like a comet

For most astronomers, the answer is straightforward: 3I/ATLAS looks and behaves like a classic comet. It is forming a coma and tail, ejecting dust and gas, and gradually brightening as the Sun heats it. Using NASA’s SPHEREx telescope, astronomers have even detected clear carbon dioxide signatures within its coma—a classic hallmark of comets. As Tom Statler, NASA’s chief scientist for small solar system bodies, summarized: “It looks like a comet and is doing what comets do.”

3I/ATLAS poses no threat to Earth

While dramatic “alien ambush” scenarios make for compelling science fiction, astronomers emphasize that 3I/ATLAS’s actual orbit is well-understood. Precise calculations based on data from ground-based telescopes and space observatories confirm its closest approach to Earth will remain over 270 million kilometers—nearly twice the distance between Earth and the Sun. Astronomers stress that 3I/ATLAS behaves entirely like a natural comet. Its only planned “invasion” is into your telescope’s eyepiece.

So is 3I/ATLAS a secret alien spacecraft on a mission, or just a comet passing through our “neighborhood”? Most scientists’ stance is unequivocal: they firmly choose the second explanation.

Comet 3I/ATLAS: Latest Updates

September 20, 2025: 3I/ATLAS Turns Green—What Happened?

In early September 2025, astronomers observed a green glow around Comet 3I/ATLAS. Such coloring is typically caused by molecules like carbon dimers fluorescing under sunlight, but previous spectral analyses of this comet showed no clear evidence of such molecules. Other molecules may be responsible for the comet’s green hue. This glow remains under investigation as scientists probe which gases produced this unexpected feature. Such discoveries offer valuable clues about the chemical composition of comets formed in other star systems.

September 23: Forget Alien Ships! New Theory: 3I/ATLAS May Be a “Planetary Seed”

Researchers Susanne Pfalzner and Michele Bannister propose that 3I/ATLAS may be more than just a comet: it is likely “a planetary seed.” Traditional planet formation models struggle to explain how gas giants rapidly grew to enormous sizes before their gas disks dissipated. Hard interstellar objects like 3I/ATLAS could serve as ready-made planetary cores, accelerating this process by accreting surrounding material. If confirmed, this theory suggests the interstellar visitor might be potential building blocks for other planets.

September 23: Comet 3I/ATLAS to Encounter Solar Storm

A coronal mass ejection (CME) from the Sun is predicted to collide with Comet 3I/ATLAS between September 24 and 25, 2025. When ordinary comets encounter solar storms, their tails can bend, break, or even detach entirely. This time, astronomers will gain the first opportunity to observe an interstellar comet’s behavior within a solar storm. Incidentally, we are fortunate this eruption occurs precisely now: just one week later, Comet 3I/ATLAS will approach the Sun and vanish from our view. This means astronomers possess only a brief yet extraordinarily valuable observation window. All eyes are on Comet 3I/ATLAS: curiosity surrounds how this interstellar visitor will fare after encountering the solar storm.

September 25: Harvard Professor Insists—3I/ATLAS Is More Than Just a Comet

Harvard astronomer Avi Loeb, known for his bold theories on interstellar objects, has once again shared his perspective on Comet 3I/ATLAS. This time, he emphasizes that the comet shows no significant non-gravitational acceleration—meaning its orbit hasn’t been altered by gas jets, unlike other comets. Loeb explains that this indicates Comet 3I/ATLAS must be exceptionally massive, possessing a nucleus several kilometers in diameter and a total mass reaching hundreds of billions of tons.

However, Loeb also proposes a more radical hypothesis. He suggests that if subsequent observations reveal abrupt trajectory changes unexplainable by natural gas effects, the only possibility would be the operation of some kind of engine. According to him, the real mystery lies in whether 3I/ATLAS is merely an exceptionally massive comet with unique chemical composition or some far more extraordinary entity.

September 28: Could 3I/ATLAS be the source of that unexplained signal from 48 years ago?

