Third Interstellar Visitor Arrives in the Inner Solar System
October 29, 2025, marks a significant milestone in human astronomical history—the third confirmed interstellar object, comet 3I/ATLAS, reached its perihelion (closest approach to the Sun) at a distance of just 1.36 astronomical units (approximately 203 million kilometers), positioned between the orbits of Earth and Mars. This mysterious visitor from beyond our solar system has captured intense global scientific attention since its discovery on July 1, 2025, by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Chile.
According to official data from NASA’s Science Mission Directorate, 3I/ATLAS entered the solar system with an inbound velocity of 44 km/s, far exceeding the speeds of the previous two interstellar visitors—1I/’Oumuamua at 26 km/s and 2I/Borisov at 32 km/s. This exceptionally high relative velocity directly confirms its interstellar origin, as no object gravitationally bound to the Sun could achieve such speeds.
Breakthrough Discovery: Extreme Nickel Vapor Emission Phenomenon
The most intriguing characteristic of 3I/ATLAS is undoubtedly its anomalous metal vapor emission behavior. On July 20, 2025, when the comet was still 3.88 astronomical units from the Sun, the Very Large Telescope (VLT) spectrographs at the European Southern Observatory first detected significant atomic nickel (Ni I) vapor signals in the comet’s coma. This discovery immediately shocked the academic community, as metals typically should not vaporize at such distant, cold locations.
In-depth reporting by Space.com indicates that ultraviolet spectroscopic analysis detected both cyanide (CN) gas and atomic nickel vapor, but even more puzzling is that observational data shows prominent nickel emission lines with virtually no evidence of iron. This differs completely from the chemical signatures of all known comets—typically, iron abundance should far exceed that of nickel. Only when the comet approached within 2.64 astronomical units of the Sun were faint iron emission lines detected.
Theoretical Hypothesis from Harvard Astrophysicist
Professor Avi Loeb, Chair of Harvard University’s Astronomy Department, proposed a widely discussed theory in his Medium research notes: the nickel vapor may originate from photodissociation of short-lived metal carbonyl compounds or organometallic compounds. In August 2025, the Keck Cosmic Web Imager further confirmed gas emissions from cyanogen (CN) and nickel (Ni), with Ni concentrated closer to the nucleus, supporting Loeb’s organometallic source hypothesis.
This chemical anomaly has sparked intense academic debate. Traditional comet theory suggests that cometary nuclei primarily consist of water ice, dry ice, and dust, with extremely low metal content existing in solid particle form. The high nickel-to-iron ratio and extremely early metal vaporization exhibited by 3I/ATLAS suggest it may possess fundamentally different chemical composition and formation history compared to solar system comets.
Age Estimation: 3 Billion Years Older Than the Solar System
Based on orbital dynamics analysis, astronomers speculate that 3I/ATLAS may be the oldest comet ever observed. Its orbital characteristics suggest the comet may have formed approximately 7.6 billion years ago—3 billion years before our 4.6-billion-year-old Solar System. This means 3I/ATLAS could be a relic from first-generation stellar systems, carrying precious information about the chemical environment of the early universe.
Research by the European Space Agency (ESA) notes that observations from the James Webb Space Telescope (JWST) and SPHEREx space observatory reveal that 3I/ATLAS is rich in carbon dioxide ice, indicating its formation in extremely frigid regions far from its parent star, where temperatures were cold enough to solidify carbon dioxide. This chemical composition resembles Kuiper Belt objects in our solar system, but differences in metal content hint at the uniqueness of different galactic environments.
Tail Evolution: The Mystery of the Anti-Solar Tail
Another anomalous feature exhibited by 3I/ATLAS is the dramatic change in its tail morphology. During July and August 2025, astronomers observed an “anti-solar tail” pointing toward the Sun—an extremely rare phenomenon. According to latest images from the Nordic Optical Telescope in the Canary Islands, this anti-solar tail transformed into a normal tail pointing away from the Sun in September.
