The universe has just delivered a new photo of awe: the James Webb Space Telescope (JWST) seems to have captured the interstellar comet 3I/ATLAS making a flyby of Mars. This stunning observation provides new hints at the origins of this mysterious visitor from outside our own solar system — and how it acts as it speeds through the cosmos. Here's what we have so far.
A Brief Introduction to 3I/ATLAS
Discovery & identity
The comet was originally detected on 1 July 2025 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Chile.
Its official designation is 3I/ATLAS (the "3I" is to indicate that it's the third known interstellar visitor to our system).
Interstellar origin & trajectory
Unlike most comets that are gravitationally tied to the Sun, 3I/ATLAS is on a hyperbolic path, meaning that it is passing through and will not be staying in the solar system.
It's moving at incredibly high velocity — more than 130,000 mph (≈ 210,000 km/h) — as it enters, passes through, and leaves our solar neighborhood.
Significance
Previously, only two other interstellar objects have been confirmed: 1I/'Oumuamua and 2I/Borisov. 3I/ATLAS is gaining attention for being the largest and most chemically fascinating of the three.
How JWST Got in the Game
JWST's first look
On 6 August 2025, JWST aimed its Near-Infrared Spectrograph (NIRSpec) at 3I/ATLAS, recording its first high-fidelity infrared observations.
The wavelengths of the data range from approximately 0.6 to 5.3 µm, enabling astronomers to examine volatile gases, dust, and ice.
What JWST discovered
The coma of the comet is carbon dioxide (CO₂)-dominant. The ratio of CO₂-to-H₂O is extremely high — approximately 8.0 ± 1.0 — ranking among the most CO₂-dominant comets ever detected.
Aside from #CO₂, water (H₂O) spectral signatures, carbon monoxide (#CO), OCS (carbonyl sulfide), water ice, and dust were observed.
The COMA is not symmetrical, with increased outgassing on the sunward side — in other words, the side towards the Sun is more active.
Such a structure implies that 3I/ATLAS possibly formed close to a CO₂ ice line in its birth system, or otherwise was subjected to more intense radiation environments than comets within our system.
Since the comet was being seen while still relatively far from the Sun (at 3.32 au), the dust and gas activity is less dramatic than one would encounter near perihelion. But even in this subdued form, the chemical imprint is dramatic.
The Mars Flyby: Why Mars, Why Now?
Mars approach & observations
On 3 October 2025, 3I/ATLAS came relatively close to Mars — within ~19 million miles (≈ 30 million kilometers) of the planet.
The European Space Agency's ExoMars Trace Gas Orbiter (TGO) employed its CaSSIS instrument (Colour and Stereo Surface Imaging System) to image the comet as a faint "fuzzy blob" in the data.
Challenges & limitations
The comet is very faint — between 10,000 and 100,000 times fainter than the targets TGO typically measures.
Due to its faintness and exposure time limits, the instruments were not able to distinguish the comet's nucleus apart from the rest of the coma.
Mars Express also tried to image it, but with the shorter exposures (max 0.5 seconds) not showing the object.
What Mars imagery contributes
These images taken from Mars are the closest in-situ observations of 3I/ATLAS currently possible with either Earth-orbit or ground telescopes. Though not as spectrally data-rich as JWST's observations, they do constrain the object's trajectory, spatial dynamics, and brightness at near-Mars range.
What the Findings Imply (and What We Don't Know Yet)
Impressive CO₂ richness
The extremely high CO₂-to-H₂O ratio is exceptional in
comets and implies that 3I/ATLAS's nucleus has volatiles less typical of
ordinary solar system comets. It could have originated in a region of its
native system that preferentially condensed CO₂.
Clues about its origin
Since interstellar comets bring the "fingerprints" of a different system, these results can aid researchers in inferring where and how 3I/ATLAS originated — and how others are different from our own. Dominance by CO₂ may be an indicator of formation near a star's frost line (for CO₂) or experiencing more intense radiation.
Evolution as it approaches the Sun
As 3I/ATLAS heads towards its perihelion (nearest the Sun) — due about 30 October 2025 — solar heating will become stronger. Its activity (gas and dust emission) and tail can get stronger, showing more.
Questions to be answered & future direction
It is not known precisely how big, how shaped, and what state of rotation the nucleus is in.
How does its dust-to-gas ratio compare with that of solar system comets?
Will fresh observations in ultraviolet, radio, or longer wavelengths uncover more species or activity?
How common — or uncommon — is this mix among interstellar travelers?
Astronomers around the world are already booking telescopes (space- and ground-based) to keep following 3I/ATLAS on its journey.
Why This Matters
Every time an interstellar object comes, it presents a fleeting opportunity to glimpse another star system's chemistry and history. Interstellar comets have a different history compared to comets that formed in our Sun's neighborhood — and maybe, a different ingredient list for volatile material, dust, and organics.
By combining JWST's power spectrum with Mars-based
imagery, there is a synergistic window to 3I/ATLAS's nature and trajectory.
Even as the comet moves outward, the data gathered here will illuminate
cometary physics models, planetary system formation, and cosmochemical
diversity.
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