Interstellar Comet 3I/ATLAS Shrouded in CO₂ Fog – NASA’s SPHEREx Reveals a Cosmic Visitor’s Secrets

• 3I/ATLAS (C/2025 N1 ATLAS) was discovered July 1, 2025 by the ATLAS survey telescope in Chile and is on a hyperbolic, unbound path with eccentricity ~6.2, making it the third confirmed interstellar object after 1I/‘Oumuamua and 2I/Borisov.
• Early estimates suggested a diameter of several kilometers, and Hubble data later indicated a nucleus of about 5–6 km across, making 3I/ATLAS the largest interstellar object observed.
• SPHEREx detected a huge CO₂ cloud around 3I/ATLAS, with the coma extending at least 348,000 km from the nucleus, alongside evidence of water ice in the nucleus.
• JWST measurements found a CO₂-to-H₂O ratio of roughly 8:1 in the coma and a CO-to-H₂O ratio of about 1.4:1.
• The CO₂-rich, CO-poor signature suggests 3I/ATLAS formed in a relatively warm region or beyond the CO₂ ice line in its home system, with CO depleted.
• Perihelion is expected around October 29, 2025 at about 1.4 AU, after which it will retreat back into interstellar space.
• 3I/ATLAS is traveling at about 210,000 km/h (130,000 mph) and has a high hyperbolic excess velocity, making spacecraft interception effectively impossible.
• SPHEREx conducted observations August 7–15, 2025 across 0.75–5.0 microns, mapping 102 colors to identify molecular fingerprints such as CO₂ and H₂O ice.
• Compared with ‘Oumuamua, which had no visible coma, and Borisov, which was CO-rich, ATLAS shows CO₂ dominance and water ice, with a nucleus larger than Borisov’s estimated <1 km.
• The findings underscore that interstellar comets can resemble solar-system comets in ice composition, spur future surveys and missions like ESA’s Comet Interceptor, and make SPHEREx data publicly available for further study.

The third-ever interstellar comet, 3I/ATLAS, has burst into our solar system cloaked in a cloud of carbon dioxide gas. Discovered in July 2025, this alien comet – the largest and brightest interstellar object yet – is giving scientists an unprecedented look at material from another star system ^{[1] [2]}. NASA’s brand-new SPHEREx space telescope has detected an abundance of CO₂ gas in 3I/ATLAS’s fuzzy coma (its thin atmosphere) along with water ice in the nucleus ^[3]. The findings suggest this visitor has a lot in common with ordinary comets in our own solar system ^[4], providing tantalizing clues about how and where it formed. Other observatories – including the James Webb Space Telescope (JWST) – confirm 3I/ATLAS’s coma is unusually rich in carbon dioxide, with one of the highest CO₂-to-water ratios ever seen ^[5]. Researchers are racing to study this rare cosmic interloper before it slingshots around the Sun in October and vanishes into interstellar space, carrying secrets of its origin. Here’s an in-depth look at Comet 3I/ATLAS, what SPHEREx discovered, and why astronomers (and the public) are so excited about this alien wanderer.

Overview: What Is Comet 3I/ATLAS?

Comet 3I/ATLAS (also designated C/2025 N1 ATLAS) is a one-of-a-kind visitor from beyond our solar system. It was first spotted on July 1, 2025, by the ATLAS survey’s telescope in Chile ^[6]. Almost immediately, its path stood out as peculiar: 3I/ATLAS is not gravitationally bound to the Sun, meaning it’s on a one-way hyperbolic trajectory through our solar system ^[7]. In fact, its orbital eccentricity is about 6.2, far above 1, which confirms an interstellar origin (for comparison, the first interstellar object 1I/‘Oumuamua had e ≈ 1.2 and the second, 2I/Borisov, e ≈ 3.6 ^[8]). This makes 3I/ATLAS only the third known interstellar object ever observed after ‘Oumuamua in 2017 and Borisov in 2019 ^{[9] [10]}.

What really has astronomers buzzing is the comet’s size and brightness. Early estimates suggested a diameter on the order of several kilometers, vastly larger than ‘Oumuamua (~100–200 m) and even bigger than Borisov (≲1 km) ^{[11] [12]}. Subsequent Hubble Space Telescope data indicate a nucleus perhaps up to ~5–6 km across ^[13], which would still make 3I/ATLAS the largest interstellar object ever seen ^[14]. It’s also intrinsically bright, with a substantial surrounding halo of dust and gas (hence its comet designation). Soon after discovery, telescopes revealed a clear coma and tail, confirming 3I/ATLAS was actively outgassing like a normal comet ^[15]. For scientists, this was exciting news: unlike ‘Oumuamua (which had no visible coma), 3I/ATLAS offers a chance to directly sample the gases and ices from another star system by analyzing its coma.

Even more intriguing, 3I/ATLAS may be extraordinarily old. Based on its chemical makeup and trajectory, one team of astronomers estimates this comet formed roughly 7 billion years ago, making it about 3 billion years older than our 4.5-billion-year-old solar system ^{[16] [17]}. “This is an object from a part of the galaxy we’ve never seen up close before,” said Oxford astronomer Matthew Hopkins, whose team suspects 3I/ATLAS could be the oldest comet ever observed ^{[18] [19]}. In other words, 3I/ATLAS might have been orbiting its home star long before Earth even existed, until some gravitational jolt sent it flying outward on a multimillion-year journey through the Milky Way.

