Key points to remember
3I/ATLAS is the third confirmed interstellar object, found on July 1, 2025 by the NASA-funded ATLAS survey in Chile.
Its closest approach to Earth was on December 19, 2025, at about 1.8 AU (roughly 269 million km), so it was never a threat.
The nucleus size is still uncertain: a Hubble detection gives an effective radius near 1.3 km, while other methods suggest a smaller core.
Its likely origin in the Milky Way's thick disk points to an age above 7 billion years, a model-based estimate rather than a direct measurement.
Very CO2-rich gas, jets and a measurable non-gravitational push fit a natural comet. The cryovolcanism idea remains a hypothesis.
NASA and ESA treat 3I/ATLAS as a comet. Avi Loeb's technological hypothesis is a minority view.
Summary
Interstellar comet 3I/ATLAS crossed the solar system in 2025 and 2026, only the third visitor from another star ever caught in the act. Astronomers found it in July 2025, watched it swing past the Sun on October 29, and tracked it past Earth on December 19 and Jupiter in March 2026. A fleet of telescopes and at least 25 spacecraft recorded its jets, its unusual carbon dioxide-rich gas and its fading after perihelion. This article reviews what is solid and what is still debated: how big the nucleus is, how old the comet may be, whether "ice volcanoes" are real, and why the alien-spacecraft claims did not convince the agencies that watched it. It also explains where the comet is now and why its visit changes expectations for future discoveries, since the survey that found it, and newer observatories, should now catch more such visitors.
Table of contents
1. What is interstellar comet 3I/ATLAS?
Interstellar comet 3I/ATLAS is a small icy body that formed around another star, was ejected into interstellar space, and drifted until it happened to cross our solar system. Its discovery was reported to the Minor Planet Center on July 1, 2025, by the NASA-funded ATLAS survey telescope in Rio Hurtado, Chile, according to NASA's quick facts page. The name explains itself: "3" because it is the third interstellar object found, "I" for interstellar, and ATLAS for the survey that spotted it. Before the name was assigned it carried the provisional designation C/2025 N1 (ATLAS), a detail recorded in the spectroscopic literature.
Astronomers knew almost immediately that the object did not belong to the Sun. Its orbit has an eccentricity of about 6.1 in the JPL solution, and any value above 1 describes an unbound, hyperbolic path. The comet arrived with an excess speed near 58 km/s and has a retrograde orbit that lies close to the plane of the planets. NASA's team judged its color, speed and direction consistent with what is expected from a comet, and the coma of gas and dust seen in early images confirmed it.
It follows two earlier visitors. 'Oumuamua (2017) showed no obvious coma, a puzzle we describe in our article on the first interstellar object, 'Oumuamua, while 2I/Borisov (2019) was an ordinary-looking comet. 3I/ATLAS is the first of the three to combine clear cometary activity, a large coma and months of high-quality data. If you want the simplest facts: it is a comet, it is not from here, and it is leaving for good. Anyone looking for the NASA live tracker can use NASA's "Eyes on the Solar System" simulation, which shows its current position and path.
2. Why is 3I/ATLAS significant, and how old is it?
Three interstellar objects in eight years is already a statistical statement: such visitors are not rare curiosities but a steady stream of leftovers from planetary systems. The Hubble team that detected the nucleus argued that several objects resembling 3I/ATLAS probably passed unnoticed before 'Oumuamua was found, unless interstellar objects follow a steep size distribution. That is why astronomers call 3I/ATLAS unusual and rare in the same breath. For a deeper look at how other star systems form planets and small bodies, see our guide to exoplanets and the search for alien life.
The age question is the most quoted one, and also the most misunderstood. A model built by Matthew Hopkins and colleagues at Oxford combined Gaia stellar data with galactic chemistry and concluded that the comet's velocity and steep path through the galaxy suggest an origin in the Milky Way's thick disk, a population of old stars. On that basis the Oxford group estimated 3I/ATLAS to be older than 7.5 billion years, compared with 4.6 billion for the Sun. The estimate is statistical, inferred from motion, not measured from the rock itself. Some chemistry hints at extreme old age, but those claims are still being tested.
The size is equally uncertain. A Hubble analysis of images from December 2025 to January 2026, published in The Astrophysical Journal Letters, detected the nucleus for the first time and found an effective radius of about 1.3 km if the surface is as dark as a typical comet. Independent work based on the comet's non-gravitational acceleration prefers a smaller core near one kilometer. The visual below shows how far apart the estimates remain.
