Massive Interstellar Comet 3I/ATLAS Sparks Debate – Harvard Astronomer: ‘Could Be Alien Tech’

Third Interstellar Visitor: 3I/ATLAS is a newly discovered comet from outside our Solar System – only the third known interstellar object after ʻOumuamua (2017) and 2I/Borisov (2019) ^[1]. It was first spotted on July 1, 2025, by NASA’s ATLAS survey telescope in Chile ^[2] ^[3].
Giant Size & Speed: Estimates suggest 3I/ATLAS is enormously large – on the order of 5–20 km in diameter (up to ~12 miles) and over 33 billion tons in mass ^[4] ^[5]. It’s racing toward the inner Solar System at about 60 km/s (over 210,000 km/h), making it one of the fastest comets ever observed ^[6] ^[7].
No Threat to Earth: Astronomers emphasize that 3I/ATLAS poses no danger to our planet. It will never come closer than ~240 million km (1.5 times the Earth–Sun distance) and during its nearest approach in late 2025 it will be on the far side of the Sun ^[8] ^[9].
Unusual and Active: Unlike prior interstellar visitors, 3I/ATLAS is active, venting gas and dust as it warms. It’s shedding large amounts of carbon dioxide, water vapor, and dust ^[10] ^[11], yet shows almost no deviation from a purely gravitational path – implying an “anomalously massive” nucleus that hardly budges from outgassing ^[12] ^[13].
Alien Probe Speculation: Famed Harvard astronomer Avi Loeb and colleagues have raised a controversial hypothesis that 3I/ATLAS could be artificial – an alien craft or probe – due to its sheer size, rare trajectory and minimal acceleration from outgassing ^[14] ^[15]. Loeb notes we “should not judge a book by its cover,” cautioning that a comet-like exterior doesn’t rule out an engineered object beneath ^[16]. This theory remains unproven but has ignited lively debate in the scientific community.

A Visitor From Beyond the Solar System

Astronomers around the world are buzzing over the appearance of comet 3I/ATLAS, an object that entered our Solar System from interstellar space. This cosmic interloper – formally designated 3I/ATLAS (with “3I” signifying the third interstellar object ever recorded) – was first detected on July 1, 2025 by the NASA-funded ATLAS survey telescope in Río Hurtado, Chile ^[17] ^[18]. Its discovery was quickly confirmed by observatories worldwide when its highly eccentric trajectory signaled an origin beyond our Solar System ^[19]. Every other comet, asteroid, planet, and moon we’ve observed shares a common birth in our Sun’s vicinity, but interstellar objects like 3I/ATLAS are true outsiders carrying clues from distant star systems ^[20].

The name “3I/ATLAS” reflects this status: the “3I” means it’s the third Interstellar object catalogued, and “ATLAS” credits the survey that found it ^[21]. (By contrast, the first interstellar visitor was named 1I/ʻOumuamua and the second 2I/Borisov.) Such sightings are extremely rare – until 2017, no one had ever seen an interstellar object up close. Now, within a few years, we have three on record ^[22]. Each offers a unique chance to study material formed around other stars. “These comets are absolutely foreign,” notes the European Space Agency – they carry “clues about the formation of worlds far beyond our own” ^[23].

Unprecedented Size and Record Speed

What truly sets 3I/ATLAS apart is its enormous scale. Initial observations by telescopes including Hubble and James Webb hinted it could span a few kilometers across ^[24] ^[25]. Subsequent analysis by Avi Loeb’s team, using precise tracking data and outgassing measurements, concluded the solid nucleus must be at least ~5 kilometers (3+ miles) in diameter ^[26] ^[27] – far larger than the previous interstellar visitors. For comparison, the first-known interstellar object, ʻOumuamua, was a mere ~0.4 km long (about a quarter-mile) and Borisov was roughly 1 km across ^[28]. In other words, 3I/ATLAS could be thousands of times more massive than ʻOumuamua or Borisov ^[29]. “This makes 3I/ATLAS three to five orders of magnitude more massive than the previous two interstellar objects we’ve observed,” Dr. Loeb noted in a recent report ^[30]. Its sheer bulk is “anomalously massive” by known comet standards ^[31].

