Initial estimates placed the comet’s diameter at about 11 kilometers (7 miles). However, subsequent data from the Hubble Space Telescope indicated that 3I/ATLAS is likely closer to the maximum size of 5.6 kilometers (3.5 miles). In any case, it is likely the largest interstellar object ever observed.
From October 1 to 7, 2025, the European Space Agency’s ExoMars Trace Gas Orbiter (TGO) and Mars Express observed the interstellar comet 3I/ATLAS as it passed close to Mars. The two Mars orbiters had the closest view of the comet of any ESA spacecraft. At its closest approach to the Red Planet on October 3, the interstellar visitor was 30 million kilometers away.
Each spacecraft used its own camera to observe the comet’s flyby. Both cameras are designed to photograph the bright surface of Mars at depths of just a few hundred or thousands of kilometers. Scientists were unsure what to expect from observing a relatively dim target so far away, according to the European Space Agency press service.
Comet 3I/ATLAS is the slightly fuzzy white dot moving downward near the center of the image. This dot is the comet’s center, encompassing its icy, rocky core and the surrounding shell.
CaSSIS was unable to distinguish the core from the coma because 3I/ATLAS was too far away. Obtaining an image of this kilometer-wide core would have been as impossible as seeing a mobile phone on the Moon from Earth.
ESA’s Mars and Jupiter missions are monitoring comet 3I/ATLAS.
But the coma, several thousand kilometers in diameter, is clearly visible. It forms as 3I/ATLAS approaches the Sun. Heat and solar radiation awaken the comet, causing it to release gas and dust, which accumulate in a halo around the nucleus.
CaSSIS was unable to measure the full size of the coma because the dust brightness rapidly decreases with distance from the nucleus. This means the coma is lost in the background noise in the image.
Typically, the coma’s material stretches into a long tail, which can reach millions of kilometers as the comet approaches the Sun. The tail is much dimmer than the coma. We don’t see the tail in the CaSSIS images, but it may become more visible in future observations as the comet continues to warm and release more ice.
Nick Thomas, principal investigator of the CaSSIS camera, explains: “This was a very challenging observation for the instrument. The comet is approximately 10,000 to 100,000 times fainter than our usual target.”
3I/ATLAS hasn’t shown up in Mars Express images yet, partly because they were taken with exposures of just 0.5 seconds (the maximum for Mars Express) compared to five seconds for ExoMars TGO.
Scientists will continue to analyze data from both orbiters, including combining multiple images from Mars Express, to try to detect the faint comet.
They also attempted to measure the light spectrum of comet 3I/ATLAS using the OMEGA and SPICAM spectrometers on Mars Express, as well as the NOMAD spectrometer on ExoMars TGO. It is currently unknown whether the coma and tail were bright enough to characterize the spectrum.
Scientists will continue to analyze the data over the coming weeks and months to try to learn more about what 3I/ATLAS is made of and how it behaves as it approaches the Sun.
ExoMars TGO image of comet 3I/ATLAS. ESA
Colin Wilson, ESA’s Mars Express and ExoMars Project Scientist, says: “While our Mars orbiters continue to make impressive contributions to Mars science, it’s always particularly exciting to see how they respond to unexpected situations like this. I look forward to seeing what the data reveals after further analysis.”
Comet 3I/ATLAS, which originated outside the solar system, is only the third interstellar comet ever observed, following 1I/Oumuamua in 2017 and 2I/Borisov in 2019.
These comets are completely alien creatures. All planets, moons, asteroids, comets, and life forms in our Solar System share a common origin. But interstellar comets are true outsiders, carrying clues about the formation of worlds far beyond our own.
Comet 3I/ATLAS was first discovered on July 1, 2025, by the ATLAS (Asteroid Earth Impact Alert System) telescope in Rio Hurtado, Chile. Since then, astronomers have been using ground-based and space-based telescopes to track its progress and learn more about it.
Based on the comet’s trajectory, astronomers speculate that 3I/ATLAS may be the oldest comet ever observed. It could be three billion years older than the solar system, which is already 4.6 billion years old.