3. The path of 3I/ATLAS and its closest approach to Earth
The path of 3I/ATLAS is one of the best measured of any interstellar object. It dived in on a retrograde orbit, passed inside the orbit of Mars, and reached perihelion, its closest point to the Sun, on October 29, 2025, at about 1.36 AU, roughly 203 million km. At that moment it moved near 209,000 km/h. From Earth it was hidden behind the Sun for weeks, which is why spacecraft such as SOHO, STEREO and GOES-19 provided the only continuous coverage.
The date many people asked about was December 19, 2025, when 3I/ATLAS made its closest approach to Earth. The distance was about 1.8 AU, close to 269 million km or 170 million miles, around 700 times the distance to the Moon. That is why every "will it hit Earth" headline was wrong: the comet never came near our planet and its orbit does not intersect ours in any dangerous way. If you want to reproduce such numbers, our kinematic equations guide covers the motion basics, and our article on gravity and the solar system explains why a fast object only bends slightly when it passes a planet.
The last major encounter was Jupiter. On March 16, 2026, 3I/ATLAS passed about 54 million km from the giant planet, a distance close to its gravitational sphere of influence, so the deflection was modest. The Juno spacecraft observed it during that window, ending the campaign described in a Europlanet 2026 abstract.
4. Outburst, color change and jets: what the tail revealed
Comets announce themselves through their activity, and 3I/ATLAS was no exception. As it approached the Sun, ground-based and space-based observers reported a rapid brightening, which many described as an outburst. Sublimation, the direct change of ice into gas, ramps up sharply as the ices warm, so a brightness surge near perihelion is expected. After perihelion the comet faded faster than it had brightened. The Hubble nucleus study measured an activity index of 4.5 on the outbound leg and called it unremarkable among solar system comets.
Jets were the other headline. Stacked amateur images in November 2025 showed several jets, including streams pointing toward the Sun, which produce the so-called anti-tail. Seeing a growing tail on a distant comet is normal, and tail images of 3I/ATLAS changed shape as the viewing geometry shifted. A measurable non-gravitational acceleration was also reported in the orbit solutions, the small push expected when outflowing gas acts like a weak thruster. That is a standard effect for comets, and researchers used it to estimate the nucleus mass and size.
The color change story is more subtle. Early spectra and images showed a red comet, then several reports described it as bluer near perihelion, and observers later posted images with a green glow, common in comets whose gas contains diatomic carbon. But a dense photometric study with the Unistellar network, using professional and citizen telescopes on both sides of perihelion, found that the coma stayed optically red with little evidence of systematic broadband color evolution. A separate analysis saw a possible bluing trend that was not statistically significant. In short, the color change was real in some snapshots, but it has not been shown to be a lasting, dramatic transformation.
5. Water loss, CO2 and cryovolcanism: the chemistry
The chemistry is where 3I/ATLAS looked most different from solar system comets. On August 6, 2025, the James Webb Space Telescope used its NIRSpec instrument to read the comet's gas. The result, summarized by Live Science, was a coma dominated by carbon dioxide, with one of the highest CO2-to-water ratios ever recorded in a comet. Reports of the data put the release near 130 kg of CO2 per second against only 6 to 7 kg of water. The authors suggested that the nucleus is intrinsically CO2-rich, either because its ices were exposed to more radiation than ours or because it formed near the CO2 ice line of its home disk.
Water was never absent. Spectroscopy by Yang and colleagues found water-ice grains in the coma, and water production rose as the comet warmed, so the phrase "leaking water" is fair. Later preprints reported that the CO2-to-water ratio dropped after perihelion. Spectra of the dust resembled D-type asteroids more than ultrared objects from the outer solar system, and reports from large telescopes, including the VLT, described atomic nickel vapor in the coma, which fits a metal-bearing nucleus.
The most dramatic claim is cryovolcanism. A Spanish-led team proposed that sublimating CO2 could react with nickel and iron sulfides inside the nucleus, driving eruptions of "ice volcanoes". At the time it was a non-peer-reviewed preprint, and similar images may also be explained by ordinary jets from a bare, icy surface. It is an interesting hypothesis, not a result.
6. Hubble, JWST, the VLT and the spacecraft that watched it
No interstellar object has been observed this thoroughly. According to the Europlanet campaign review, at least 25 spacecraft recorded 3I/ATLAS during its passage, and 11 interplanetary spacecraft collected infrared, visible, ultraviolet and X-ray data. Psyche observed it on September 25, 2025, and Juno finished the campaign in March 2026. NASA's PUNCH, STEREO, SOHO and GOES-19 solar observatories monitored its brightness when Earth-based telescopes could not.