The comet is also moving unusually fast. Hurtling in from the direction of the Milky Way’s center, 3I/ATLAS travels about 60 kilometers per second ^[32] – that’s ~210,000 km/h (130,000 mph). ESA reports this is “the highest ever recorded for a Solar System visitor” ^[33]. Such extreme speed likely means it has been voyaging through interstellar space for billions of years ^[34], long enough to escape any home star’s gravity and wander the galaxy. By the time it was first seen, 3I/ATLAS was roughly 420 million miles out (670 million km) ^[35], inbound toward the inner Solar System. Despite its headlong approach, Earth was never in its sights – the comet is not on a collision course.

No Threat – But a Rare Spectacle for Scientists

Experts are adamant that Earth is safe from this celestial visitor. The comet’s path will keep it no closer than about 150 million miles from Earth (240 million km) at all times ^[36]. That’s over one and a half times the distance between Earth and the Sun. In fact, during its closest pass by Earth, expected in late November 2025, 3I/ATLAS will actually be positioned behind the Sun relative to us ^[37] ^[38]. “It poses no danger to our planet or any other planets in the Solar System,” the European Space Agency confirms ^[39]. NASA likewise emphasized that the comet “will never come closer than 150 million miles away” ^[40]. So despite tabloid buzz about a “massive comet hurtling toward Earth,” there’s no apocalypse in the offing – only a fascinating fly-by on a cosmic scale.

For astronomers, 3I/ATLAS represents an extraordinary opportunity, not a threat. Its large size and long lead time (it was detected more than a year before its perihelion) give scientists a chance to observe an interstellar comet in unprecedented detail. “Beyond [its discovery] we do not know very much, and there are many efforts underway to observe this object with larger telescopes to determine composition,” said Larry Denneau, co-investigator of the ATLAS survey ^[41]. Already, a global campaign is underway: observatories on Earth are tracking it nightly, while space telescopes and planetary probes prepare to take a peek as it nears. ESA’s Planetary Defence Office fast-tracked efforts to follow 3I/ATLAS, even digging through old images (“precovery”) to pin down its orbit ^[42]. Multiple spacecraft will join the watch: between October 1–7, NASA’s Mars Reconnaissance Orbiter (HiRISE camera) and ESA’s Mars Express and ExoMars Trace Gas Orbiter will target 3I/ATLAS when it passes about 30 million km from Mars ^[43] ^[44]. In November, as the comet swings by Jupiter’s vicinity, ESA’s new JUICE probe (Jupiter Icy Moons Explorer) will train its instruments on it ^[45]. These remote observations can glean data on the comet’s composition, activity, and maybe even images (though at tens of millions of km away, it will appear only as a speck) ^[46] ^[47].

Scientists are eagerly anticipating 3I/ATLAS’s closest approach to the Sun (around late October 2025) when it will be just inside Mars’s orbit ^[48]. As it heats up, the comet could become much more active. “It currently has a faint coma, but the coma and tail may increase dramatically as the object comes closer to the Sun. Its closest approach… will be later this year, when it comes inside the orbit of Mars. We don’t know what will happen, so that’s exciting,” said ATLAS astronomer Larry Denneau ^[49]. Such excitement is palpable – this is the first time we can observe an interstellar comet developing as it nears the Sun. After perihelion, 3I/ATLAS will whip back out into deep space, not to return, so researchers are keen to gather as much data as possible during this one chance.

A Strange, Active Comet Defies Expectations

Though it hails from another star system, 3I/ATLAS so far looks and behaves much like a normal comet, only bigger. It sports a coma – a fuzzy cloud of gas and dust around its nucleus – and even a faint tail detected in late August images ^[50]. As sunlight warms the surface, frozen volatiles sublime (turn to gas) and spew out dust, just as with comets originating in our Oort Cloud. In fact, Hubble Space Telescope images showed dust streaming off the sunlit side of 3I/ATLAS, and James Webb Space Telescope spectroscopy has identified familiar cometary molecules in the haze: carbon dioxide (CO₂), water vapor (H₂O), carbon monoxide (CO), carbonyl sulfide (OCS), and more ^[51]. Recent JWST data indicate the coma’s composition is dominated by CO₂ (~87% by mass) with smaller fractions of CO (~9%) and H₂O (~4%) ^[52]. These proportions suggest 3I/ATLAS might be unusually rich in carbon dioxide ice, but otherwise it’s exhibiting the expected “melting” behavior of a comet as it nears the Sun.

