The Truth About 3I ATLAS and Comet SWAN

A comet the size of a city. Another streaking in from the dark, hundreds of times brighter. And YouTube headlines warning that one is “100× bigger” and “targeting” the other. If you have seen these claims, you are not alone. Social media has lit up with speculation about two very different celestial visitors: 3I/ATLAS, the third confirmed interstellar object, and C/2025 R2 (SWAN), a newly discovered bright comet.

The drama writes itself. Two travelers from the outer dark, arriving in quick succession, separated by just a couple of months. To the public imagination, that looks like a cosmic showdown. To astronomers, it is an incredible stroke of luck. It is a rare chance to study both an interstellar wanderer and a flashy, homegrown comet almost at the same time. The real story, as usual, is stranger and more interesting than the headlines. Yes, one comet is dramatically brighter. No, they are not on a collision course. And no, there is no evidence of alien probes signaling across the Solar System. What we are watching is a natural and deeply exciting chapter in planetary science. These two objects offer a double opportunity. They allow us to test what we know about comets, compare local and interstellar material, and refine the way we detect and interpret newcomers to our Solar neighborhood.

Over the next few sections, we will unpack what has actually been observed. We will explain where the “100× bigger” claim comes from. Finally, we will show what this double feature tells us about comets, interstellar visitors, and the scientific process itself.

Discovery Timelines

The story of these two objects begins with patient sky surveys and a bit of luck.

3I/ATLAS (C/2025 N1) was first spotted on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System, better known as ATLAS. This is a network of telescopes in Hawaii and South Africa that scans the skies for near-Earth objects. The initial data showed an unusual orbit. Follow-up observations confirmed that the object was not bound to the Sun. Its trajectory is hyperbolic, which means it is just passing through our Solar System before continuing back into interstellar space. That made it only the third interstellar visitor ever detected, after ʻOumuamua in 2017 and Borisov in 2019. Astronomers quickly turned the world’s best instruments toward 3I/ATLAS. The Hubble Space Telescope, the Gemini Observatory, and other major facilities gathered data to measure its brightness, shape, and composition. Early reports showed a faint but active comet. There was a developing coma and tail, and signs of gas release. By August 2025, the excitement was already building.

Then, in September 2025, a second comet appeared on the scene. C/2025 R2 (SWAN) was discovered on September 11 in images from the Solar Wind ANisotropies instrument aboard the SOHO spacecraft. Amateur astronomers and professional skywatchers noticed it almost immediately because it was brightening rapidly. Within days it was visible with binoculars and small telescopes. This timing set the stage for confusion. In the span of just two months, the public heard about one comet coming from interstellar space and another that suddenly appeared and outshone the first. Some people linked the two events, even though the comets have very different orbits and origins. The discovery timelines highlight how modern astronomy works. Automated surveys pick up faint moving objects. Observatories around the world confirm positions and refine orbital paths. Once an object is flagged as unusual, larger telescopes and spacecraft instruments join in. The process is quick, collaborative, and transparent.

In this case the process also collided with viral internet culture. Scientific reports about orbital fits and photometric measurements transformed into headlines about “giant comets arriving” and “cosmic collisions.” The truth is more subtle but also more rewarding. By sheer chance, astronomers now have two new visitors to study almost back to back. One is a natural comet from another star system. The other is a bright long-period comet from our own distant Oort Cloud. Both arrived at the right moment to grab our collective attention.

Orbital Mechanics and What Interstellar Means

Every comet follows a path shaped by gravity. Most comets travel in long elliptical orbits that carry them far from the Sun and then bring them back after thousands or even millions of years. These are the long-period comets that originate in the Oort Cloud, a vast shell of icy bodies surrounding the Solar System.

Some comets have shorter orbits, looping back in only a few decades or centuries. These usually come from the Kuiper Belt beyond Neptune. In either case the defining feature is that the orbit is closed. The comet is bound to the Sun and will return. An interstellar object is different. Its orbit is not closed but open. Instead of an ellipse, the path is a hyperbola. That means the object approaches the Sun once, swings around, and then continues on to infinity. Gravity cannot pull it back. This is what makes 3I/ATLAS special. When astronomers calculated its orbital parameters, they found that its eccentricity was greater than one. In simple terms, eccentricity is a number that describes the shape of an orbit. A perfect circle has eccentricity zero. An ellipse has a value between zero and one. A parabola is exactly one. A hyperbola is greater than one. For 3I/ATLAS the eccentricity is clearly hyperbolic. That is proof it came from outside the Solar System.