Wow Signal

The original computer printout of the “Wow!” signal.

Big Ear Radio Observatory and North American Astrophysical Observatory (NAAPO)

In 1977, a radio telescope in Ohio captured a mysterious 72-second radio burst now known as the “Wow! Signal”—one of the strongest candidates for a potential transmission from an extraterrestrial civilization to date. Astronomers have been unable to explain it, leaving it a cosmic mystery for decades.

Harvard astronomer Avi Loeb now speculates that 3I/ATLAS may hold clues. As this comet traverses the same region where the Wow! Signal was detected, it raises the question: Could this interstellar visitor be the signal’s true origin?

September 29: Comet 3I/ATLAS Easily Weathered a Solar Storm

Big solar flare? No big deal! Recently, Comet 3I/ATLAS encountered a solar storm but appears unscathed. At least, no reports indicate significant changes to its condition. By contrast, when Comet Encke faced a similar storm in 2007, its magnetic field completely collapsed and its tail was severed.

September 30: Comet 3I/ATLAS is about to vanish from view until late November

Soon, 3I/ATLAS will fade into the Sun’s glare, temporarily disappearing from our telescopes. Due to orbital alignment, Earth and the comet will be positioned on opposite sides of the Sun—not ideal for observations. Currently, the comet is positioned just a few degrees from the Sun, making it difficult to distinguish against the bright background. By October 29, when it reaches perihelion, it will be completely obscured by the Sun’s glare. But don’t worry—this isn’t a final farewell. Comet 3I/ATLAS is expected to make a comeback in late November, faintly glowing in the predawn sky.

October 1: Comet 3I/ATLAS Approaches Mars

Comet 3I/ATLAS is heading toward the Red Planet. On October 3, it will pass Mars at a distance of 0.19 astronomical units (28 million kilometers)—an extremely close encounter in astronomical terms. NASA plans to seize this opportunity, deploying both orbital and surface missions—including the Mars Reconnaissance Orbiter and the Curiosity and Perseverance rovers—to gather unique data on this interstellar visitor. Might the alien inhabitants of the comet wave back at us? We’ll have to wait and see!

October 5: New Images of Comet 3I/ATLAS from Mars Orbit

The real deal: Comet 3I/ATLAS captured by NASA’s Perseverance rover and processed by Simeon Schmauß. This interstellar traveler appears as a faint speck against the starry backdrop. Don’t be fooled—this is the genuine photo, not those viral fake images.

Astronomy enthusiast Simeon Schmauß processed a new image of the interstellar comet 3I/ATLAS, combining 20 exposures of 30 seconds each. The result shows the comet as a faint, diffuse glow of approximately magnitude 8—just as scientists expected. Currently, 3I/ATLAS is about 30 million kilometers from Mars.

While it appears dim, this is unsurprising: it is small (at most 5.6 kilometers in diameter) and receives only about 40% of the sunlight Earth does at this distance. Remember—Perseverance is no Hubble Telescope, so expecting it to capture crisp cosmic portraits is unrealistic. Meanwhile, social media is flooded with purported “Perseverance photos” showing a bright cylindrical object—but most have been debunked as misidentified images.

Many mistook this bright streak for Comet 3I/ATLAS, but it was actually Phobos, one of Mars’ moons, streaking across the sky. Long exposure blurred it into a “cylinder,” leading to online misidentifications as 3I/ATLAS—some even called it an alien spacecraft.

October 7: ESA spacecraft captures new images of Comet 3I/ATLAS

Image of Comet 3I/ATLAS captured by ExoMars TGO from Martian orbit. Despite its camera

A fresh batch of 3I/ATLAS comet images from the European Space Agency’s ExoMars and Mars Express missions has just been transmitted back! As the comet swept past Mars, ExoMars’ Trace Gas Orbiter (TGO) successfully captured a faint, hazy halo—its coma, composed of gas and dust surrounding an icy nucleus. The comet itself is too distant and small to be clearly imaged, but detecting its faint halo represents a major breakthrough. Scientists are currently stacking and processing the data to reveal more details.