CO₂ in the Coma: SPHEREx’s Big Discovery

In August 2025, NASA trained its new SPHEREx space telescope on 3I/ATLAS – and the comet did not disappoint. SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) is uniquely suited to sniff out cosmic chemicals, and it quickly detected a huge cloud of carbon dioxide gas surrounding 3I/ATLAS ^{[20] [21]}. The CO₂ coma extends at least 348,000 km outward from the nucleus ^[22] – nearly the distance from Earth to the Moon! Along with the CO₂, SPHEREx’s infrared data revealed a wealth of water ice in the comet’s nucleus ^[23] (evidenced by absorption features of H₂O ice). Meanwhile, it found little to no carbon monoxide gas (CO) in the vicinity ^[24]. This chemical profile – CO₂-rich, H₂O-rich, but CO-poor – immediately caught scientists’ attention.

Why is the carbon dioxide detection such a big deal? For one, it’s the first time we’ve clearly identified CO₂ in an interstellar comet’s coma, and in great abundance ^[25]. Carbon dioxide is one of the primary ices in comets, but it’s notoriously tricky to observe from Earth (our atmosphere blocks many of CO₂’s infrared signatures ^[26]). With SPHEREx above the atmosphere, astronomers could directly “see” the CO₂ sublimating off 3I/ATLAS as it warmed up. “SPHEREx’s finding very large amounts of vaporized carbon dioxide gas around 3I/ATLAS told us it could be like a normal solar system comet,” explained Carey Lisse, a Johns Hopkins University astronomer on the SPHEREx science team ^[27]. Comets in our solar system are typically made of the same three major ices – water (H₂O), carbon dioxide (CO₂), and carbon monoxide (CO) – just in different proportions ^[28]. Finding these familiar ingredients around an interstellar comet suggests comets from other star systems can closely resemble our own ^[29].

However, 3I/ATLAS’s coma isn’t exactly standard. In fact, follow-up observations by JWST show the comet’s CO₂-to-H₂O ratio is extremely skewed – on the order of 8:1, the highest ever measured in any comet ^[30]. Typically, water is the dominant outgassing molecule for comets near the Sun, with CO₂ only a minor component; but in 3I/ATLAS, CO₂ dominates. This ratio is about six standard deviations above the norm for comets ^[31]. By contrast, JWST found the carbon monoxide to water ratio in 3I/ATLAS was about 1.4:1, which is more in line with previous comet observations ^[32]. In other words, 3I/ATLAS is unusually CO₂-rich but not unusually CO-rich, at least at the distances observed. The absence of detectable CO gas alongside a strong CO₂ glow is a crucial clue: CO is more volatile (it boils off at lower temperatures) than CO₂, so a lack of CO suggests the comet’s most volatile ices have already been depleted ^[33].

Lisse put it vividly: SPHEREx’s carbon dioxide coma – “notably one that lacks carbon monoxide” – plus the icy nucleus indicate that 3I/ATLAS was “well baked and boiled” before it ever left its parent star system ^[34]. In plainer terms, this comet likely spent time in a relatively warm environment where much of its CO (and other ultra-volatile compounds) boiled away, leaving behind mostly H₂O and CO₂ ice. “3I/ATLAS is behaving like a normal, well-thermally processed, natural solar system cometary object does,” Lisse said ^[35]. Indeed, some comets born in the inner regions of our solar system show a similar pattern – they’ve lost their most volatile ices after repeated trips near the Sun, so they outgas mainly water and CO₂ ^[36]. The fact that 3I/ATLAS mirrors this pattern suggests commonality in comet evolution across planetary systems.

Importantly, detecting specific molecules like CO₂ can tell the story of the comet’s origin. The high CO₂ content in 3I/ATLAS might reflect the conditions of its home system. Scientists offer a few hypotheses: perhaps the comet formed in a region beyond the CO₂ “ice line” around its original star, where carbon dioxide ice was especially abundant compared to water ^[37]. Alternatively, it may have been exposed to intense ultraviolet radiation in its youth, which could chemically alter and deplete water ice while leaving a higher proportion of CO₂ ^[38]. “Our observations are compatible with an intrinsically CO₂-rich nucleus,” wrote the research team, indicating 3I/ATLAS may contain ices that experienced higher radiation levels or formed close to the CO₂ ice line in its protoplanetary disk ^[39]. In either case, the comet’s chemistry is a fingerprint of its birthplace. By studying 3I/ATLAS’s coma, scientists are effectively peering into the conditions of a distant star’s planetary nursery – a place and time far removed from our own solar system.

Beyond origin clues, the CO₂ discovery is a milestone for comet science. It marks a “major step forward” in comparing interstellar comets to local ones ^[40]. Until now, we’ve had precious few data points: ‘Oumuamua offered no compositional data (no coma to analyze), and Borisov was only observed in a handful of molecules (mostly cyanide, water, and CO). With 3I/ATLAS, astronomers are getting the first detailed chemical portrait of an interstellar comet, and it turns out to have significant overlap with comets born in our solar neighborhood ^[41]. That’s a fascinating finding – it implies the basic building blocks (water, carbon dioxide, etc.) and processes (solar heating, outgassing patterns) might be universal to comets everywhere ^[42]. At the same time, the unusual CO₂ dominance reminds us that each star’s comets have their own quirks and history.