Next month, ESA will observe the comet using the Jupiter Icy Moons Explorer (Juice). Although Juice will be farther from 3I/ATLAS than our Mars orbiters did last week, it will see the comet just after its closest approach to the Sun, meaning it will be in a more active phase. Juice’s observations are expected to be available no earlier than February 2026.
Images of comet 3I/ATLAS obtained by ESA’s ExoMars TGO
Icy wanderers like 3I/ATLAS provide a rare, tangible link to the wider galaxy. Visiting such an object would allow humanity to establish a much broader connection with the universe. To this end, the ESA is developing the Comet Interceptor mission.
Comet Interceptor is scheduled to launch into a parking orbit in 2029, where it will await a suitable target—an undisturbed comet from the distant Oort cloud surrounding our solar system, or, in an unlikely but highly attractive scenario, an interstellar object like 3I/ATLAS.
Michael Koeppers, a scientist on the Comet Interceptor project, explains: “When the Comet Interceptor project was selected in 2019, we knew of only one interstellar object—1I/ʻOumuamua, discovered in 2017. Since then, two more similar objects have been discovered, displaying a wide variety of appearances. Visiting one of them could be a breakthrough in understanding their nature.”
A controversial paper questions whether the interstellar object 3I/ATLAS is a potentially dangerous alien probe. However, experts have dismissed this as “nonsense.”
The newly discovered interstellar object 3I/ATLAS may be a disguised example of “possibly hostile” extraterrestrial technology, according to a controversial study by a small team of scientists that includes a renowned alien-hunting astronomer.
Studying 3I/ATLAS could help unravel the mysteries of other Milky Way star systems. M. Hopkins/Ōtautahi-Oxford team. Basemap: ESA/Gaia/DPAC, Stefan Payne-Wardenaar, CC-BY-SA 4.0
Their paper, which has not been peer-reviewed, echoes similar claims made previously regarding ‘Oumuamua, the first-ever space interloper discovered in 2017.
However, experts told Live Science that the new claims are “nonsense” and “offensive,” and insist that the available evidence indicates the object is entirely natural.
Most experts are unconvinced and unimpressed by the sensational new claims about 3I/ATLAS. David Rankine/Catalina Sky Survey
3I/ATLAS was discovered on July 1, 2025, hurtling toward the Sun at over 210,000 km/h (130,000 mph), and was confirmed to be an interstellar object less than 24 hours later. Initial observations strongly suggest it is a large comet surrounded by a cloud of ice, gas, and dust called a coma, extending up to 24 kilometers (15 miles) across. A computer model simulating its origin suggests it may be 3 billion years older than our solar system, potentially making it the oldest comet ever observed.
However, in a new paper uploaded to the arXiv preprint server on July 16, 2025, three researchers question whether the comet is actually some form of hidden alien technology sent here by an advanced and potentially aggressive extraterrestrial civilization.
3I/ATLAS, formerly known as A11pl3Z, will fly through the solar system, making close encounters with Mars, Venus, and Jupiter. David Rankine/Catalina Sky Survey
The researchers described the new work as a “pedagogical exercise” or thought experiment and provide no clear evidence of alien intervention. Instead, they point to the comet’s “anomalous characteristics” and propose alternative theories to explain them.
The study’s most prominent author is Avi Loeb, a Harvard University astronomer known for his ideas about connections between extraterrestrial objects and intelligent extraterrestrials. He leads the Galileo project, which aims to detect evidence of technosignatures and UFOs. In 2023, he led a controversial expedition that he claimed to have recovered fragments of possible alien technology left behind by an unconfirmed interstellar meteorite in the Pacific Ocean. (These claims have since been largely debunked by independent researchers.)
Loeb was also the lead researcher who argued that the unusual shape and non-gravitational acceleration of the interstellar object ‘Oumuamua were signs that it was an alien probe.