October 7: Comet 3I/ATLAS Ejecting Massive Amounts of Water

Astronomers have confirmed that interstellar comet 3I/ATLAS is continuously ejecting water vapor into space—at a rate of approximately 40 kilograms per second (about 2.5 tons per minute), comparable to a fire hose operating at full pressure. This discovery comes from NASA’s Swift Observatory, which detected faint **hydroxyl (OH)** ultraviolet signals—a chemical signature produced when sunlight breaks down water molecules.

Surprisingly, this phenomenon occurred at a location where solar heat struggles to readily convert ice into gas. Swift detected the signal when the comet was nearly three times farther from the Sun than Earth. This suggests the comet is either unusually active or its surface material contains more volatile compounds than scientists anticipated.

This marks the first confirmed detection of water on an interstellar comet, a discovery already reshaping astronomers’ understanding of these visitors from other star systems. Compared to the previously observed “dry” ‘Oumuamua and carbon-rich 2I/Borisov, 3I/ATLAS stands out as a true “water carrier”—demonstrating that the fundamental building blocks of life may be widespread throughout the Milky Way.

Whether it’s an “oddball” ordinary comet or some more mysterious signal, 3I/ATLAS continues to defy scientists’ expectations—but as always, the most reliable answers will come from new data, not speculation. For now, the comet has dimmed and temporarily vanished from view, but it will return after mid-November, offering astronomers fresh opportunities to study this interstellar traveler.

Frequently Asked Questions About Comet 3I/ATLAS

Why is Comet 3I/ATLAS so special?

First, it is a true interstellar object, meaning it did not form in our solar system but originated from another star system. Such objects are extremely rare, making each new discovery exciting. Additionally, early estimates suggest its nucleus could be up to 5 kilometers in diameter, making it one of the largest interstellar objects discovered to date. Best of all, this cosmic traveler will be visible in the night sky this year.

Is Comet 3I/ATLAS the only interstellar object discovered so far?

Not at all! In fact, 3I/ATLAS is the third confirmed interstellar object to enter our solar system. The first was the peculiar object ‘Oumuamua, discovered in 2017—a slender rock that was once speculated to be an alien spacecraft. The second was 2I/Borisov in 2019, which behaved more like a typical comet with a bright coma and long tail.

How do we know 3I/ATLAS isn’t from our solar system?

Its unusual trajectory was the first major clue—the orbit isn’t circular but hyperbolic, leading astronomers to suspect it’s not a local object. Subsequent orbital calculations confirmed this: 3I/ATLAS travels at approximately 210,000 kilometers per hour—a speed typical of interstellar objects and far exceeding any body bound by the Sun’s gravitational pull.

Will Comet 3I/ATLAS collide with Earth?

On October 29, 2025, Comet 3I/ATLAS will reach its closest approach to the Sun at about 1.4 astronomical units (approximately 210 million kilometers). Before that, it will enter the inner region of Mars’ orbit, passing by the Red Planet at a distance of just 30 million kilometers. But don’t worry—it won’t come dangerously close to Earth.

No need to worry—while Comet 3I/ATLAS will traverse the inner reaches of our planetary system, it will remain at a very distant distance from Earth. Its closest approach to our planet will be about 1.8 astronomical units—roughly 270 million kilometers, nearly twice the average distance between Earth and the Sun.

Is 3I/ATLAS really a comet? Could it be an alien spacecraft?

Most likely, 3I/ATLAS is indeed a comet. It possesses an icy nucleus that releases gas and dust, forming a coma and tail—exemplifying classic cometary behavior (you can verify this in our comet feature article). We might hope to meet extraterrestrial friends, but this time, it’s unlikely.

People are always eager to believe, so many ordinary things get mistaken for UFOs—from balloons to satellites in the night sky. To help you tell the difference, we’ve compiled some of the most common misidentifications into an infographic. Don’t be fooled so easily!