The European Space Agency's ExoMars Trace Gas Orbiter has captured unprecedented close-range data on the interstellar object 3I/ATLAS (C/2025 P1) as it passed perihelion-its closest approach to the Sun-at roughly 1.35 to 1.40 astronomical units (AU), or just over 200 million kilometers. The object, now hidden behind the Sun in a phase known as solar conjunction, has temporarily disappeared from Earth's view, but spacecraft near Mars continued observing its activity.
ESA officials confirmed that ExoMars was in a uniquely favorable position to monitor the object as it moved near Mars earlier this month. The orbiter's high-resolution imaging and spectroscopic data are being described as among the most detailed ever collected for an interstellar visitor, providing new insight into its volatile composition and dynamic behavior.
Preliminary mission reports indicate that 3I/ATLAS exhibited significant outgassing activity even at its relatively distant perihelion. The NASA Neil Gehrels Swift Observatory detected hydroxyl gas (OH) within the object's coma - a byproduct of water molecule dissociation - confirming the release of water vapor despite the object's unusually cold and remote orbit. Scientists say this indicates an early or distant activation phase not typically observed in Solar System comets.
Adding to the intrigue, spectral data reveal anomalous gas ratios. The detected proportion of carbon dioxide (CO₂) to water (H₂O) in the comet's atmosphere was far higher than that seen in comets originating from the Kuiper Belt or Oort Cloud. Researchers believe this chemical imbalance may indicate that 3I/ATLAS formed in a colder, more volatile-rich region beyond the Solar System or that it has preserved pristine ices since entering interstellar space.
Spectral data show that 3I/ATLAS displays an unusually high ratio of carbon dioxide (CO₂) to water (H₂O) compared with comets originating from the Kuiper Belt or Oort Cloud, according to mission reports. Researchers say this chemical imbalance may indicate that the object formed in a colder, more volatile-rich region beyond the Solar System or that it has preserved pristine ices since entering interstellar space.
Although the majority of astronomers classify 3I/ATLAS as a natural comet, Harvard astrophysicist Avi Loeb has again drawn attention for entertaining a different hypothesis - that the object's hyperbolic path, reflective anomalies, and early activation might point to artificial origins. ESA and NASA have since emphasized that no data currently support this theory, and all observations remain consistent with natural cometary behavior.
The next observation window is scheduled for mid-November 2025, when 3I/ATLAS re-emerges from behind the Sun. ESA and NASA plan coordinated monitoring through several major instruments, including the James Webb Space Telescope, Hubble Space Telescope, Mars Reconnaissance Orbiter, and Mars Express. Further studies are expected in March 2026, when NASA's Juno spacecraft will capture data as the comet passes within 59 million kilometers of Jupiter.