Meet SPHEREx: NASA’s New Cosmic Molecule Mapper

The dramatic CO₂ detection was made possible by NASA’s SPHEREx mission, a state-of-the-art space telescope that launched in March 2025 ^[43]. SPHEREx isn’t a traditional observatory taking pretty pictures; instead, it’s designed to perform an all-sky spectral survey – essentially mapping the entire sky in 102 different colors (wavelength bands) from the visible deep red into the near-infrared ^[44]. Over its two-year mission, SPHEREx will scan every patch of sky multiple times, collecting low-resolution spectra for billions of celestial sources. Its primary science goals span cosmology (probing the early universe’s structure and the epoch of reionization) and galactic science (especially the distribution of water ice and organic molecules in our Milky Way) ^{[45] [46]}. The mission’s mouthful of a name – Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer – hints at this broad agenda.

One of SPHEREx’s strengths is its ability to detect and identify molecules by their spectral “fingerprints.” When SPHEREx observed 3I/ATLAS from August 7–15, 2025, it collected data from 0.75 to 5.0 microns in wavelength ^[47]. Within this range, many important cometary molecules have telltale signatures. For example, CO₂ has a strong emission feature around 4.26–4.3 µm, and indeed SPHEREx saw a “huge uptick” in brightness at those wavelengths corresponding to copious CO₂ gas ^[48]. Water ice, on the other hand, absorbs sunlight at specific near-IR bands (around 1.5 and 2.0 µm), and SPHEREx detected those subtle absorption dips indicating solid H₂O in the nucleus ^[49]. By splitting the incoming light into dozens of narrow color channels, SPHEREx can clearly distinguish these molecular signals from the overall glow of the comet. “SPHEREx’s forte is creating 102 different wavelength maps of the entire sky at the deep red to mid-infrared (0.75–5.0 µm),” said Lisse, “[and] many of these wavelengths cannot be studied from Earth due to atmospheric absorption” ^[50]. For comet scientists, this opens up unprecedented access to key chemical fingerprints that were previously hard to catch ^[51] – like carbon dioxide.

Another advantage of SPHEREx is its wide field of view. It can image a relatively large patch of sky in one go, which proved useful for 3I/ATLAS’s extended coma. The SPHEREx team noted that the CO₂ gas envelope was so expansive (10+ arcminutes on the sky) that only a wide-field instrument like SPHEREx could map it in its entirety ^{[52] [53]}. The spacecraft’s spectral resolution nailed down the CO₂ emission, while its broad gaze traced the coma’s full extent – an ideal combination for this discovery ^{[54] [55]}. In essence, SPHEREx provided both the chemical identification and the global context (size and shape) of the comet’s atmosphere in one dataset.

Though SPHEREx was primarily built for cosmology and galactic surveys, catching an interstellar comet in action is a spectacular bonus. It showcases the mission’s versatility. “By mapping the entire sky in 102 colors, we can study all kinds of celestial objects, from distant galaxies and nearby stars in the Milky Way to comets in our own solar system, whether interstellar visitors or not,” said Olivier Doré, SPHEREx project scientist at NASA’s Jet Propulsion Laboratory ^[56]. “It really underscores the transformative power of the dataset we are now generating. The richness is extraordinary, and it will fuel discoveries across the astronomical community since our data are being continuously released.” ^[57] In practical terms, this means SPHEREx data on 3I/ATLAS (and everything else it observes) will be publicly available, enabling scientists worldwide to scour the spectra for insights.

The 3I/ATLAS findings were quickly written up in a Research Notes of the AAS publication by the SPHEREx science team ^{[58] [59]}. And the mission isn’t done with 3I/ATLAS yet – as the comet moves, SPHEREx can revisit it during routine sky scans. In late October 2025, 3I/ATLAS will reach perihelion (closest point to the Sun) at roughly the distance of Mars’s orbit ^{[60] [61]}. As it draws nearer, even more ice will vaporize. “Scientists expect the water ice in its nucleus to sublimate, producing a large water coma to match the carbon dioxide coma, as well as a much stronger dust coma and tail than we see now,” Lisse explained ^[62]. SPHEREx, along with many other telescopes in NASA’s fleet, will be watching closely at perihelion to catch those changes ^[63]. It’s a prime opportunity to see how an interstellar comet behaves as it experiences intensifying solar warmth – will it brighten dramatically? Break apart? Only continuous monitoring will tell. The fact that SPHEREx can scan the whole sky repeatedly means it won’t miss the show even if 3I/ATLAS’s trajectory carries it near the Sun’s glare.

In summary, SPHEREx has proven to be a game-changer for comet chemistry, right out of the gate. Within months of launch, it bagged a historic detection of CO₂ from an interstellar visitor. This bodes well for the mission’s future: every six months, SPHEREx will complete another all-sky map ^[64], potentially catching other comets (or who knows what else) with unusual spectra. And if another interstellar object comes speeding through, SPHEREx will be ready to dissect its light just as it did for 3I/ATLAS.

How Does 3I/ATLAS Compare to ‘Oumuamua and 2I/Borisov?

