A Starry Intruder
3I/ATLAS (also known as C/2025 N1) is the third established interstellar object to pass through our Solar System, following in the footsteps of 'Oumuamua and 2I/Borisov. Found on July 1, 2025, by the ATLAS system in Chile, its hyperbolic orbit and astounding velocity instantly identified it as a traveler from outside our Solar System.
What's Inside? A Coma Full of CO₂ and Unexpected Gases
Recent observations made with the James Webb Space Telescope (JWST) showed that the coma surrounding 3I/ATLAS is made up of a predominantly carbon dioxide (CO₂)—a very high percentage that makes it different from other Solar System comets
The coma also has minor amounts of water vapor (H₂O), carbon monoxide (CO), carbonyl sulfide (OCS), water ice, dust, and even traces of the heavier isotope of carbon-13
. In particular, the CO₂/H₂O mixing ratio is approximately 8.0 ± 1.0, one of the highest ever recorded and significantly greater than observed trends in long-period or Jupiter-family comets
A Relic of the Ancient Galaxy
Observations suggest that 3I/ATLAS is extremely old, possibly 3-14 billion years old, possibly older than our Solar System itself
. Its orbital path and makeup imply that it could have formed from the galactic thick disk, perhaps from a time of high star formation activity called the "Galactic Cosmic Noon" (~9–13 billion years ago)
A Reddish Coma and a Faint Tail
High-resolution photographs—such as those of the Hubble Space Telescope and Gemini South—have shown a reddish coma for 3I/ATLAS, probably caused by irradiated organic molecules. Dust grains ejected from the nucleus are of varying size and velocity: tiny particles leave quickly, but larger ones travel more slowly
The comet has a pale, diffuse tail extending away from the Sun, which was progressively more observable over late August 2025. The tail will get progressively brighter as the observing geometry improves towards perihelion late in October
Swift Flyby and Encounters with Spacecraft
3I/ATLAS zooms through our Solar System at very high speed—estimated by some sources at about 150,000 mph
Since it travels past Mars and not Earth, it's no threat. Interestingly, its trajectory takes it close to a number of spacecraft: Psyche on Sept 4, Mars-orbiting missions on Oct 3, and JUICE on Nov 4 might catch its tail—not too close, but close enough to allow useful observations
Why This Discovery Matters
New window into ancient material: Its CO₂-rich composition could mean formation in unusually cold parts of the universe or hard radiation exposure—insights into early planetary disks outside our own
Galactic history revealed: As a probable thick-disk remnant, research on 3I/ATLAS assists tracing the chemical and dynamical history of our Milky Way
Spacecraft "visits" on a small scale: Flybys by current spacecraft provide a unique opportunity to see an interstellar visitor in place—potentially transforming our knowledge of such objects
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