C/2025 R2 (SWAN) tells a different story. Its orbit is very elongated but still closed. The best fit places its origin in the Oort Cloud, tens of thousands of times farther away than Pluto. It is making one of its rare plunges into the inner Solar System after a journey that took millions of years. Unlike 3I/ATLAS, it is still a member of our Solar family. Orbital mechanics provide the key to separating fact from speculation. The two comets are not traveling together. They are not aligned on the same path. One is an interstellar visitor on a one-time flyby. The other is a local comet returning after a long absence. Their arrival in the same season is a coincidence of timing, not coordination.

Understanding orbits is also the first step in understanding the bigger picture. The hyperbolic path of 3I/ATLAS makes it a messenger from another star system. The elliptical path of SWAN connects it to the ancient reservoir of icy bodies around our own Sun. Together they demonstrate how celestial mechanics can reveal deep histories written in the sky.

Brightness vs Nucleus Size

When the public hears that one comet is “100 times bigger” than another, the natural assumption is that scientists have measured the solid core and found it vastly larger. That is not the case. The difference lies mainly in how bright each comet appears, and brightness can be a misleading guide to size.

A comet is not just a solid ball of ice and rock. As it approaches the Sun, heat causes volatile ices to sublimate. Gas and dust escape, forming a glowing coma around the nucleus and eventually a long tail. What telescopes measure most directly is the total light reflected or emitted by this cloud of material, not the bare nucleus itself. The nucleus of a comet is usually only a few kilometers across. For 3I/ATLAS, the best constraints from Hubble suggest a maximum diameter of about 5.6 kilometers. It might be smaller, even as little as a few hundred meters, but it cannot be larger than that upper limit. For C/2025 R2 (SWAN), there is not yet a precise nucleus measurement. What we see is a rapidly expanding coma that makes the comet appear bright even in small telescopes. Brightness is expressed in magnitudes, a logarithmic scale where each step of 5 magnitudes represents a factor of 100 in brightness. At discovery, SWAN was several magnitudes brighter than ATLAS. That difference translates to a brightness ratio of hundreds of times. This is where the phrase “100 times bigger” came from. Yet brightness is not a direct measure of physical size. A dusty comet with active jets can look vastly brighter than a more compact comet with a similar or even larger nucleus.

To illustrate, imagine two campfires in the night. One burns clean and small, producing little smoke. The other throws off sparks and a huge cloud of glowing ash. From far away, the smoky fire looks much bigger and brighter. The size of the woodpile, however, might be the same. Comets behave in a similar way. Their appearance depends not just on the size of the core but also on how much gas and dust they release. Astronomers use careful modeling to separate coma brightness from nucleus reflection. They combine photometry with high-resolution imaging and sometimes radar. Only with this combination can they estimate the actual size of the nucleus. For now, we can say with confidence that 3I/ATLAS has a nucleus smaller than 6 kilometers. For SWAN, the nucleus size remains uncertain. It is almost certainly not a hundred times larger than ATLAS. Its impressive glow is mainly a result of active outgassing and a favorable orbit that brings it closer to Earth.

The lesson is clear. Brighter does not always mean bigger. The coma and tail can exaggerate the scale of a comet in the sky. Headlines that treat brightness ratios as size differences risk spreading confusion. In reality, both comets are fascinating, but neither one dwarfs the other by a factor of a hundred in physical bulk.

What Observatories Have Actually Seen

Once the discoveries of 3I/ATLAS and C/2025 R2 (SWAN) were confirmed, the world’s best instruments quickly turned toward them. These are not casual skywatching events. They are opportunities to gather data that might not come again in our lifetimes.