The discovery of 3I/ATLAS offers a chance to compare all three known interstellar interlopers – and they’ve each been surprisingly different so far. Here’s how 3I/ATLAS stacks up against its predecessors, 1I/‘Oumuamua and 2I/Borisov, in terms of behavior and composition:

• ‘Oumuamua (1I, 2017): The first interstellar object stunned astronomers with its oddities. Detected in October 2017, ‘Oumuamua was very small (about 100–200 meters long) and had no observable coma or tail, appearing like a bare rock ^[65]. It exhibited an unusual, elongated shape or extreme aspect ratio (inferred from its light curve), and most mysteriously, it showed a slight non-gravitational acceleration as it exited the inner solar system – as if something was gently pushing it, even though no gas emissions were seen. This led to wild speculation (from hydrogen icebergs to alien solar sails), because unlike 3I/ATLAS or Borisov, ‘Oumuamua behaved more like an asteroid than a comet ^[66]. Scientists now lean toward natural explanations – possibly that ‘Oumuamua was a fragment of nitrogen or water ice that sublimated invisibly, or a fluffy aggregate that outgassed very subtly. But the truth is we caught ‘Oumuamua on its way out and never got a spectrum, so its exact nature remains ambiguous ^[67]. What’s clear is that ‘Oumuamua did not display the rich gas-and-dust signature that 3I/ATLAS does. If 3I/ATLAS can be called a “typical” comet (albeit interstellar), ‘Oumuamua was the oddball – more akin to an inert shard from another solar system, with no coma to analyze. Its hyperbolic path (e ≈ 1.2) was only mildly above escape velocity ^[68], meaning it was moving relatively slowly and we barely noticed it before it was gone.
• Borisov (2I, 2019): In contrast to ‘Oumuamua, Comet 2I/Borisov looked much more familiar. Discovered in August 2019 by amateur astronomer Gennadiy Borisov, this object clearly had a coma of dust and gas and even a tail – essentially a normal comet, just on a hyperbolic orbit ^[69]. Borisov’s trajectory (eccentricity ~3.36) confirmed it came from interstellar space ^[70]. Astronomers were thrilled to study Borisov’s composition; using telescopes, they detected water vapor, hydrogen cyanide, and a surprisingly large amount of carbon monoxide (CO) in its coma. In fact, Borisov turned out to be very CO-rich and relatively water-poor compared to typical solar system comets ^[71]. Estimates put its CO/H₂O ratio between about 35% and 100%, vastly higher than the ~4% average in Sun-born comets ^[72]. This means Borisov likely formed in an extremely cold region (or around a cooler star), where volatile CO ice was plentiful ^[73]. Apart from the chemistry, Borisov’s nucleus was quite small – Hubble observations suggested a diameter under 1 km (perhaps ~0.5 km) ^{[74] [75]}. It was also shedding material rapidly; Borisov lost a few hundred kilograms of water per second as it neared the Sun ^[76], and it even fragmented a bit as it left (the nucleus broke into pieces in 2020). In essence, 2I/Borisov was the first real interstellar comet observed up close, and it showed that other solar systems can produce cometary bodies very much like those here, albeit with some different ice mixtures.
• ATLAS (3I, 2025): Now along comes 3I/ATLAS, which in many ways is more like Borisov than ‘Oumuamua – it’s an active comet – but with its own twists. The size is one big difference: 3I/ATLAS seems to be orders of magnitude larger than ‘Oumuamua and at least several times larger than Borisov ^[77]. A tentative estimate of ~5–15 km diameter (with uncertainty) makes it a giant among interstellar objects ^[78]. This bodes well for data-gathering, since a bigger nucleus typically means a brighter comet and more material to detect. Sure enough, 3I/ATLAS has been bright enough to study with multiple major telescopes simultaneously. Its composition, as discussed, is notable for carbon dioxide dominance and lots of embedded water ice ^{[79] [80]}, with little carbon monoxide gas at this stage ^[81]. That’s almost the mirror image of Borisov’s composition (which was CO-rich and, as far as we know, not unusually high in CO₂). Unfortunately, CO₂ is hard to measure in comets without space-based instruments, so we don’t have a comparable CO₂ reading for Borisov – but nothing in 2019 suggested Borisov had as extreme a CO₂/H₂O ratio as ATLAS. As one astronomer put it, each interstellar comet so far has “hid new insights” in its coma ^[82]. Borisov’s surprise was an overabundance of carbon monoxide, whereas ATLAS’s surprise is an overabundance of carbon dioxide. Both interstellar comets seem “relatively depleted in water” compared to most local comets ^{[83] [84]}, which might indicate that comets ejected from other systems tend to come from colder, outer regions (rich in CO or CO₂ ices). Meanwhile, poor ‘Oumuamua had no detectable volatiles at all, which could mean it was a very different kind of object (perhaps a fragment of a planetary crust, or a comet that lost all its ice). The three known interstellar visitors thus appear to originate from “distinct stellar populations with different ages and galactic locations,” according to one analysis ^[85] – in other words, each might hail from a very different type of star or protoplanetary disk.

