Comet 3I/ATLAS was detected on July 1, 2025 by the NASA-funded ATLAS telescope in Río Hurtado, Chile. This object represents the third confirmed interstellar visitor to the Solar System, after 1I/ʻOumuamua and 2I/Borisov. The hyperbolic trajectory confirms its external origin, without a closed orbit around the Sun.
Pre-discovery observations date back to June 14, 2025, collected from archives of ATLAS telescopes at multiple locations and the Zwicky Transient Facility in California. Astronomers classified the comet due to its initial coma and gas ejections.
- Initial speed: around 61 km/s, or 221 thousand km/h.
- Current distance from the Sun: approximately 4.5 AU at detection.
- Naming: “3I” indicates the third interstellar, with “ATLAS” for the discovery system.
Experts monitor the object to map its composition, which differs from local comets.
Trajectory and safe approach to the Sun
The orbit of 3I/ATLAS takes it to a minimum distance from the Sun of 1.4 astronomical units, or 210 million kilometers, on October 30, 2025. This proximity is just within the orbit of Mars, without risk to Earth.
The comet remained at a distance of at least 1.8 AU from Earth, equivalent to 270 million kilometers. Its passage activates NASA’s planetary defense protocols, coordinated with the International Asteroid Warning Network.
Astronomers predict greater activityand with solar heating, including possible dust ejections. Terrestrial visibility persists until September 2025, with a reappearance scheduled for December.
Chemical composition defies known models
The James Webb Space Telescope detected carbon dioxide dominant in the 3I/ATLAS coma in August 2025. Other gases include carbon monoxide, carbonyl sulfide, and water ice.
These elements suggest formation in an ancient stellar environment, possibly in the thick disk of the Milky Way. Studies indicate an age between 3 and 11 billion years, higher than that of the Solar System.
- Core diameter: upper limit of 5.6 km, minimum of 440 meters.
- Early activity: observed at 6.4 AU from the Sun in May 2025.
- Enrichment: high levels of iron and nickel in ejected dust.
Webb data reinforce hypotheses about planetary formation in distant systems.
Space telescope observations advance
Hubble captured images on July 21, 2025, revealing an icy core with a drop-shaped dust cocoon. The exhibitionses show background stars striated by the comet’s hyperbolic velocity.
SPHEREx observed between August 7th and 15th, confirming the release of volatile compounds. The TESS mission recorded cometary activity months before official detection.
Other instruments, such as Swift and Parker Solar Probe, track changes to the surface. Collaboration with ESA includes missions such as Juice and SOHO for spectral data.
Instruments on Mars contribute to analysis
Mars probes, including Perseverance and Curiosity, have adjusted sensors to capture emissions from 3I/ATLAS during its close pass. The Mars Reconnaissance Orbiter provided high-resolution images in October 2025.
These observations detect interactions between the comet and the solar wind. Preliminary data shows unusual polarization in the ion tail, observed by ground-based telescopes in November.
The integration of terrestrial rovers expands the scope, measuring impacts on planetary atmospheres. Analysis continues to refine gaseous ejection models.
Recent anomalies intrigue the scientific community
The brightness of 3I/ATLAS increased fivefold between September and October 2025, according to measurements by STEREO-A and GOES-19. Color changes and non-gravitational acceleration of 1.1×10⁻⁶ AU/day² were recorded by ALMA.
These variations include deviation of four arc seconds from the predicted route. Observations on November 9, 2025, in Spain captured ion tailbent by the solar wind, despite lunar interference.
Astronomers note the absence of explosive outbreaks until August, but monitor potential fragmentation. The object reappears visible from November 11, 2025, in the pre-auroral eastern sky.
Global reactions and training exercises
The International Asteroid Warning Network began an exercise in October 2025 to improve orbital measurements. Participants with MPC codes register by November 7th, focusing on comet astrometry.
Astronomical communities in Chile and Hawaii reported marginal comas in July. The Vera C. Rubin Observatory captured serendipitous images in June, narrowing down the core’s diameter.
Studies on ArXiv, from September, highlight enrichment in heavy metals. IAWN emphasizes the opportunity for global telescope calibration.
Future of post-perihelion observations
3I/ATLAS follows an exit trajectory, without returning to the inner Solar System. Telescopes such as Europa Clipper and Lucy plan spectral collections in December 2025.
Data from SOHO during solar conjunction on October 21 recorded afterglow. Final analyzes estimate total mass and composition, based on observed ejections.
The passage concludes the study window in mid-2026, with files open for research. Amateur contributions via remote observatories such as PixelSkies add details to the database.