Astrophysicist Avi Loeb explains that the anti-solar tail may be a visual phenomenon created by large dust particles under orbital geometry effects, or it may be related to the comet’s rotational axis orientation and jet activity. As the comet approached perihelion, the influence of solar radiation pressure and solar wind intensified, causing the tail morphology to change and eventually revert to typical ion tail and dust tail structures.
Coordinated Observations by Multiple Space Missions
The perihelion passage of 3I/ATLAS triggered a rare multi-mission coordinated observation campaign in space exploration history. On October 3, 2025, ESA’s ExoMars Trace Gas Orbiter (TGO) conducted close-range observations of the comet from 30 million kilometers away as 3I/ATLAS passed near Mars orbit. This marked the first time a Mars orbiter has observed an interstellar comet.
Even more exciting, according to Space.com reporting, NASA’s Europa Clipper Jupiter probe is predicted to potentially traverse the ion tail of 3I/ATLAS during the period of October 30 to November 6, 2025. This unexpected “tail crossing event” will allow the probe’s particle detection instruments to directly sample the plasma environment of an interstellar comet, providing unprecedented in-situ measurement data.
Safety Assessment for Earth
Despite widespread public attention and some media sensationalism surrounding 3I/ATLAS, both NASA and ESA have clearly stated that this comet poses no threat to Earth. Orbital calculations show that the comet’s closest approach to Earth will occur on December 19, 2025, at a distance of 1.80 astronomical units (approximately 269 million kilometers), far exceeding any potentially hazardous safety distance.
With a brightness of only around magnitude 20, 3I/ATLAS can only be observed through professional astronomical telescopes and is completely invisible to the naked eye. This contrasts sharply with spectacular naked-eye comets like Hale-Bopp in 1997 or NEOWISE in 2020. Astronomy enthusiasts wishing to track this interstellar visitor will need telescopes with at least 30 cm aperture and precise celestial positioning systems.
Milestone Significance for Interstellar Object Research
3I/ATLAS is the third confirmed interstellar object identified by humanity, preceded by the cigar-shaped 1I/’Oumuamua discovered in 2017 and comet 2I/Borisov discovered in 2019. Each interstellar visitor has brought new surprises and challenges to the astronomical community: ‘Oumuamua exhibited anomalous acceleration and an extreme axis ratio; Borisov was the first confirmed interstellar object with typical cometary characteristics; and now 3I/ATLAS, with its unique metal chemistry and extreme age, is once again rewriting our understanding of interstellar object diversity.
Experts at The Planetary Society note that with next-generation survey programs like the LSST (Vera C. Rubin Observatory) coming online in the coming years, several interstellar objects are expected to be discovered annually. This will fundamentally transform our understanding of interstellar material exchange, planetary system formation, and the origins of life. The detailed observational data from 3I/ATLAS will become a crucial benchmark for establishing standard models of interstellar objects.
Unsolved Mysteries and Future Research Directions
Despite the wealth of observational data already provided by 3I/ATLAS, many fundamental questions remain unanswered:
- The exact source mechanism of nickel vapor: Is it photodissociation of metal carbonyl compounds? Or does the nucleus contain some unknown organometallic mineral?
- The cause of the high nickel-to-iron ratio: Does this reflect unique chemical abundances in its parent stellar system?
- The actual size and mass of the nucleus: Currently estimated at approximately 500 meters in diameter, but precise measurements remain challenging
- Rotation period and shape: Light curve analysis suggests possible irregular rotation
- Verification of true age: Can the 7.6-billion-year estimate be verified through other methods?
As 3I/ATLAS gradually moves away from the Sun in November and returns to interstellar space, astronomers are seizing the final observation window period, using the Hubble Space Telescope, JWST, and ground-based large telescopes for high-resolution imaging and spectroscopic analysis. These data will continue to generate new scientific discoveries over the coming years, establishing a solid foundation for the emerging field of interstellar object studies.
The perihelion passage of 3I/ATLAS is not merely an astronomical observation event, but symbolizes humanity’s continuous expansion of the boundaries of cosmic knowledge. Every visitor from interstellar space is a “postcard” the universe sends us, telling stories of distant stellar systems and the mysteries of cosmic evolution.