Beyond composition, their dynamics also differ. 3I/ATLAS is barreling through on a flatter, faster trajectory than the others. It’s coming in roughly along the plane of the Milky Way (rather than a high angle) ^[86] and zipping by the Sun at about 210,000 km/h (130,000 mph) ^[87], considerably faster than Borisov’s speed at a similar distance. It will reach perihelion around October 29, 2025, at ~1.4 AU from the Sun ^[88], then head back out for good. For comparison, Borisov passed about 2 AU from the Sun at closest approach ^[89], and ‘Oumuamua went inside 1 AU (it was discovered post-perihelion at ~0.25 AU distance). Yet 3I/ATLAS’s hyperbolic excess velocity is greater, meaning it was probably traveling faster relative to the Sun long before entering our solar system. Its high speed and large mass mean no existing rocket could catch it – it’s simply moving too fast for us to mount a space mission in time (more on that later) ^[90].

The bottom line: All three interstellar objects have given us surprises. ‘Oumuamua challenged our ideas of what small bodies can look like (thin, tumbly, and comet-like in trajectory but asteroid-like in appearance). Borisov reassured us that other systems make comets that, at least superficially, resemble those we know, while also hinting at chemical differences. And now 3I/ATLAS is showing an even closer kinship to solar system comets (with water and CO₂ present), yet with an extreme composition ratio that has never been observed before ^[91]. Each one is a tiny sample of a distant star system’s leftovers – and each has taught us something new about the diversity of planetary building blocks in our galaxy ^[92].

Comas and Clues: Why CO₂ Detection Matters for Formation Theories

One of the reasons astronomers are so keen to measure molecules like CO₂ in comets is that a comet’s “coma” is a chemical time capsule. The coma is the thin, fuzzy atmosphere that forms around a comet’s solid nucleus when it approaches a star and its ices begin to sublimate (turn directly from solid to gas). As sunlight heats the comet, ices like water, CO₂, and CO vaporize and stream out, dragging dust with them, which creates a glowing cloud around the nucleus. This temporary atmosphere grows larger and brighter the closer the comet gets to the Sun (or any star), an effect known as outgassing ^[93]. The coma can range from thousands to hundreds of thousands of kilometers in diameter, enveloping the comet in a misty halo of its own material. (For instance, 3I/ATLAS’s CO₂ coma spans at least 348,000 km across, as noted earlier ^[94]!) Eventually, pressure from the solar wind can blow parts of the coma into a long tail streaming out behind the comet.

In essence, the coma is the comet “coming to life” under the Sun’s heat – and by analyzing the coma, scientists can identify what ices the comet is made of. This is incredibly important for understanding cometary and planetary formation. Comets are often called dirty snowballs or icy time capsules because their cores contain pristine materials from the early days of their star system. In our solar system, many comets originated in the cold outskirts (the Kuiper Belt or Oort Cloud) and contain primordial ices from the solar nebula. When they swing inward and outgas, they release those ancient ingredients for us to measure.

Detecting specific molecules in a coma (like H₂O, CO₂, CO, methane, etc.) thus reveals the comet’s composition, which is a direct reflection of the conditions where it formed. Each volatile substance has a characteristic “freeze-out” temperature – for example, water ice is stable out beyond the water snow line (~150 K, typically around 3 AU from the Sun), CO₂ ice is stable at even colder temperatures (farther out), and CO ice requires it colder still. If a comet formed in the very distant, frigid regions of a protoplanetary disk, it should have trapped a lot of CO and CO₂ along with water. If it formed closer in, or spent eons in warmer regions, the more volatile CO might have escaped, leaving mainly water (which has a higher boiling point) and maybe CO₂. “The amounts of each [ice] depend on where and when the comet was formed and evolved,” Lisse noted ^[95]. “A comet formed at the very edges of our dawning solar system and then thrown quickly far away… should have all three ices in abundance,” he explained. “By contrast, a comet formed close to the sun and/or residing for a long time after its formation will lose its carbon monoxide and contain mainly water and carbon dioxide.” ^[96]

This principle is key to planetary scientists: by reading a comet’s chemical makeup, we read its history. In our solar system, we see comets that fit this schema – some long-period comets arriving from the far Oort Cloud are rich in CO (preserving their deep-freeze composition), whereas short-period comets that spent more time near the Sun often have depleted CO but still vent CO₂ and H₂O ^{[97] [98]}. Now, with interstellar comets like 3I/ATLAS, we can apply the same logic to a comet from another star. The detection of abundant CO₂ and water but scant CO in 3I/ATLAS’s coma strongly suggests it formed in a region where it got “baked” – either relatively close to its star or subjected to warming events that drove off the CO ^[99]. That, in turn, might hint at the structure of its original planetary system (perhaps a system where comets spend time migrating inward, or a star that had a strong early UV output).

Furthermore, certain molecules can serve as tracers of specific processes. For example, JWST’s observations of 3I/ATLAS also measured the ratio of two isotopes of carbon (^12C/^13C) in the CO₂, finding it similar to Earth’s value ^[100]. This suggests the comet’s carbon was processed in an environment not too unlike the solar protoplanetary disk in terms of isotope chemistry. Such details help researchers compare the chemistry of different star systems. Every new molecule detected (be it familiar ones like CO₂ or more exotic species like OCS or amines) refines our picture of what ingredients exist out there and how they might combine to form planets and maybe even the precursors for life.

