Traveling at speeds exceeding 30 kilometers per second along a hyperbolic trajectory, the comet 3I/ATLAS is crossing the solar system as the third confirmed interstellar object in history. The celestial body was identified on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS), an astronomical survey operating in Chile. Because the object originated outside our stellar neighborhood, its passage allows researchers to analyze matter formed in distant planetary environments.
Astronomers are using instruments including the James Webb Space Telescope to examine the chemical makeup of the visitor. The object poses no collision risk to Earth, maintaining a minimum distance of 270 million kilometers from the planet, while its perihelion — the point of closest approach to the Sun — takes place at a distance of 210 million kilometers.
Observations of the comet have identified three primary characteristics that define its origin and structure:
- High concentrations of frozen carbon dioxide on the surface.
- Simultaneous presence of water vapor and carbon monoxide, setting it apart from local comets.
- An orbital path indicating the body was ejected from another star system billions of years ago.
3I/ATLAS は太陽の背後にありません。
— 3I/ATLAS (@3IAtlas_Anomaly) 2025 年 10 月 28 日
天文学者らは、この星間物体は 10 月 29 日の近日点中に太陽の背後に消えるだろうと告げられました。しかし、新しい観測結果は、それが真実ではないことを示しています。
10 月 25 日、チームは 3I/ATLAS が太陽の横に隠れていないことを記録しました。
位置: 赤緯 13 時間 38 分… pic.twitter.com/cqRoIn8PGT
Telescopes in the Atacama Desert confirmed the unbound orbit
The ATLAS survey recorded the comet during routine monitoring operations in the Atacama Desert. Preliminary tracking data immediately indicated that the object was not gravitationally bound to the Sun.
Subsequent orbital calculations confirmed a velocity higher than the solar system escape speed. Within days of the initial sighting, an international team of astronomers verified its classification as an interstellar object.
Frozen carbon dioxide dominates the one-kilometer core
Spectroscopic analysis revealed that frozen carbon dioxide (CO₂) dominates the surface of 3I/ATLAS. This chemical signature indicates that the body formed in the outer regions of another star system at temperatures below -100°C.
Water vapor appears in smaller quantities than those typically observed in comets native to our solar system. The detection of carbon monoxide completes the volatile profile mapped by researchers.
Estimates based on brightness measurements show that the comet core has a diameter of approximately 1 kilometer. As the object approaches the Sun, thermal activity increases and releases gas that forms a visible coma around the nucleus.
Over 50 institutions in 20 countries track the chemical data
Ground-based observatories in Hawaii and Spain monitor daily changes in the comet structure. Meanwhile, the James Webb Space Telescope captures infrared imagery to map the spatial distribution of the ejected material.
The measurements collected during the passage feed directly into computer models of chemical evolution in protoplanetary disks, the dense rings of gas and dust that surround young stars. This scientific collaboration involves more than 50 institutions across 20 countries.
Velocity exceeds 40 kilometers per second on the outbound path
After passing its closest point to the Sun, the comet accelerates on its trajectory out of the solar system. As it leaves the gravitational influence of the Sun, its final velocity exceeds 40 kilometers per second.
The outbound path includes a crossing of the orbit of Mars in December 2025. Due to the increasing distance from Earth, optical observations of the object concluded in mid-2026.
Gas tail and chemical ratios differ from 1I/Oumuamua and 2I/Borisov
Unlike 1I/Oumuamua, the first known interstellar object which showed no cometary activity, 3I/ATLAS displays a distinct gaseous tail. When compared to the second confirmed visitor, 2I/Borisov, the comet exhibits a higher relative concentration of carbon dioxide.
These structural and chemical differences demonstrate the diversity of planetary formation processes across the galaxy. Each interstellar object provides distinct physical data regarding the environmental conditions of distant star systems.
Spectroscopy reveals absence of fine dust and rising sublimation rates
High-resolution instruments detect specific molecular emissions coming from the coma. The recorded spectrum confirmed the absence of the fine dust particles typically found in comets native to the solar system.
During its approach, the sublimation rate — the direct transition of ice into gas — increased by 20% for every 1 million kilometers the object moved closer to the Sun, with brightness models projecting a peak at magnitude 10.