What’s unexpected, given this vigorous outgassing, is how little effect it has on the comet’s motion. Typically, when comets vent jets of gas, they get a tiny push, causing their trajectory to deviate slightly from an ideal gravity-driven orbit. In 3I/ATLAS’s case, scientists find almost no measurable non-gravitational acceleration – its path is essentially a perfect open curve dictated by the Sun’s gravity ^[53] ^[54]. That is surprising because JWST estimates 3I/ATLAS is losing mass at a prodigious rate of ~150 kilograms per second in gas and dust ^[55]. All that thrust should nudge a smaller comet, but 3I/ATLAS barely flinches. The logical inference is that the comet’s nucleus is immensely massive – likely over 33 billion tons, according to Loeb’s team calculations ^[56]. In essence, 3I/ATLAS is such a heavyweight that the recoil from its sublimating gases is negligible. They estimate the nucleus is a minimum ~5 km across if made of typical icy material, aligning with the largest size estimates from telescopic imaging ^[57] ^[58].

Researchers have also noted that 3I/ATLAS’s orbit is highly tilted and even slightly retrograde (it’s moving opposite the direction of most planets’ orbits) ^[59]. It will dive through the plane of our Solar System at a steep angle, crossing the orbits of Jupiter, Venus, and Mars but from an uncommon trajectory ^[60] ^[61]. Even so, calculations show it will miss Mars by a wide margin (~1.67 million miles outside Mars’s orbit at closest) ^[62] and similarly pose no risk to other planets. The unusual path and tilt simply underscore that this object came from a very different place and angle than the typical comets we see. Combined with its great mass, 3I/ATLAS is truly an outlier. “Loeb highlighted the rarity of finding such an object,” notes one report – based on known populations, thousands of smaller interstellar objects like ʻOumuamua should appear before one the size of ATLAS ever did ^[63]. Yet here it is in our skies, suggesting either an incredible stroke of luck or that our surveys are just beginning to reveal a wider range of interstellar wanderers than expected.

Could It Be an Alien Probe? A Controversial Idea

Whenever an object from beyond the stars shows up at our doorstep, it’s bound to spur imagination. In the case of 3I/ATLAS, some scientists have openly speculated that this might not be just a comet – it could be something engineered, like a dormant alien spacecraft or probe. This provocative hypothesis has been championed most prominently by Prof. Avi Loeb, a Harvard astrophysicist known for his willingness to consider extraterrestrial explanations. Loeb, along with researchers Adam Crowl and Adam Hibberd, published a paper in July exploring the idea that 3I/ATLAS might be artificial in origin ^[64]. They point to the comet’s extraordinary mass and unusual orbital tilt, proposing that if an intelligent civilization wanted to send a large craft into our Solar System, a path like 3I/ATLAS’s could be advantageous ^[65]. For instance, its retrograde, low-inclination orbit might allow an incoming object to reconnoiter the inner planets “without major restrictions” ^[66] – essentially sneaking in under the radar of typical planet-aligned orbits.

Loeb’s team argues that 3I/ATLAS’s trajectory – which brings it relatively close to Venus, Earth’s orbit, and Mars over the next year – could be intentional if it were a probe designed to study planets. “The pathway and tilt could let intelligent life aboard the object measure planetary orbits and masses,” Loeb suggests, raising the notion of a “Trojan Horse” scenario where something that looks like a comet is actually on a scouting mission ^[67] ^[68]. They stress this is speculative, but worth examining scientifically ^[69]. The study even muses on benign vs. malign intentions if it were a craft ^[70] – though there’s absolutely no direct evidence of any technology or signals from 3I/ATLAS at this point.

The alien hypothesis got another jolt when Loeb noted a curious coincidence involving the famous “Wow! Signal.” The Wow! Signal was a strong, unexplained radio transmission detected in 1977, long suspected (though never confirmed) to be of extraterrestrial origin. Loeb pointed out that on the date of that 1977 signal, the position in the sky it came from was roughly in the same patch of sky where 3I/ATLAS would have been, far out in space ^[71] ^[72]. In August 1977, calculations show 3I/ATLAS was about 600 AU away (over 55 billion miles from Earth) in the constellation Sagittarius ^[73]. The Wow Signal arrived from Sagittarius as well, about 4° away in sky position ^[74] ^[75]. The odds of such an alignment by random chance are around 0.6%, according to Loeb ^[76]. If – and it’s a massive “if” – the signal actually came from 3I/ATLAS, Loeb notes the transmitter would’ve needed a power on the order of 0.5–2 gigawatts (similar to a big terrestrial power plant) to be detected across that distance ^[77]. The signal’s frequency was also slightly blue-shifted (i.e. increased) by an amount consistent with an object moving toward us at tens of km/s ^[78] – intriguingly close to 3I/ATLAS’s approach speed. While this is far from proof of anything (and could be coincidental), Loeb hopes these musings encourage astronomers to listen in on 3I/ATLAS as it passes through, just in case it does emanate any radio signals ^[79]. So far, no telescope has reported unusual emissions from the comet.