CO₂ is especially important because it’s thought to be a major driver of comet activity at intermediate distances. Water ice doesn’t sublimate much until a comet gets relatively close to the Sun (~2–3 AU), but CO₂ can sublimate at lower temperatures (it’s volatile around 5 AU and inward). So a comet rich in CO₂ can become active farther from the Sun. In 3I/ATLAS’s case, we saw it outgassing CO₂ strongly at ~3.3 AU distance ^[101], while its water output was still modest (most of its H₂O was likely still frozen at that distance). This early CO₂-driven activity could explain why 3I/ATLAS already had a significant coma when first observed inbound. Knowing this helps astronomers model the comet’s behavior – e.g. predicting how its brightness might surge once water sublimation kicks in closer to the Sun (as Lisse predicted, a big water coma should appear by perihelion, adding to the CO₂ coma ^[102]).

In summary, “reading” a coma’s molecular makeup is like conducting a cosmic chemistry experiment: it tells us the recipe of the comet. For interstellar comets, it’s our only way to learn about distant stellar systems without sending a probe there. Each molecule detected is a clue about that comet’s origin story – the temperature of its nursery, the radiation environment, the possible presence of certain elements or organic compounds. By comparing these clues across many comets, we build a broader understanding of planetary formation and diversity. Did other stars form comets with the same mix of water and organics as ours? Do interstellar comets carry more carbon (in CO or CO₂ form) on average? These questions feed into theories about how common Earth-like planets or life’s ingredients might be elsewhere. Thus, detecting something like CO₂ in 3I/ATLAS isn’t just about one comet – it’s a piece of the grand puzzle of how solar systems evolve and exchange material.

What Scientists Are Saying about 3I/ATLAS

The arrival of 3I/ATLAS and the discoveries by SPHEREx and JWST have generated a flurry of excitement in the astronomical community. Researchers are keen to share what this interstellar comet is teaching us. Here are a few reactions and quotes from scientists involved:

• Carey Lisse, planetary astronomer and member of the SPHEREx science team, emphasized how normal 3I/ATLAS seems in many respects. “SPHEREx’s finding very large amounts of vaporized carbon dioxide gas around 3I/ATLAS told us it could be like a normal solar system comet,” Lisse told Space.com ^[103]. He explained that comets in our solar system are largely made of the same three ices (water, CO₂, CO) and that the ratios among them reveal a comet’s formation zone ^[104]. In the case of 3I/ATLAS, the dominance of CO₂ and lack of CO led Lisse to conclude the comet had been “well baked and boiled” before being kicked out of its home system ^[105] – essentially echoing what we see in long-lived comets of the Sun that have lost their more volatile ingredients. “3I/ATLAS is behaving like a normal, well-thermally processed, natural solar system cometary object does,” he said, placing our interstellar visitor firmly in line with cometary physics we understand ^[106].
• Olivier Doré, SPHEREx project scientist at JPL, highlighted the technological triumph that these observations represent. He noted that the mission’s ability to map the sky in over a hundred wavelengths enables it to study everything from distant galaxies to nearby comets in unprecedented detail. “It really underscores the transformative power of the dataset we are now generating,” Doré said of SPHEREx’s results, “The richness is extraordinary, and it will fuel discoveries across the astronomical community since our data are being continuously released.” ^[107] In the context of 3I/ATLAS, Doré’s statement underlines that this discovery is just one of many to come – SPHEREx’s data is a goldmine that researchers will be digging through, possibly uncovering even more surprises about 3I/ATLAS or other objects serendipitously caught in its all-sky surveys.
• Martin Cordiner, an astrochemist at NASA’s Goddard Space Flight Center (and lead author of the JWST study), commented through his paper that the high CO₂ content of 3I/ATLAS could point to unique conditions in its origin. “Our observations are compatible with an intrinsically CO₂-rich nucleus,” the team wrote, suggesting that 3I/ATLAS “contains ices exposed to higher levels of radiation than Solar System comets, or… formed close to the CO₂ ice line in its parent protoplanetary disk.” ^[108]. This measured statement from the scientific report highlights the two main theories (radiation versus formation location) that could explain the comet’s chemistry, and it reflects the cautious excitement of scientists: they’re intrigued by the data, but still considering multiple explanations.
• Mark Norris, an astronomer at the University of Central Lancashire, remarked on the broader significance when 3I/ATLAS was first identified. “If confirmed, it will be the third known interstellar object from outside our solar system that we have discovered, providing more evidence that such interstellar wanderers are relatively common in our galaxy,” Norris said ^[109]. This quote came just after the discovery, but it has proven prescient – indeed 3I/ATLAS was confirmed interstellar, and its discovery supports the notion that many more such objects could be drifting among the stars. Norris’s use of “relatively common” speaks to a paradigm shift: a decade ago we had zero known interstellar objects, now we have three, and scientists suspect there are always many of them passing through undetected. His reaction captures the excitement of realizing we’ve opened a new window on celestial phenomena that might happen frequently.
• Professor Martin Barstow of the University of Leicester, speaking to Space.com, stressed the scientific treasure that interstellar comets represent. “They undoubtedly carry chemical signatures from outside the solar system, so gaining observations tells us a lot about the possibility of material traveling between planetary systems,” Barstow said ^[110]. He added that if we could ever get a sample from one, “it would be an incredible breakthrough.” ^[111]. Barstow’s reaction underscores why researchers are so eager to study 3I/ATLAS: it literally carries alien stardust – materials from another system – which might inform us about the building blocks of planets elsewhere and even the potential distribution of life’s precursors. His mention of material traveling between systems also nods to the idea of panspermia (the transfer of life or prebiotic compounds between stars via comets), a tantalizing concept in astrobiology.
• Finally, a bit of levity in the scientific discourse: whenever an interstellar object shows up, speculation runs wild. Avi Loeb, a Harvard professor, famously hypothesized ‘Oumuamua could be an extraterrestrial spacecraft; and with 3I/ATLAS, a controversial preprint in 2025 posited it might be a piece of “possibly hostile” alien technology – a claim that mainstream experts swiftly dismissed as “nonsense” ^[112]. “Here we go again!” one headline quipped about that paper ^[113], reflecting some astronomers’ exasperation with unfounded alien theories. Still, even these offbeat reactions highlight how extraordinary interstellar comets are: they capture imaginations far and wide. As Loeb himself wrote (in a more grounded tone), first ‘Oumuamua, then Borisov, and now ATLAS show that interstellar interlopers are real – and each one teaches us something new ^[114].

