The interstellar comet 3I/ATLAS, the third confirmed object from outside the Solar System, reaches perihelion on October 29, 2025, at 210 million kilometers from the Sun. Detected by the ATLAS telescope in Chile on July 1, the body travels at 209,000 km/h and originates from another star system. Astronomers track the approach with no collision risk to Earth.
The comet shows intense activity with dust and gas jets forming a 10,000-kilometer tail. Its CO2-rich composition challenges traditional comet formation models.
- Speed: 209,000 km/h
- Perihelion distance: 210 million km
- Estimated age: over 7 billion years
Designation and hyperbolic orbit
The 3I prefix marks it as the third interstellar object after 1I/ʻOumuamua and 2I/Borisov. The hyperbolic orbit confirms origin beyond the Solar System.
Entry occurred in May 2025 at 6.4 astronomical units from the Sun. TESS satellite observations detected early activity at that stage.
Initial detection and imaging
Telescopes in Chile and Spain captured images in July showing a diffuse coma with reddish color. Zwicky Transient Facility pre-discoveries date back to June.
Cometary activity confirmation came on July 2 with subtle elongations indicating a tail. Initial data enabled precise speed and direction calculations.
Chemical composition details
The coma features CO2 levels eight times higher than water. Thermal barriers in the nucleus explain low water presence.
Swift telescope observations captured ultraviolet hydroxyl glow in July and August. Limited ice decomposition supports the findings.
European studies from August noted progressive coma reddening through July. Surface evolution aligns with prior interstellar comets but with greater intensity.
🚨 3I/ATLAS Over Moscow: A Strange Green Visitor Appears! 🟢👁️
— World News X (@WorldNews_X_) October 28, 2025
Comet 3I/ATLAS reaches its closest point to the Sun on October 28–29, then becomes visible from Moscow (Oct 30–Nov 17) as a faint green glow in the evening sky. 🌌 pic.twitter.com/lp4Ri0U1uC
Nucleus size and mass
The nucleus diameter ranges from 300 meters to 11 kilometers. Mass estimates reach billions of tons per Hubble data.
Micrometer-sized dust grains contribute to the observed reddish hue. James Webb infrared instruments map organic materials like irradiated tholins.
Planetary close approaches
The comet passed Mars on October 3 at 28 million kilometers. Probes including Trace Gas Orbiter and Perseverance recorded it as a bright point.
In November, ESA’s Juice probe will capture images using cameras and spectrometers. NASA’s Europa Clipper coordinates observations in late October.
Ionic tail particles may interact with Clipper instruments. Jupiter passage happens in March 2026 for further Juice analysis.
Jet activity and dust output
Gas and dust jets point sunward due to ice sublimation. Dust production rate hit 30 kg per second in July with 100-micrometer grains.
Negative light polarization suggests irregular surface. Nordic Optical Telescope confirmed active diffusion since July.
Steady brightness prevents fragmentations seen in other comets. Models predict stability through perihelion focusing on dust measurements.
Ongoing monitoring and departure
NASA coordinates with the International Asteroid Warning Network for training from November 2025 to January 2026. The comet crosses Jupiter’s orbit post-October and exits the Solar System.
It becomes unobservable from ground telescopes in December. Apps like Sky Tonight guide amateur views at magnitude 12.
The global campaign adds data to interstellar object catalogs. Annual detections forecast with future LSST operations.
