Astronomers at The object, discovered on July 1, 2025 by the ATLAS telescope at Chile, is the third confirmed interstellar visitor to the solar system, after ‘Oumuamua and 2I/Borisov. The structural change, which included the disappearance of the antitail and the emergence of a conventional tail of more than 56,000 kilometers, occurred on November 5, sparking debate about its chemical composition.
The detection took place during routine observations, with no immediate impact on Terra, as the comet maintains a minimum distance of 270 million kilometers. Cientistas point out that the speed of 58 km/s and the hyperbolic orbit confirm its external origin. Nasa coordinated missions such as Hubble and James Webb to analyze the phenomenon.
Initial detection and interstellar confirmation
Comet 3I/ATLAS appeared in dense fields near the galactic center, which delayed its identification until July 2025. Preliminary Observações from the ATLAS telescope on Río Hurtado, Chile, indicated cometary activity, such as marginal coma and 3 arcsecond elongation.
On July 2, Telescópio Óptico Nórdico confirmed diffuse activity, with a reddish color in the coma, similar to 2I/Borisov. Equipes of Instituto of Astrofísica of Canarias used the Telescópio Duplo of
The 3I designation came from Minor Planet Center on July 2, marking it as interstellar for its high W vertical velocity relative to nearby stars.
Physical characteristics and composition revealed
The composition includes dominant carbon dioxide, water, traces of cyanide and an unusual nickel alloy, detected by James Webb on August 6, 2025. Espectroscopia of Very Large Telescope on August 21 confirmed cyanide and nickel in the coma.
The core measures between 440 meters and 5.6 kilometers in diameter, estimated by Hubble on July 21. Imagens showed a dust droplet-shaped cocoon surrounding the icy core.
In October, the Perseverance probe at Marte captured long-exposure images, recording the comet crossing the sky with stars as traces.
The 13% mass loss after perihelion on October 29-30, 2025 suggests atypical outgassing, with acceleration not compatible with standard sublimation.
Observations from multiple space missions
Nasa missions have contributed crucial data since September. The Lucy probe observed on September 16, while STEREO-A recorded from September 11 to 25.
MAVEN captured images on October 9, and Esa/Nasa’s SOHO monitored on October 15 and 16. The TESS satellite has indicated possible activity since May 7, 2025, at 6.4 astronomical units from Sol.
- Hubble performed ultraviolet spectroscopy in November to measure gas emissions and sulfur-to-oxygen ratio.
- James Webb scheduled new observations in December 2025, post-perihelion.
- Vera C. Rubin Observatory imaged serendipitously from June 21st to July 3rd.
These efforts form a mosaic of data, allowing comparisons with solar comets.
Changes in tail structure and color
The antitail, a jet of solar material, appeared in July and August, but disappeared abruptly in November. Substituiu itself by a conventional tail, growing to 56 thousand kilometers.
The color changed to intense blue tones, linked to ionized carbon monoxide, observed by Hubble and James Webb. Extreme Polarização in reflected light, unprecedented in comets, suggests unusual particle alignment.
Erratic variations in the nucleus indicate activity beyond that expected for helated bodies. Astrofotógrafo Michael Jäger captured image on November 28, 2025, showing sharp anti-tail, defying cometary physics.
These transformations occur as the comet crosses Virgem and Leão in December.
Scientific debates on anomalous acceleration
The 3I/ATLAS acceleration exceeds outgassing models, raising questions about dynamics. Avi Loeb, from Harvard, listed 12 anomalies in November 2025, including solar jet and ecliptic alignment with 0.2% probability.
Other astronomers, such as Chris Lintott, criticize the speculation, stating that the object exhibits classic comet signatures. Scott Manley and David Kipping point out flaws in the statistical analysis of Loeb.
On November 13, radio astronomy techniques detected emissions for the first time, validating active volatile reserves. Very Large Array plans further observations.
- Tilted trajectory suggests origin in the thick disk of Via Láctea, estimated to be 7.6 to 14 billion years old.
- Cyanide emission detected on August 22 by Observatório Lowell.
- No technology signature confirmed by Nasa in November briefings.
The discussion continues in bulletins such as MPEC 2025-U142.
Current trajectory and visibility
Perihelion occurred at 1.36-1.4 astronomical units from Sol, within the orbit of Marte. Aproximação minimum to Terra on December 19, 2025, at 1.8 astronomical units.
In March 2026, it will pass within 50 million kilometers of Júpiter, possibly altering its exit. Magnitude apparent below 12 requires telescopes for observation.
- Visible in the pre-dawn sky until spring 2026.
- It passes behind Sol in October, reappearing in December.
- Probe Hera can immerse in the ion tail from October 25 to November 1, 2025.
Amateur astronomers anywhere in Terra can track it now.
Implications for interstellar studies
3I/ATLAS offers a time capsule of another system, with methanol-rich chemistry detected by ALMA. Isso suggests formation in a protoplanetary disk with conditions different from those of Sol.
Comparisons with ‘Oumuamua and Borisov highlight variations in composition and activity. The detection reinforces the importance of networks like ATLAS for planetary defense.
Continued observations by JWST in December target post-perihelion emissions. The object traveled millions of years through interstellar space before entry.
Studies indicate that 68% confidence points to an advanced age of stars in the thick disk. Dados of radio emissions on November 13 confirm preserved primordial material.