In summary, scientists are both enthusiastic and thoughtful in their reactions. The consensus is that 3I/ATLAS offers an unprecedented peek into another solar system’s chemistry, confirming some expectations (e.g. presence of common ices) while posing new questions (e.g. why so much CO₂?). There’s a palpable excitement that we finally have a big, bright interstellar comet to scrutinize with the best instruments available. At the same time, researchers are careful to place these findings in context – not jumping to conclusions, but considering how this one comet fits into the emerging picture of many possible interstellar objects. The quotes above capture the mix of wonder and scientific curiosity that 3I/ATLAS has inspired.

Why Interstellar Comets Fascinate Scientists and the Public

Just a few years ago, the idea of discovering an interstellar comet was the stuff of science fiction. Now, we’re on our third one – and each discovery has caused a sensation. Interstellar comets (and asteroids) ignite so much interest for several reasons:

1. They are messengers from afar: An interstellar object is literally material from a completely different star system, wandering into our cosmic backyard. For scientists, this is a dream opportunity – it’s as if a spacecraft from a distant world delivered a sample to us for free. We can study the object’s composition and properties without leaving home. As Mark Norris mused, “Thanks to these visitors from outside our solar system, we may not have to travel that far to sample star systems beyond our own” ^[115]. Instead of launching a probe on a 50,000-year journey to another star, an interstellar comet is coming to us, offering a shortcut to learn about alien worlds. This aspect fascinates scientists, who hope to glean clues about how planets and comets form around other suns, and whether our solar system’s history is unique or common.

2. Rarity and “firsts”: Only three interstellar objects have ever been detected, so each one is a rare event. The public is naturally intrigued by rare phenomena, and the scientific community mobilizes swiftly to observe them from every angle. In the case of 3I/ATLAS, as soon as it was announced, astronomers around the globe (and in orbit) sprang into action. NASA quickly coordinated a “multi-mission” observation campaign ^{[116] [117]} – including telescopes like Hubble, JWST, SPHEREx, Swift, and others – to scrutinize 3I/ATLAS before it fades. The European Space Agency’s comet experts also tracked it. This all-hands-on-deck approach underscores how precious the opportunity is; the object will be observable for only a few months in total. The intensity of observations, and the stream of new findings (like the CO₂ coma detection), have kept both scientists and space enthusiasts riveted.

3. They could be the “tip of the iceberg”: The discoveries of ‘Oumuamua, Borisov, and ATLAS in quick succession suggest that interstellar visitors might be more common than we realized. We simply hadn’t been able to detect them until recently. Now with better surveys (like ATLAS, Pan-STARRS, and soon the Vera Rubin Observatory), we expect to find many more. In fact, the new Rubin Observatory – which just began testing its powerful wide-field telescope – “could discover many more interstellar objects like 3I/ATLAS” during its upcoming decade-long sky survey ^[118]. A 2020 study estimated that at any given time, there might be an interstellar object larger than 1 km somewhere inside the orbit of Saturn; we just need to look hard enough. The prospect of routinely finding interstellar comets in coming years is hugely exciting to scientists. It means we’ll be able to build up a sample size of these objects, not just study one oddball at a time. Each new find like 3I/ATLAS boosts public interest further, as people realize this isn’t an isolated fluke – there’s a whole population of interstellar nomads out there. Our solar system is not a closed box; it intermingles with the rest of the galaxy.

4. Mystery and the allure of the unknown: Interstellar objects carry an aura of mystery. They hail from unknown distant stars – perhaps from a serene Sun-like star, or maybe a red dwarf, or a binary system, etc. Because they are not immediately traceable to any known source, they invite speculation. The public and press often ask, “Where did it come from? What’s it made of? Could it be alien technology?” (the latter question pops up frequently, as we saw with both ‘Oumuamua and 3I/ATLAS’s speculation). While scientists approach these questions with data and skepticism, the mystery itself generates public fascination. It’s a real-life cosmic whodunit – we have an object and we’re trying to deduce its origin story through clues. That narrative, of uncovering the secrets of an interstellar visitor, captivates many. The fact that 3I/ATLAS turned out to have unexpected properties (like the CO₂ fog) only adds to the intrigue, showing that these visitors can defy expectations and deepen the mystery of how nature works in other star systems.