3I/ATLAS is currently moving toward the Sun at more than 210,000 km/h. It will reach perihelion on October 30. Olivier Hainaut et al./European Southern Observatory
Today, the general consensus is that ‘Oumuamua was an asteroid spewing gas into space, like a comet. However, Loeb and his colleagues continue to argue for its potential extraterrestrial origin and propose missions to track this wandering space object.
Loeb’s co-authors of the new study are both associated with the Initiative for Interstellar Exploration (i4is), a UK institute planning future missions to distant star systems.
In a blog post explaining the new work, Loeb wrote that if 3I/ATLAS is a “technological artifact,” it could be evidence for the dark forest hypothesis, which suggests that the reason we haven’t found evidence of extraterrestrial life is because they remain silent to remain invisible to potential predators or prey.
“The consequences, if the hypothesis proves correct, could be catastrophic for humanity and may require protective measures,” he wrote.
Most of the new paper’s findings focus on 3I/ATLAS’s unusual trajectory. The object is moving significantly faster than the only other known interstellar objects—’Oumuamua and Comet Borisov, spotted in 2019—and entered the Solar System at a different angle than its predecessors, approaching the Sun from the side of our star’s orbit through the Milky Way.
Many astronomers have observed a fuzzy coma around 3I/ATLAS, strongly suggesting it is a comet. International Gemini Observatory/NOIRLab/NSF/AURA/K. Mech (IfA/U. Hawaii). Image processing: Jen Miller and Mahdi Zamani (NSF NOIRLab)
Loeb wrote that the object’s trajectory “offers various advantages to extraterrestrial intelligence,” which could use it for covert surveillance of Earth. One such advantage is that 3I/ATLAS will approach relatively close to three planets: Jupiter, Mars, and Venus. The minimal distances between the object and these worlds, Loeb wrote, would allow aliens to undetectedly deploy their “gadgets” there.
3I/ATLAS will also be hidden on the side of the Sun opposite Earth when it approaches our star in late October. “This may be done intentionally to avoid detailed observations with ground-based telescopes during the object’s period of maximum brightness or when instruments from this hidden observation point are sent back to Earth,” Loeb wrote.
Researchers say 3I/ATLAS’s unusually high speed also makes it significantly more difficult for spacecraft to launch to intercept it before it leaves the solar system, which could be another potential sign of secret alien activity.
Based on the current velocity, Loeb also noted that 3I/ATLAS would have entered the outer reaches of the solar system around 8,000 years ago, which “corresponds roughly to the time when human technology became advanced enough to begin documenting Earth’s history.” Although it’s unclear what he’s trying to say by that.
The researchers also argue that there is not enough evidence to definitively prove that 3I/ATLAS is a comet, especially given that researchers have yet to identify any “volatiles” or specific chemicals associated with comets in its coma.
Since 3I/ATLAS’s discovery, researchers have been trying to identify it, and the overwhelming consensus is that it is a comet.
Astronomers discovered 3I/ATLAS on July 1. Less than 24 hours later, its status as an interstellar object was confirmed. ATLAS/University of Hawaii/NASA
“Numerous telescopic observations of 3I/ATLAS have shown that it exhibits classic signs of cometary activity,” Darryl Seligman, an astronomer at the University of Michigan who led the first study to quantify 3I/ATLAS, told Live Science in an email.
“All the evidence points to this being a normal comet ejected from another solar system, just as countless billions of comets have been ejected from our solar system,” added Samantha Lawler, an astronomer at the University of Regina in Canada who specializes in solar system dynamics.
Any assumptions about the absence of volatiles in the object are premature. “The object is still quite far from the Sun, so we generally don’t expect direct evidence of volatiles,” Seligman said. However, he added, these compounds will likely become detectable in the coming weeks and months.
Loeb acknowledges that the alien technology scenario is unlikely: “By far the most likely outcome is that 3I/ATLAS is a completely natural interstellar object, likely a comet,” he wrote in a blog post.
Loeb is no stranger to this kind of criticism and defends his position, writing that “the hypothesis itself is an interesting exercise and is interesting to explore, regardless of its probable validity.”