Mainstream astronomers approach these claims with a healthy dose of skepticism. The prevailing view is that 3I/ATLAS, like Borisov, is simply a very large comet – remarkable, but naturally occurring. Many note that extraordinary claims demand extraordinary evidence, and nothing definitively “artificial” has been observed about 3I/ATLAS to date beyond its unexpected size. Loeb himself acknowledges his alien probe scenario is just a hypothesis to test, not a confirmed discovery ^[80]. His aim, he says, is to remind the scientific community to keep an open mind and investigate anomalies rigorously rather than dismissing them outright ^[81] ^[82]. “We should not decide about the nature of 3I/ATLAS based on the chemical composition of its skin…we should not judge a book by its cover,” Loeb argues ^[83], implying that even if the object looks like a comet on the surface, we shouldn’t entirely rule out deeper surprises. To that end, he has urged NASA to use every tool available – even suggesting the HiRISE camera on Mars orbiter try to get a pixel-scale glimpse of the nucleus – to better constrain 3I/ATLAS’s true nature ^[84].

Comparing 3I/ATLAS to ʻOumuamua and Borisov

3I/ATLAS invites comparison with its two interstellar predecessors, and the three could not be more different in some respects. ʻOumuamua (1I), discovered in 2017, was small and utterly mysterious – it had no visible coma or tail, appeared cigar-shaped (or possibly pancake-shaped) and tumbling, and exhibited a slight non-gravitational acceleration that sparked debate for years ^[85] ^[86]. Some scientists hypothesized ʻOumuamua was propelled by outgassing of invisible hydrogen or other volatiles, while others (like Loeb) controversially suggested it might have been an alien light-sail due to its odd acceleration and reflectivity. To this day, ʻOumuamua’s exact nature remains unresolved, though recent studies propose it could have been a shard of a Pluto-like exoplanet (made of nitrogen ice) or a water-rich object releasing hydrogen ^[87] ^[88]. Importantly, ʻOumuamua was roughly 100–150 meters in size – tiny compared to 3I/ATLAS – and it left the Solar System before extensive observations could be made.

2I/Borisov, discovered in 2019, by contrast looked just like a garden-variety comet. It had a gaseous coma, a tail, and a composition similar to long-period comets from our own Oort Cloud. Borisov was a few hundred meters across (estimates around 0.5–1 km) ^[89]. Scientists detected water vapor, cyanide, diatomic carbon – typical comet chemicals – and noted nothing anomalous in its trajectory. Essentially, Borisov proved that interstellar comets can closely resemble native comets, implying planetary systems elsewhere eject objects like ours do. Borisov was smaller and dimmer than 3I/ATLAS, and it was only observed for about a year as it passed through and exited.

Now 3I/ATLAS seems to combine aspects of both: it behaves like a normal comet (outgassing abundantly like Borisov did) yet has peculiar dynamical characteristics (minimal acceleration, huge size) that set it apart. “The comet has some similarities to 2I/Borisov in that it appears to be an icy comet, but it is much larger, possibly 10 km in diameter,” noted Larry Denneau of the ATLAS project ^[90]. In fact, Borisov and other known comets are dwarfed by ATLAS – one reason scientists are so intrigued. Another difference is detectability: 3I/ATLAS was discovered while still far away (beyond Jupiter’s orbit) because it’s intrinsically brighter (owing to its size and activity). ʻOumuamua, being tiny and inert, was only spotted when it was already zipping back out after perihelion; Borisov was caught on its way in but not long before closest approach. With ATLAS, astronomers have the luxury of time to coordinate observations across many platforms.

All three interstellar objects underscore that interstellar space is populated by debris of varying sizes. Based on ʻOumuamua and Borisov, researchers had expected the typical interstellar visitor to be on the smaller end (sub-kilometer scale). The arrival of a “Manhattan-sized” comet like 3I/ATLAS (some liken its bulk to the size of Manhattan island) ^[91] is thus a surprise. It suggests either that rare big ones do exist and we got lucky, or that perhaps our detection biases misled us about what’s common. Some astronomers believe that as sky surveys improve (e.g. the upcoming Vera Rubin Observatory), we’ll start finding many more interstellar objects, potentially revealing a whole spectrum of sizes. In any case, 3I/ATLAS has emphatically expanded the sample size – and our sense of what’s out there between the stars.

