Astronomers from the International Astronomical Union confirmed on July 2, 2025 the interstellar origin of comet 3I/ATLAS, initially detected by the ATLAS system in Chile. The object, with a hyperbolic trajectory, represents the third visitor from outside the Solar System, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. This rarity drives observations coordinated by space agencies.
The discovery occurred on July 1, when the telescope recorded the body 20-30 kilometers in diameter between the constellations of Serpent and Sagittarius. Its activity, confirmed by fuzzy images, attracted immediate focus from the scientific community to analyze materials formed in another star system.
The comet passes perihelion in October 2025, approaching the Sun at 1.37 AU, releasing gases and dust for detailed study.
- Hyperbolic trajectory indicates no return to the Solar System.
- Core estimated at 320 meters to 5.6 km by Hubble observations.
- Current speed of 1.5 inches per minute, with observed acceleration.
Initial detection details
The NASA-funded ATLAS system identified 3I/ATLAS as a pweak point in observations from June 2025, retrieved from archives. Confirmed on July 2 by the Nordic Optical Telescope, the object exhibited marginal coma and tail elongation, active cometary characteristics.
Subsequent research in Chile and Arizona validated its interstellar nature, with academic studies predicting interactions with space missions.
The mobilization included ground and space telescopes to map its evolution.
Chemical composition revealed by James Webb
The James Webb Space Telescope observed the comet in August 2025, detecting a coma dominated by carbon dioxide (CO₂) in an 8:1 ratio with respect to water (H₂O). This composition exceeds local comet standards, suggesting formation in cold regions of another system.
High levels of carbon monoxide (CO), cyanide and nickel vapor were recorded, with water ice in smaller quantities. Experts analyze whether cosmic radiation altered these elements during the trip.
The crust irradiated from the core, estimated at 15-20 meters, converts CO to CO₂ in sunlight, explaining the observed dominance.
These detections occur 277 million miles from Earth, with data processed to compare isotopic signatures.
Observations from probes on Mars
The European Space Agency recorded 3I/ATLAS between October 1 and 7, 2025, using ExoMars TGO and Mars Express 19 million miles away. Stacked imagesrevealed stellar traces, confirming stable activity without outbreaks until September.
NASA’s Swift UVOT detected hydroxyl (OH) in July and August, tracing water vapor in the coma. SPHEREx observed from August 7 to 15, identifying carbonyl sulfide among the gases released.
These close passes to Mars provide data on atmospheric interactions.
- ExoMars TGO: Focus on trace gases.
- Mars Express: Dust and Ion Analysis.
- Swift UVOT: Ultraviolet to volatile spectra.
Global monitoring campaign
Hubble captured an image on July 21, 2025, showing a drop-shaped dust cocoon around the nucleus, with stars streaked by the hyperbolic trajectory. TESS and Martian probes are part of the network, coordinated by NASA and ESA.
Ongoing studies are examining the ionic tail, with Clipper expected to cross Europe between October 30 and November 6, although without official confirmation of maneuvers.
The comet reaches maximum proximity to Earth on December 19, at 268 million kilometers, allowing terrestrial observations limited by solar brightness.
Current trajectory and visibility
3I/ATLAS travels from ancient regions of the Milky Way, estimated to be 10 billion years old, speeding past local comets. Its elongated, almost symmetrical shape appears in imageshigh resolution, possibly reshaped by radiation.
Visible at magnitude 11.7 on November 5, with a bright coma and faint tail opposite the Sun, the object remains distant from Earth at 2.23 AU.
Astronomers use the passage to train threat detection, improving impact alerts.
The post-perihelion proximity in mid-November will reveal the post-solar phase, escaping the stellar glare.
Advances in interstellar studies
The third extrasolar comet offers clues about planetary diversity, with chemical anomalies challenging cometary ebullition models. Infrared observations from JWST highlight its potential for mapping distant stellar origins, integrating data from multiple missions for a complete profile.
This ongoing analysis, with no detections of new elements to date, reinforces the understanding of formative processes beyond the Solar System, with a focus on volatiles and dust for future comparisons.