Images from the James Webb Space Telescope reveal something strange is happening with the interstellar comet 3I/ATLAS. NASA’s James Webb Space Telescope has conducted its first observations of the interstellar comet 3I/ATLAS, revealing that the comet appears to be unusually rich in carbon dioxide.
Astronomers have spotted something strange in the first images of the interstellar comet 3I/ATLAS taken by the James Webb Space Telescope as it hurtles toward our Sun, according to a new study.
Initial telescope observations revealed that 3I/ATLAS has one of the highest ratios of carbon dioxide (CO₂) to water (H₂O) ever recorded in a comet. If confirmed, this unusual chemical composition could shed light on 3I/ATLAS’s mysterious origins beyond our solar system.
Since its discovery in July 2025, scientists have been using a variety of telescopes to learn everything they can about Comet 3I/ATLAS. This extremely rare comet is only the third confirmed interstellar object ever recorded, and researchers are eager to study its composition before the asteroid streaks past our Sun in October and leaves the Solar System forever.
Comets that fly past stars develop an atmosphere, or coma. This cloud of gas and dust grows larger and brighter as the comet approaches the star, while ice and other materials on the comet heat up and release gas in a process called outgassing. According to the study, images from the JWST telescope showed that 3I/ATLAS’s coma consisted primarily of carbon dioxide.
On August 6, researchers used NASA’s James Webb Space Telescope to observe 3I/ATLAS. NASA/James Webb Space Telescope
The researchers noted that the high carbon dioxide content could be related to radiation exposure or the location of the comet’s formation relative to the distance at which CO2 freezes (the CO2 ice line) around its parent protoplanetary disk—the swirling gas and dust that surrounds young stars and from which planets, comets, and asteroids are born.
“Our observations are consistent with the CO2-rich core, which may indicate that 3I/ATLAS contains ices that have been exposed to stronger radiation than Solar System comets, or that it formed near the CO2-ice line in its parent protoplanetary disk,” the researchers wrote in the study.
With each new observation, astronomers are learning more about Comet 3I/ATLAS. Their data currently indicate that the comet is traveling at over 210,000 km/h along an unusually flat and straight trajectory, unlike any other in the Solar System.
Initial estimates placed the comet’s diameter at approximately 11 kilometers (7 miles). However, subsequent data from the Hubble Space Telescope indicated that 3I/ATLAS is likely closer to the maximum size of 5.6 kilometers (3.5 miles). In any case, it is likely the largest interstellar object ever observed.
The comet’s speed, which is the fastest ever recorded for a visitor to the solar system, suggests 3I/ATLAS has been on the move for billions of years, gaining momentum through a gravitational slingshot effect as it zips past stars and nebulae, according to a NASA statement released earlier following observations with the Hubble Space Telescope.
“Nobody knows where the comet came from,” said David Jewitt, an astronomer at the University of California, Los Angeles, and the leader of the Hubble Space Telescope team. “It’s like seeing a rifle bullet for a thousandth of a second. There’s no way to project that with any precision to determine where it started.”
New images show interstellar comet 3I/ATLAS beginning to develop a distinctive tail as it approaches the Sun ahead of its close approach to Earth this fall.
A stunning new telescopic image reveals the growing tail of interstellar comet 3I/ATLAS. The glowing limb is beginning to take shape as the icy interloper draws ever closer to the Sun on its one-way journey through the Solar System.
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3I/ATLAS is a comet approximately 11 kilometers in diameter, first spotted in early July. It’s hurtling toward us from beyond the asteroid belt between Jupiter and Mars. Scientists quickly realized that this ultra-fast object didn’t originate in our cosmic neighborhood. It was likely ejected by a distant star within the Milky Way and is now passing us on its way through the galaxy.