3I/ATLAS drew immediate attention because of its interstellar status. The Hubble Space Telescope took images that placed a hard upper limit on the size of its nucleus at about 5.6 kilometers. Observations from the Gemini Observatory and the NOIRLab facilities showed a growing tail and a visible coma. The object is actively shedding gas and dust, which marks it as a comet rather than a dry asteroid. Spectroscopy revealed something even more interesting. Instruments detected the signature of cyanogen, a molecule of carbon and nitrogen. This is a common component of cometary comae, but it was striking to see it in an interstellar comet. It suggests that comets in other star systems carry some of the same volatile materials as those in our own Solar System. Later measurements indicated possible carbon monoxide and carbon dioxide as well. These gases tell scientists about the thermal history of the comet and about how ices survive in interstellar space. Images also showed a tail extending more than half a million kilometers. That tail developed in a matter of weeks as the comet approached the Sun. The rapid growth confirmed strong activity despite the nucleus being relatively small. This is a valuable comparison point with 2I/Borisov, which had a larger and more sustained coma, and with 1I/ʻOumuamua, which showed no visible tail at all.

C/2025 R2 (SWAN) has provided a very different spectacle. Discovered in September 2025, it quickly brightened to binocular visibility. Amateur astronomers reported a greenish glow, typical of comets rich in diatomic carbon. Large telescopes confirmed this and recorded a dramatic, fan-like tail that continues to expand. Some reports describe the tail stretching several degrees across the sky, which means it covers a region larger than ten full Moons as seen from Earth. Unlike 3I/ATLAS, SWAN is close enough and bright enough that many smaller observatories can contribute data. Backyard astronomers have photographed its evolution night after night, providing a continuous record of brightness changes. Professional facilities add precision photometry and spectroscopy. Together, this creates a real-time scientific campaign that spans both professionals and citizen scientists.

The contrast between the two comets is striking. ATLAS is faint but exotic, an interstellar wanderer with a small nucleus and a scientific value far beyond its brightness. SWAN is bright and dramatic, a classical comet from our own Oort Cloud with an enormous tail that delights skywatchers. One carries the thrill of mystery. The other carries the beauty of spectacle. Both have already taught us important lessons. From ATLAS we learn that interstellar comets can carry familiar molecules, linking the chemistry of distant systems with our own. From SWAN we are reminded how volatile-rich comets behave when they plunge inward, shedding vast amounts of gas and dust. Each adds to the catalog of comet behavior, and each sharpens our understanding of how small icy bodies evolve.

What observatories have actually seen is therefore very different from what viral headlines suggest. No evidence points to collisions, artificial signals, or alien intent. The data show natural cometary activity, with details that match established physics and chemistry. The excitement lies in the chance to compare two rare visitors side by side. One from our neighborhood’s icy frontier, and one from the wider galaxy.

The Alien Probe Conversation

Whenever an unusual object enters the Solar System, speculation follows. The discovery of 1I/ʻOumuamua in 2017 sparked widespread discussion about whether it might be an artificial probe. That conversation was renewed with 3I/ATLAS. Some commentators and even a few scientists suggested the possibility that it could be a technological artifact from another civilization.

One of the most vocal figures in this debate has been Avi Loeb of Harvard University. He argued that it is scientifically valid to consider non-natural explanations when an object shows unusual properties. He pointed out that ʻOumuamua had an odd shape and acceleration that some models could not fully explain. By analogy, he raised the question of whether 3I/ATLAS might deserve similar consideration. Mainstream astronomers take a different view. The observations of 3I/ATLAS so far are consistent with a natural comet. It has a visible coma, a long tail, and gas emissions that match well-known cometary chemistry. These are exactly the features one would expect from a small icy body heated by the Sun. NASA and other research groups have been clear in their statements. There is no evidence of artificial behavior.

The alien probe idea persists because it captures imagination. It provides a narrative hook that spreads quickly in the media. However, science requires more than imagination. An extraordinary claim demands extraordinary evidence. For 3I/ATLAS the evidence points toward a conventional comet. The uncertainties that remain involve its precise size and composition, not its origin as a machine.This does not mean that scientists dismiss the possibility of detecting technology from other civilizations. It means that the burden of proof is high. If a future object shows unusual signals, structured emissions, or non-gravitational maneuvers that cannot be explained by outgassing, then the conversation will shift. Until then, the best scientific explanation for 3I/ATLAS is that it is a natural comet.