The telescopes added the detail. The first Hubble image dates from July 21, 2025, when the comet was 277 million miles away and showed a teardrop-shaped cocoon of dust. Hubble returned on November 30, 2025, at about 286 million km, and again after perihelion to isolate the nucleus. Webb and NASA's SPHEREx mapped its gas and ices, ESA used XMM-Newton and Juice, Japan's XRISM joined the X-ray work, and the Very Large Telescope delivered high-resolution spectra of cyanide and nickel. Mars orbiters even captured the comet from their own vantage point, a rare perspective.
So where is it now, and what is its current position? As of October 2026, 3I/ATLAS is well beyond Jupiter's orbit and still receding. It remains an extremely faint object, visible only to large telescopes, and Hubble and JWST are expected to follow it until roughly the end of 2028. You can check the live position in NASA's Eyes on the Solar System tool or compare orbits in our planets explorer. Reaching such a visitor with a spacecraft would demand a leap in speed, a topic we cover in space propulsion physics and future tech.
7. Avi Loeb, UFO claims and what the evidence say
Almost from discovery, 3I/ATLAS became a magnet for alien speculation. Harvard astrophysicist Avi Loeb argued that a list of oddities, including its large inferred size, its orbit within a few degrees of Earth's plane, its encounters near Venus, Mars and Jupiter, and later its metal signatures, made a technological origin worth considering. He also said, in interviews, that if the comet did not break up at perihelion, the natural explanation would need a second look. Queries such as "3I/ATLAS UFO" and "3I/ATLAS radio signal" followed.
The agencies that actually pointed instruments at it disagree. NASA's small-bodies lead described it as overwhelmingly a natural body, and a senior NASA official said that all the evidence points to a comet. European Space Agency scientists reached the same view. The comet has a coma, a tail, jets, ordinary gas species and a post-perihelion fade consistent with typical comets, as the Hubble team documented. Rumors of radio signals circulated online, but none of the agency observations reviewed here reported an artificial signal.
Loeb's role is still useful to understand. One report noted that he frames part of the argument as a pedagogical exercise, a way to test how seriously the community weighs low-probability ideas. That is a legitimate exercise as long as the evidence is weighed honestly. To see how the scientific method handles anomalies like these, read our piece on the science of UAP, physics and AI. The practical lesson from 3I/ATLAS is that strange does not mean artificial: an object from another star will not look like our comets, and unfamiliar chemistry is exactly what a different planetary system should produce.
8. Common mistakes about 3I/ATLAS
Believing it could hit Earth. The scare came from sensational titles that mixed up perihelion and the Earth approach. The correct fact is a closest approach of about 1.8 AU on December 19, 2025, roughly 700 Moon distances away, as in the 269 million km figure above.
Reading every color change as a sign of design. People notice one blue image and jump to conclusions. Dense photometry shows the coma stayed mostly red, and a shift in a single frame is better explained by gas, dust and viewing angle, such as the green glow seen in December.
Treating the age as measured. Headlines say "7 billion years" as a fact. In reality it is a kinematic estimate from a galactic model, the kind of number that changes with better data. A safe phrase is "likely older than 7.5 billion years, according to the Oxford model".
Calling cryovolcanism settled. A preprint is a proposal, and peer review comes later. Ordinary jets may mimic the same images, so the careful wording is "possible cryovolcanism", not "ice volcanoes erupted".
Using stale "live" pages. Many 2025 tracker pages still say the comet is approaching. It has been leaving since late 2025, so a page that promises a future flyby is outdated and a current position tool should be used instead.
9. FAQ about 3I/ATLAS
When was the closest approach of 3I/ATLAS to Earth?
The closest approach to Earth happened on December 19, 2025, when the comet passed at roughly 1.8 AU, about 269 million km or 170 million miles. That is far beyond the orbit of Mars at its nearest to Earth and about 700 times the Earth-Moon distance. It was too faint to see with the naked eye, so observing it required telescopes. Its closest approach to the Sun, perihelion, was earlier, on October 29, 2025, at about 1.36 AU. Those two dates are often confused: perihelion is about the Sun, while the December date is about Earth. After December the comet kept moving outward, passing Jupiter on March 16, 2026, and it will not return, because its orbit is hyperbolic.
Will 3I/ATLAS hit Earth?