A Cosmic Time Capsule – and Ongoing Mysteries

Comet 3I/ATLAS is more than just a fleeting news headline; it’s a scientific goldmine and a messenger from a distant past. Given its likely age of billions of years, this object could predate the formation of Earth, condensing around an alien sun before our Solar System even existed ^[92]. In that sense, it’s possibly the oldest comet humanity has ever observed, a pristine relic of another star’s planetary system ^[93]. Analyzing its chemical makeup (like the ratios of gases and ices) can offer clues about the environment in which it formed eons ago. Already, the unusually high fraction of CO₂ detected in 3I/ATLAS hints at the conditions of its origin – perhaps it formed in a colder, outer region of a protoplanetary disk where CO₂ ice was abundant, or it might have a different evolutionary history than typical Oort Cloud comets.

Over the coming weeks and months, expect a flood of new data on 3I/ATLAS. Telescopes worldwide are monitoring its brightness and spectrum. Space-based observatories and planetary probes will report their findings as the comet makes its swing through the inner Solar System. By late October 2025, 3I/ATLAS will reach perihelion (closest point to the Sun) just inside Mars’s orbit ^[94]. It may put on a bit of a show for astronomers if its activity spikes – though it’s too distant to be seen with the naked eye, even the Hubble and Webb telescopes might capture dramatic jets or fragmentation if the comet’s nucleus breaks apart (as sometimes happens with sizable comets). After perihelion, 3I/ATLAS will begin its long exit back into interstellar darkness, not to return for millions of years (if ever). Essentially, we have this one chance to study it before it fades away. The comet will disappear from view sometime in early 2026 as it recedes; late November 2025 offers one more observing window when it re-emerges from behind the Sun ^[95].

Will 3I/ATLAS turn out to be just a remarkable natural comet, or something more exotic? The safe bet is on the former: all observations so far support it being made of rock and ice, behaving like a comet and nothing overtly “engineered.” Yet the mysteries remain – its unexpectedly large mass, its rarity, and the tantalizing what-ifs posed by thinkers like Loeb. Even skeptical scientists concede that 3I/ATLAS is strange enough to warrant every bit of scrutiny we can muster. If nothing else, the discussion around alien probes has galvanized interest in getting as much information on this object as possible. And that is a win for science: whether the comet is natural or not, we stand to learn something new and profound by studying an emissary from another star. As Loeb put it, even if the odds of it being extraterrestrial technology are low, “we must consider the possibility of a black swan event…like a Trojan Horse” from afar ^[96]. The vast majority of scientists will be focusing on the comet’s physical and chemical secrets, but keeping an open mind is part of the process.

In the end, 3I/ATLAS reminds us how much we still have to discover about our universe. Just a decade ago, we had never seen an interstellar object; now we’re tracking a giant one with cutting-edge instruments. Whether it’s carving a new chapter in planetary science or (however improbably) heralding an encounter with alien technology, 3I/ATLAS has already changed the game. “We’re gonna need a bigger telescope,” joked one science writer ^[97] about its discovery, and indeed, 3I/ATLAS is pushing astronomy to new frontiers – from rapid response observation campaigns to lively debates about life in the cosmos. This interstellar visitor has come a long way to reach us, and as it streaks past the Sun and back into the stars, it carries the potential to expand our understanding of the cosmos and our place within it.

Sources:

Reuters – “Newly spotted comet is third interstellar object seen in our solar system” (Jul. 3, 2025) ^[98] ^[99]
ESA – “Comet 3I/ATLAS – frequently asked questions” (Sep. 2025) ^[100] ^[101]
The Economic Times – “3I/ATLAS interstellar object much larger than previously thought… new findings” (Jul. 2025) ^[102] ^[103]
Fox News – “Massive comet… could be alien technology, Harvard astrophysicist says” (Sep. 29, 2025) ^[104] ^[105]
Avi Loeb (Medium) – “Was the ‘Wow! Signal’ Emitted from 3I/ATLAS?” (Sep. 30, 2025) ^[106] ^[107]
The Debrief – “Interstellar comet 3I/ATLAS is ‘anomalously massive’” (Sep. 27, 2025) ^[108] ^[109]
The Economic Times – “Alien probe? Harvard scientists spark debate on 3I/ATLAS” (Sep. 2025) ^[110] ^[111]
New York Post – “‘Massive’ comet… larger than previously thought, could be alien tech” (Sep. 29, 2025) ^[112] ^[113]