The alien probe conversation is valuable in one sense. It reminds us that astronomy is not only about gathering data but also about asking bold questions. It challenges scientists to refine their models and to look carefully for anomalies. At the same time, it highlights the importance of distinguishing between speculation and evidence. In the case of 3I/ATLAS the speculation makes headlines, but the evidence points firmly to natural origins.

Broader Implications for Planetary Science and Public Communication

The simultaneous arrival of 3I/ATLAS and SWAN gives scientists a rare opportunity. Interstellar objects are vanishingly rare, and bright Oort Cloud comets are unpredictable. Having both within months of each other allows researchers to compare different kinds of messengers from space.

From a planetary science perspective, 3I/ATLAS is especially valuable. It carries material from another star system. Even if the nucleus is only a few kilometers across, the ices and dust it releases preserve conditions from the early stages of a foreign planetary system. Measuring its gases and comparing them with those of local comets tells us whether chemistry is universal or highly variable. Early results showing cyanogen and carbon-bearing molecules suggest common ground. That strengthens the idea that the building blocks of life may not be unique to our own Solar System. SWAN, in contrast, offers a powerful reminder of what our own Oort Cloud contains. Its dramatic tail shows what happens when a long-period comet wakes up after millions of years in deep freeze. This helps refine models of cometary evolution, activity cycles, and the delivery of water and organics to early Earth. The presence of such comets also keeps attention on planetary defense, since long-period comets can pass very close to Earth.

There is also a lesson in communication. Scientific announcements about brightness and orbit are careful and precise. Internet headlines often are not. The claim that one comet is “100 times bigger” than another is an example of how nuance can be lost when technical details meet the pressure for attention. Astronomers must work harder to explain the difference between brightness and size, between interstellar and local origins, and between evidence and speculation.

The public fascination is not a problem. It is an opportunity. Viral interest in comets shows that people want to be part of the cosmic story. Scientists and communicators can use that curiosity to explain how discoveries are made, how measurements are interpreted, and why skepticism is healthy. Done well, the conversation about these comets can inspire more trust in science, not less.

Conclusion

Two comets entered the public eye in 2025. One came from another star system. The other rose suddenly in brightness from our own Oort Cloud. Their timing invited comparisons and fueled viral claims. In reality, they are separate stories that together show the richness of planetary science.

3I/ATLAS is only the third confirmed interstellar object ever detected. Its hyperbolic orbit guarantees that it will never return. The gases it releases already hint at chemistry shared across star systems. Every observation adds to our understanding of how planets and comets form beyond the Sun.

C/2025 R2 (SWAN) is dramatic in a different way. It is bright, green, and visible to backyard observers. Its long tail and rapid changes give scientists a laboratory for studying cometary physics in real time. It comes from the deep reservoir of our own Solar System, a reminder of the icy material that may once have helped seed Earth with water and organic molecules.

The viral claim that one comet is “100 times bigger” than the other misses the point. Size is uncertain, brightness is misleading, and alien probe stories are speculation. What is certain is that two remarkable objects are here at once, and they are giving us a chance to learn.

For those who want to follow updates, here are the key sources:

• NASA and ESA mission pages for comet observations
• JPL Horizons and TheSkyLive for orbital data
• Hubble and Gemini press releases for scientific imaging
• Astronomy outlets such as EarthSky, Space.com, and Sky & Telescope

The real headline is simple. Two very different travelers have arrived. Each has a story to tell about origins, chemistry, and cosmic history.

Resources

Official Data and Ephemerides

• JPL Horizons Ephemeris System — Generate real-time orbital data for 3I/ATLAS and C/2025 R2 (SWAN).
• TheSkyLive — 3I/ATLAS Tracker
• TheSkyLive — C/2025 R2 (SWAN) Tracker

Space Agencies and Observatory Updates

• NASA Solar System Exploration — 3I/ATLAS Overview
• Hubble Space Telescope News Releases — Includes recent imaging and analysis of 3I/ATLAS.
• NOIRLab Newsroom — Gemini Observatory updates on comet studies.
• ESA SOHO Mission — SWAN Instrument

Scientific Context and Analysis

• EarthSky — Comet News and Skywatching Guides
• Space.com — Astronomy and Space Science News
• Sky & Telescope — Observing Resources
• arXiv.org — Astrophysics Preprints (search for “3I/ATLAS” or “C/2025 R2 SWAN” for current research papers).