No. It never posed a collision risk. The orbit solutions from NASA's Jet Propulsion Laboratory showed a closest approach to Earth of about 1.8 AU, and the actual pass matched the prediction. The scare stories came from social media posts that mixed up perihelion with the Earth approach or used misleading images. Today the comet is moving away from the Sun at tens of kilometers per second, so there is no remaining chance of an encounter with our planet. Even at its closest, it was much farther from Earth than Mars ever gets, which is why NASA described the passage as safe. The survey that found it, ATLAS, is itself a planetary defense system built to spot dangerous asteroids, and the comet simply happened to be one of its most unusual catches. Because its orbit is hyperbolic and it is moving fast, it has no way of returning to threaten anyone later.
How big is 3I/ATLAS?
The honest answer is "about one to three kilometers across, depending on the method". Early estimates from brightness suggested a very large nucleus, but the bright coma hid the true core. A Hubble study from December 2025 to January 2026 detected the nucleus and gave an effective radius near 1.3 km, which means a diameter around 2.6 km if the surface is very dark. Modeling of the non-gravitational acceleration favors a nucleus close to one kilometer. The rotation period appears to be longer than about an hour, and the shape may be elongated by at least 2:1. The coma, in contrast, stretched for hundreds of thousands of kilometers.
Is 3I/ATLAS an alien spacecraft?
There is no evidence for it. NASA and ESA scientists analyzed images and spectra from more than 20 spacecraft and many telescopes, and they describe an ordinary, if unusually CO2-rich, comet. Avi Loeb has argued that a technological explanation cannot be fully excluded, but his view is a minority position and rests on interpretations of anomalies that have natural explanations. The comet shows a coma, tail, jets, water ice and gas species that match cometary behavior. Claims about radio signals were not confirmed. Scientific caution means we keep testing, but the default explanation, supported by the observations, is a natural comet from another star.
Where is 3I/ATLAS now?
As of October 2026, 3I/ATLAS is far beyond Jupiter's orbit and moving away from the Sun on a one-way trip out of the solar system. It is extremely faint, so only large professional telescopes can still record it. Hubble and JWST are expected to keep it in range until roughly the end of 2028, which gives astronomers a rare chance to watch how a comet from another star behaves at large distances after its closest solar approach. For its exact position, use NASA's Eyes on the Solar System or an ephemeris tool such as TheSkyLive, which compute the current distance from Earth and the Sun. Amateur astronomers should not expect to photograph it any more: the bright pictures and videos of late 2025 were taken when the comet was near perihelion and Earth, and it has faded considerably since then. Professional follow-up now focuses on spectra and on measuring how its activity shuts down far from the Sun.
How does 3I/ATLAS differ from 'Oumuamua and Borisov?
'Oumuamua, found in 2017, passed without a visible coma, and its odd acceleration fueled years of debate. 2I/Borisov, found in 2019, looked much like a solar system comet. 3I/ATLAS lies between them in some ways: it shows a clear coma and jets like Borisov, but it is faster, probably larger, and unusually rich in carbon dioxide. It also seems to come from an older population of stars, the Milky Way's thick disk, while the first two did not. Together, the three objects show that interstellar visitors are diverse, and that is why each new one matters. Three data points are still too few to say what a "typical" interstellar object is, but they already rule out the idea that all of them look alike. Each one carries a record of the star system and the era in which it formed, and 3I/ATLAS, as the apparent elder of the group, may be the closest we get to examining material older than our own Sun.
10. Conclusion: what 3I/ATLAS teaches us
Interstellar comet 3I/ATLAS has left the inner solar system, but its data will keep scientists busy for years. We learned that visitors from other stars can be active, CO2-rich comets with a nucleus of a few kilometers, and that a single object can be tracked by more than 25 spacecraft. We also learned how quickly unverified claims spread: the alien and collision stories did not survive contact with the data. Astronomers now expect the Vera Rubin Observatory to find many more interstellar objects, perhaps dozens, so the next one may arrive with a better-prepared audience. For the latest news and updates on 3I/ATLAS, follow published papers rather than headlines.
Sources
Comet 3I/ATLAS facts and FAQs -NASA: https://science.nasa.gov/solar-system/comets/3i-atlas/3i-atlas-facts-and-faqs/
Nucleus and postperihelion activity of interstellar object 3I/ATLAS observed by Hubble (Hui et al.) - The Astrophysical Journal Letters, vol. 999, L37: https://doi.org/10.3847/2041-8213/ae471c (preprint: https://arxiv.org/abs/2601.21569)
Third ever detection of an interstellar object - University of Oxford, Physics: https://www.physics.ox.ac.uk/news/third-ever-detection-interstellar-object