3I/ATLAS reaches closest point to the Sun and reveals composition with carbon dioxide
Comet 3I/ATLAS, the third interstellar object confirmed to visit the Solar System, reached its perihelion on Wednesday, October 29, 2025, at a distance of 204 million kilometers from the Sun. The event occurred at 8:47 am Brasília time, positioning the comet 1.36 astronomical units from the central star. Astronomers from agencies such as NASA and ESA monitored the passage to analyze the core’s activity under solar influence.
The safe distance prevented total fragmentation, allowing solar heat to cause ice sublimation and the formation of an observable coma and tail. This configuration offers data about the comet’s origin in a distant star in the Milky Way. Initial monitoring confirms stability in the structure, with jets of gas and dust detected by space telescopes.
Discovered in July 2025 by the ATLAS system in Chile, 3I/ATLAS travels at more than 210 thousand km/h in hyperbolic orbit. Its composition, rich in carbon dioxide – eight times higher than water – differs from local comets and suggests formation conditions in remote systems. Observations from the James Webb Telescope revealed this chemical anomaly weeks before perihelion.
- Perihelion distance: 204 million km (1.36 AU).
- Average speed: 210 thousand km/h.
- Estimated core diameter: Up to 5.6 km.
Initial discovery details
Amateur astronomers processed images from the CCOR-1 coronagraph on the GOES-19 satellite to identify the comet behind the Sun. Early detection on July 1 allowed continued tracking ahead of the solar conjunction on October 21. This initial visibility revealed a reddish dust coma, similar to comet 2I/Borisov.
The object has exhibited cometary activity since May 2025, when it was 6.4 AU from the Sun, according to data from NASA’s TESS satellite. Telescopes like Gemini South captured images in August showing a 0.5 arcminute tail. These initial observations confirm that 3I/ATLAS poses no threat to Earth, maintaining a minimum distance of 269 million km in December.
Chemical composition revealed
Spectroscopic analyzes indicate a dominance of carbon dioxide in the coma, with an unprecedented concentration among known comets. This proportion, six times above the expected variation, points to origins in an ancient galactic thick disk. The Hubble Space Telescope detected a reddish color in the dust, evolving throughout July.
Studies by the Instituto de Astrofisica de Canarias, in July, confirmed diffuse activity with a tail lengthening of 3 seconds of arc. With no explosions recorded until August, the brightness remained stable at magnitude 11-12. These chemical elements provide clues about planetary formation processes in distant systems.
The higher solar escape velocity reinforces the pathinterstellar ory, with an eccentricity of 6, greater than Borisov’s 3.4. Observations by the Nordic Optical Telescope in July validated an active appearance, without asteroid features.
Activity during perihelion
Solar heat intensified sublimation, increasing the comet’s brightness to magnitude 9, according to preliminary studies. Instruments such as SOHO and STEREO-A recorded rapid increase, exceeding rates of Oort Cloud comets at similar distances. This luminous acceleration remains without complete explanation.
The geometry of the solar conjunction hid the event from Earth, but space probes captured data in real time. An unusual antitail and gas jet emerged, indicating vigorous response to heating. Platforms such as the JPL Small-Body Database have predicted peak activity without significant orbital changes.
Upcoming planetary passages
After perihelion, 3I/ATLAS moves away from the Sun, passing 97 million km from Venus on November 3, 2025. This proximity allows observations by ESA’s JUICE mission, heading to Jupiter, between November 2 and 25. IAWN global campaign begins November 27 to refine orbital measurements.
On December 19, the comet reaches its closest point to Earth, 269 million km, in the constellation Virgo. Its position advances by 3 degrees daily, making it visible at dawn with 300 mm aperture telescopes. In March 2026, Jupiter influences it at 54 million km, slightly altering the outgoing trajectory.
- Initial observability: 3 dand November, northern hemisphere.
- Expected magnitude: 11-12 in November, falling to 13-14 in December.
- Jupiterian influence: March 16, 2026.
Global monitoring ongoing
The International Asteroid Warning Network coordinates observations to map variations caused by post-perihelion gas jets. Telescopes in Namibia record ice shelves, while the National Observatory in Brazil denies rumors of risks. These efforts collect data on mass and sublimation, without deviations that indicate collision.
The IAU campaign, which began on October 21, focuses on accurate astrometry to estimate non-gravitational shifts. Observatories such as Canada-France-Hawaii have confirmed marginal coma since July. This international cooperation guarantees continuous coverage until the exit of the Solar System.