5. Visceral imagery and scale: There’s also something awe-inspiring about the visuals and scale of these events. Hubble Space Telescope images of 3I/ATLAS, for instance, showed a bright nucleus with a teardrop-shaped dust cocoon streaming off it ^[119]. It drives home that this object is actively shedding material as it hurtles through space. The sheer speed – 130,000+ mph – and the huge size of the coma – hundreds of thousands of kilometers – give a sense of a dynamic, powerful phenomenon. It’s literally a piece of another solar system blowing past us with a tail of gas and dust in its wake. Such imagery can spark public imagination. (The Virtual Telescope Project even hosted live feeds of telescopic views, letting people “watch it live online” as it moved through the stars ^[120].) In a year when comets are in the news, interstellar ones stand out as even more special.

6. Scientific payoff and the future: Lastly, interstellar comets spur discussions about future missions and technology. The scientific interest is so high that space agencies are thinking about how to capture these gifts from the stars. For example, the European Space Agency’s planned Comet Interceptor mission (launching in 2029) is designed to park in space and wait for a target – possibly even an interstellar object – to appear, so it can rapidly redirect and perform a flyby ^{[121] [122]}. When 3I/ATLAS was discovered, one immediate question was: can we send a spacecraft to meet it? Unfortunately, 3I/ATLAS is moving too fast and was discovered too late for us to mount an interception – “it’s just too fast,” as Professor Barstow put it, unless we’d had something ready and waiting in orbit ^[123]. This time, we’ll have to rely on telescopes. But the discussion itself motivates improved preparation. Scientists and engineers are using these events to argue for ready-to-launch “rapid response” probes in the future. The public, too, finds the idea thrilling: the notion of chasing down an alien chunk of rock or ice in space is straight out of a sci-fi adventure. The more interstellar objects we find, the stronger the case for attempting a robotic intercept or even sample-return in the future. Imagine physically retrieving material from an interstellar comet – it would be a scientific bonanza, potentially revealing isotopes and compounds in far greater detail than remote sensing can. Every time an interstellar visitor like 3I/ATLAS shows up, it fuels that ambition and keeps the conversation going about what humanity might do when the next one comes.

In conclusion, Comet 3I/ATLAS has captured both scientific and public imagination. It combines the allure of a cosmic mystery, the thrill of a groundbreaking discovery, and the promise of new knowledge about worlds beyond our own. As NASA and other agencies have demonstrated by pointing every instrument they can at this comet ^{[124] [125]}, the scientific community is deeply invested in learning all it can from this fleeting visitor. And judging by the media headlines and social media buzz (and even the fringe alien theories), the public is equally enthralled by the idea of an interstellar interloper in our skies.

The story of 3I/ATLAS is still unfolding – with perihelion approaching, we might yet see the comet flare in brightness or exhibit new behavior. By December 2025, it will disappear from view, heading back into the dark of interstellar space, perhaps never to return. But it will not have passed in vain. In a matter of months, 3I/ATLAS has greatly expanded our understanding of interstellar comets: it has shown us a chemical kinship with our own comets (water and CO₂ abound) ^[126], while also highlighting intriguing differences (an *8:1 CO₂/H₂O ratio! ^[127]). It’s validated the capabilities of next-generation observatories like SPHEREx and JWST in doing cosmic chemistry. And it’s reinforced the notion that the galaxy is full of wandering debris, some of which will occasionally pay us a visit. Each such visitor holds lessons about the broader cosmos. As we prepare for the possibility of many more interstellar objects in the future, the tale of 3I/ATLAS will be remembered as a landmark – the moment we detected a carbon dioxide fog around an alien comet and realized just how connected we are to the rest of the galaxy ^{[128] [129]}. The excitement surrounding this comet – from the halls of academia to internet forums – speaks to our enduring human curiosity: when a piece of another world arrives at our doorstep, we can’t help but look, wonder, and learn.

Sources:

• Space.com – “The interstellar comet 3I/ATLAS is wrapped in carbon dioxide fog, NASA space telescope reveals” ^{[130] [131] [132] [133] [134]} (Space.com, Sept. 4, 2025)
• SPHEREx/Caltech News – “SPHEREx Discovers Extended Carbon Dioxide Coma in Interstellar Object 3I/ATLAS” ^{[135] [136]} (Aug. 21, 2025)
• Universe Today – “3I/ATLAS’s Coma Is Largely Carbon Dioxide” ^{[137] [138] [139]} (Aug. 31, 2025)
• Live Science – “JWST reveals something strange with interstellar comet 3I/ATLAS” ^{[140] [141] [142]} (Aug. 27, 2025)
• Space.com – “New interstellar object 3I/ATLAS: Everything we know so far” ^{[143] [144] [145] [146]} (July 7, 2025)
• Space.com – “Astronomers say 3I/ATLAS is ‘very likely to be the oldest comet we have ever seen’” ^{[147] [148]} (July 11, 2025)
• Wikipedia – “2I/Borisov” ^{[149] [150]} (retrieved 2025) – on Borisov’s discovery and composition.
• NASA Science – “NASA’s New SPHEREx Mission Observes Interstellar Comet” ^[151] (Aug. 25, 2025) – official NASA update.
• Space.com – “Why scientists are so excited about the newfound interstellar comet 3I/ATLAS” (op-ed) and various news articles on 3I/ATLAS ^{[152] [153]}, for expert quotes and context.

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