On August 27, 2025, astronomers at the Gemini South telescope in the Chilean Andes obtained a new detailed image of comet 3I/ATLAS, clearly revealing the comet’s tail for the first time. This trail of ice and dust is being blown away by the solar wind—a stream of charged particles emanating from the Sun. The tail is only just beginning to emerge as the comet’s frozen core, or nucleus, absorbs increasing amounts of solar radiation, causing it to eject ever more particles from its icy surface. The tail will continue to grow as the comet approaches the Sun in the coming months, eventually becoming several times wider than the comet itself.
Gemini International Observatory/NOIRLab/NSF/AURA/Shadow the Scientist. Image processing: J. Miller and M. Rodriguez (Gemini International Observatory/NSF NOIRLab), Provost TA (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)
The new image also shows a fuzzy cloud of ice and dust surrounding the comet. This cloud, known as a coma, will continue to expand as the comet warms in the Sun. This will allow the comet to reflect more light, making it appear brighter in the night sky, though it won’t be visible to the naked eye.
These classic cometary features provide further evidence that 3I/ATLAS is a natural object and not an extraterrestrial probe, as some scientists have controversially suggested with little supporting evidence.
3I/ATLAS is the third and likely the largest interstellar object ever discovered. It follows observations of the mysterious object ‘Oumuamua in 2017, which was also misidentified as a potential alien spacecraft, and Comet Borisov in 2019, which also sprouted a stunning tail.
Comet 3I/ATLAS will reach perihelion, its closest point to the Sun, on October 29. However, it will be on the opposite side of our star, Earth, meaning we will lose sight of it at this time and may not see its tail at its peak. The comet will reach its closest approach to Earth in December, when it will come within 275 million kilometers of our planet—about 700 times farther than Earth is from the Moon—before beginning its long journey back out of the Solar System.
Astronomers are eager to study the object in as much detail as possible over the next year to learn more about its origins and how various star systems form and evolve. Recent observations from the James Webb Space Telescope indicate that 3I/ATLAS contains unusually high amounts of water and carbon dioxide compared to other known comets. Additional images of the comet, including a detailed image from the Hubble Space Telescope and a colorful image from the Gemini North telescope in Hawaii, have also shed light on its composition.
Each new image of the comet also serves as a constant reminder of this rare cosmic encounter.
A NASA telescope has discovered a rare interstellar comet two months before its official “discovery.” The Transiting Exoplanet Survey Satellite (TESS), a planet-hunting telescope, observed the rare interstellar comet 3I/ATLAS two months before its official “discovery,” and these early observations show that the comet is surprisingly active.
One of the advantages of having so many telescopes observing large swaths of the sky is that if astronomers discover something interesting, the data archives of other satellites will likely contain images of the object taken before its official discovery that no one bothered to examine. This is precisely what happened with our newest interstellar visitor, 3I/ATLAS, which, although discovered in early July, was visible to other telescopes as late as May 2025.
The Transiting Exoplanet Survey Satellite (TESS) isn’t designed to search for interstellar objects or anything faint. As its name suggests, it’s designed to observe stars (which are bright) and exoplanets passing in front of them, while also monitoring the decline in the light curve of their host star. But data is data, and since TESS this year accidentally observed a patch of sky where 3I/ATLAS was previously expected to be, researchers Adina Feinstein and Darryl Seligman of the University of Michigan, and John Noonan of Auburn, decided to search telescope archives for any information about it.
A Hubble Space Telescope image of 3I/ATLAS taken in late July. NASA/ESA
They succeeded, it turns out, beginning on May 7, 2025, during two separate observing periods. Because TESS takes an image every 200 seconds, and 3I/ATLAS moves much faster than the traditional stars TESS is designed to detect, the team had to use a technique known as “shift-stacking.” They predicted where the interstellar object (ISO) would be in each image, shifted the images so that the ISO was in the same place in each image, and then stacked multiple images together to obtain a clear signal from an object that would otherwise be too faint to detect in a single image.
3I/ATLAS began its observing period at approximately 6.35 AU and reached a distance of approximately 5.47 AU by the end of the second observing window, on June 2. During this time, its flux increased by a factor of 5, although a decrease in distance would have resulted in a brightness increase of only approximately 1.5 times.
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