NASA and other observatories are racing to study 3I/ATLAS, only the third known interstellar object, after data showed a carbon-dioxide-dominated coma and other unusual chemical signals.
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Astronomers studying interstellar comet 3I/ATLAS say new observations point to an unusual chemical mix in its coma—especially a very high carbon dioxide-to-water ratio—just days before the comet’s closest approach to Earth on Dec. 19.
NASA says the object poses no danger and will remain far away, but the flyby is a rare chance to compare material from another planetary system with comets born around the Sun.
The comet, discovered on July 1, is being tracked across the solar system by multiple NASA spacecraft and space telescopes, along with major ground-based observatories.
What is 3I/ATLAS, and why it matters
Comet 3I/ATLAS is only the third known object confirmed to have entered the solar system from interstellar space, making it an unusually valuable target for planetary science.
NASA says the comet was first reported by the NASA-funded ATLAS survey on July 1, 2025, triggering rapid follow-up by space- and ground-based telescopes.
Because interstellar visitors spend limited time close enough to study in detail, researchers are using this window to measure its composition, activity, and trajectory as precisely as possible.
One reason for the intense interest is that comets preserve frozen material from their formation environment, meaning an interstellar comet can act as a sample of chemistry from a different star system.
NASA describes the current campaign as a broad, “solar system-wide” observing effort that aims to identify how 3I/ATLAS differs from “home-grown” comets.
Earth flyby: when, how close, and how fast
NASA says 3I/ATLAS will make its closest approach to Earth around Friday, Dec. 19, at about 170 million miles (270 million kilometers), roughly 1.8 astronomical units.
That distance is nearly twice the Earth–Sun distance, and NASA emphasizes there is no danger to Earth from the flyby.
At discovery, the comet was moving at about 137,000 miles per hour (221,000 kilometers per hour), highlighting how quickly interstellar objects traverse the inner solar system.
NASA also reports that the comet was observed from Mars at much closer range earlier in its passage, when it went by Mars at about 19 million miles, allowing spacecraft there to collect valuable images and ultraviolet data.
After the Earth flyby, NASA says the comet will continue outward and is expected to pass the orbit of Jupiter in spring 2026.
Key dates and observations
| Milestone | Date (2025–2026) | Why it matters |
| Discovery reported by ATLAS survey | July 1, 2025 | Start of global follow-up and orbit confirmation. |
| Hubble nucleus size constraint (based on Hubble observations) | Aug. 20, 2025 | NASA estimates the nucleus is at least ~440 m and at most ~5.6 km in diameter. |
| JWST infrared spectroscopy (inbound) | Aug. 2025 (reported in published/preprint results) | Finds a CO2-dominated coma and measures an unusually high CO2/H2O ratio. |
| Closest approach to Earth | Dec. 19, 2025 | Best geometry for many observations; still safely distant. |
| Expected passage near Jupiter’s orbit | Spring 2026 | Marks its continued exit through the outer solar system. |
The “unusual chemistry” scientists are seeing
The most discussed result so far comes from infrared spectroscopy with the James Webb Space Telescope, which found a coma dominated by carbon dioxide and detected multiple volatiles and components including H2O, CO, water ice, dust, and a tentative detection of OCS (carbonyl sulfide).
In the same JWST analysis, researchers reported a coma CO2/H2O mixing ratio of about 7.6 ± 0.3, described as among the highest observed in any comet and significantly above typical trends seen in many solar-system comets.
The study argues the measurements are consistent with a nucleus intrinsically rich in CO2 ice and discusses scenarios such as formation near a CO2 ice line in its parent disk or differences in radiation processing compared with solar-system comets.
Other observing teams have highlighted additional chemical clues at different wavelengths, suggesting a complex and evolving coma as the comet responds to solar heating.
A Space.com report summarizing European Southern Observatory Very Large Telescope work describes detections of atomic nickel vapor at large solar distances and notes the unusual aspect of nickel appearing without iron above instrument limits, raising questions about how metals are being released at relatively low temperatures.
The same report describes later detections of cyanogen (CN), a common cometary emission, as activity increased on the way toward the Sun.
Observers have also tracked visible-color changes as different molecules fluoresce in sunlight, which can help researchers infer what is being released and how the coma chemistry evolves with temperature and solar radiation.
Sci.News, citing a Gemini North observing effort, reports that the comet showed a faint greenish glow in later images, attributed to light emitted by gases in the coma including diatomic carbon (C2), a common source of green emission in comets.
How NASA is studying it (and what comes next)
NASA says at least a dozen of its assets have contributed images or data since discovery, including spacecraft stationed across the solar system as well as space telescopes.
The agency notes that Mars-based observations came from multiple missions, including imagery from Mars Reconnaissance Orbiter and ultraviolet measurements from MAVEN, plus a faint surface view by the Perseverance rover.
NASA also says heliophysics missions were able to track the comet when it appeared close to the Sun in Earth’s sky—an interval when many ground telescopes could not observe it—broadening coverage across the most challenging part of its trajectory.
In addition to composition, scientists are still trying to narrow down basic physical properties, including the size of the nucleus, the dust and gas production rates, and how the comet’s activity changes as it leaves the inner solar system.
NASA’s current public size constraint, based on Hubble data, places the nucleus somewhere between about 440 meters and 5.6 kilometers in diameter, reflecting both the challenge of measuring an active comet and the urgency of gathering more data before it fades.
After the Dec. 19 Earth flyby, the comet will steadily become harder to observe as it recedes, so many teams are prioritizing multi-wavelength work now to lock down the chemical “fingerprint” while the signal remains strong.
The approach of 3I/ATLAS offers an unusually timely opportunity to study interstellar material with modern instruments, and early results already suggest a coma chemistry unlike most comets typically measured in the solar system.
With the Dec. 19 flyby still safely distant, observatories are expected to continue coordinated observations to connect JWST’s CO2-rich measurements with visible and ultraviolet tracers, including emissions linked to carbon-bearing species and metals.
NASA says the campaign will continue as the comet heads outward toward Jupiter’s orbit in 2026, helping researchers place 3I/ATLAS in context with the earlier interstellar objects and refine models of how comet chemistry varies across planetary systems.
