The James Webb super telescope has mapped a hardened layer between 15 and 20 meters thick covering the interstellar comet 3I/Atlas. This outer shell formed due to the continuous bombardment of galactic cosmic rays over billions of years of travel through deep space. Being the third visitor from outside our system already cataloged by science — following Oumuamua and Borisov — the celestial body underwent detailed analyzes in August 2025, at which time it was sailing 445 million kilometers from our planet.
Scientists from the Royal Institute of Space Aeronomy in Belgium lead the survey that details how carbon monoxide turned into carbon dioxide on the outside of the object. Originating from a distant star system, the rocky and icy body reached its closest point to the Sun, known as perihelion, on October 29, 2025. At a distance of 1.36 Astronomical Units from the star, the heat began the sublimation process of this hardened shell.
The visitor’s movement through space occurs in a hyperbolic trajectory at a speed exceeding 210 thousand kilometers per hour, a flight rate that attests to its origins from another region of the galaxy.
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- The initial identification took place on July 1, 2025, through the ATLAS observation complex, installed in Chile.
- The gas cloud around the nucleus has a predominance of carbon dioxide, exceeding the volume of water by eight times.
- Calculations indicate that the space rock could be up to 7 billion years old, which makes it older than the Solar System itself.
Continuous radiation action shaped the comet’s external structure
Extremely high-energy particles from other parts of the universe, known as galactic cosmic rays, have hit the object’s ice reserves for eons. This constant chemical interaction forced the conversion of monoxide to carbon dioxide, generating thick surface armor up to 20 meters deep.
Computational models based on data from comet 67P, previously studied by the Rosetta spacecraft, show that one billion years of exposure would be the minimum time needed to create this barrier. As the object traveled through deep space without the magnetic protection that surrounds our system, the impact of these particles occurred directly and severely.
Lack of solar magnetic shield altered the visitor’s chemistry
During its crossing of the Milky Way, 3I/Atlas did not have the protective bubble of the heliosphere, responsible for blocking much of the radiation that reaches the local planets and asteroids. This vulnerability scenario left the celestial body exposed to uninterrupted shocks that rewrote the chemical composition of its surface.
Celestial bodies formed in our cosmic neighborhood manage to keep their original characteristics intact precisely because of this solar barrier. In the case of the interstellar traveler, the outer shell functions as a historical record of galactic impacts, while the interior remains preserved.
Current mapping proves the transformative impact of prolonged radiation on bodies roaming deep space. Astronomers now plan to cross-reference this unprecedented information with the existing database on comets in our own system.
Maximum approach to the Sun caused the release of gases and dust
The passage through perihelion in late October 2025 caused high solar temperatures to melt part of the irradiated crust, ejecting material into space. This superficial melting opens a window of opportunity for observation equipment to access the internal, untouched layers of the rock.
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Astrophysics teams are comparing light signatures captured before and after maximum heating to try to decipher the rock’s original chemical recipe. The object’s luminosity gave a sudden jump, accompanied by a strong bluish color that indicates the burning of highly volatile compounds.
The gas halo surrounding the nucleus displays a massive amount of carbon dioxide, a behavior completely different from the pattern seen in comets neighboring Earth. Jets of ejecta were tracked by solar monitoring satellites such as SOHO and STEREO-A, even when the object was still in very cold zones.
Precision instruments detect anomalies in chemical composition
The NIRSpec spectrometer, operated aboard the James Webb Telescope, quantified the exceptional levels of CO2 in the comet’s temporary atmosphere. This abundance suggests that the body formed in an extremely cold, carbon-saturated zone on the outskirts of its star of origin.
Although the outer layer has been severely mutated, the comet’s heart holds primitive building blocks from other regions of the galaxy. The bright bluish hue that caught the researchers’ attention results directly from the accelerated heating of these carbon-based molecules.
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- The ratio between carbon dioxide and water reached 7.6 ± 0.3, the highest ratio ever documented in the history of astronomy.
- The nucleus expelled about 129 kilograms of CO2 per second during peak activity.
- The cloud around the object contained microscopic water ice particles less than 1 micrometer in size.
A detailed reading of the light bands confirms the intense chemical processing caused by cosmic rays, recording values for the proportion between carbon monoxide and water of around 1.65 ± 0.09.
Escape route and size of the nucleus impress researchers
Initially located in July 2025 in the direction of the Sagittarius constellation, 3I/Atlas follows a single-pass trajectory and will be ejected back into interstellar space after circumventing the Sun. The extreme speed of 210 thousand kilometers per hour acts as the definitive signature that it does not belong in our neighborhood.
Combined photographic records from the Hubble and James Webb telescopes revealed the structure of the tail and estimated the core’s diameter to be between 440 meters and 5.6 kilometers. A reddish color on the surface and strong ultraviolet light emissions began to be captured by the lenses in August 2025.
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The first signs that the ice was melting appeared in May 2025, when the visitor was 6.4 astronomical units away from our star. Even with the gradual loss of brightness after perihelion, the celestial body will continue to be monitored by large professional observatories.
The mathematical parameters used to calculate the evaporation of comets needed to be recalibrated to deal with this unusual chemistry, which even includes traces of heavy metals like nickel. Cross-checking data with 2I/Borisov, the previous interstellar visitor, reveals very similar patterns of extreme volatility.
Fundamental differences separate external visitors from local comets
Travelers from other stars, exemplified by 3I/Atlas, carry much more carbon dioxide than water in their temporary atmospheres, reversing the logic of comets that were born here. The radiation-burned crust hides an interior that functions as a veritable time capsule of galactic chemistry.
Measurements consolidated in August 2025 attest to this clear separation of categories and reinforce the destructive power of space radiation on nomadic objects. Terrestrial observation bases continue to monitor the evolution of the tail and the fluctuations in light emission.
The passage of this celestial body provides a golden chance to adjust theoretical models about the degradation of materials exposed to the galaxy’s hostile environment. Uninterrupted tracking of their exit route will help fill in historical gaps about how planetary systems form and evolve.