Scientists led by the Southwest Research Institute announced on Monday (26) that the interstellar comet 3I/ATLAS releases microscopic aggregates of water ice and dust measuring about one micron or smaller. As the third confirmed object from outside our celestial neighborhood, the body travels on a hyperbolic trajectory through the Solar System at speeds exceeding 30 km/s, having peaked at 68.3 km/s during its closest solar approach. First spotted on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS), a NASA-funded survey facility in Rio Hurtado, Chile, the visitor provides astronomers with physical samples of planetary building blocks forged around a distant star.

International research teams rely on the James Webb Space Telescope to examine the chemical elements escaping from the traveling body. The trajectory posed zero collision risk to Earth, maintaining a minimum separation of 270 million kilometers—equivalent to 1.8 astronomical units—on December 19, 2025. Observational campaigns tracked the object through its perihelion, the point of closest approach to the Sun at 210 million kilometers (1.4 astronomical units), reached in late October 2025.

Spectroscopic measurements show a heavy concentration of frozen carbon dioxide alongside carbon monoxide and water vapor in ratios that set the body apart from native comets. Furthermore, the new study published in The Astrophysical Journal Letters on October 26, 2026, indicates that its tiny ice and dust grains share spectral traits with icy worlds beyond Neptune. The steep angle of its orbital path indicates that gravitational forces ejected the object from an alien star system billions of years ago.

Survey in the Atacama Desert and confirmation of the unbound orbit

Astronomers first logged the object—also designated C/2025 N1 (ATLAS)—during routine sky patrols conducted in Chile’s Atacama Desert, while archival images later traced its appearance back to May and June 2025. Those initial positional measurements immediately pointed to an open orbit free from the Sun’s gravitational capture.

Within days of the alert, planetary defense networks and global observatories verified its interstellar status after orbital calculations proved the body exceeded the solar escape velocity. The International Astronomical Union assigned the “3I” prefix to mark the third interstellar visitor in history.

Carbon dioxide frost, methanol levels and the one-kilometer core

Spectroscopic readings collected across multiple wavelengths confirmed that frozen carbon dioxide dominates the outer crust of 3I/ATLAS, pointing to an origin in the frigid outer regions of another protoplanetary disk at temperatures below -100°C. The volatile inventory also includes carbon monoxide, a smaller volume of water vapor than typically seen in local comets, and elevated methanol levels detected by the Atacama Large Millimeter/submillimeter Array (ALMA) and reported by the National Radio Astronomy Observatory on September 9, 2026.

See also: 3I/ATLAS interstellar comet shows high carbon dioxide levels during solar system passage

Brightness measurements place the solid nucleus at approximately 1 kilometer in diameter, with Hubble and non-gravitational estimates narrowing the core between 0.32 and 0.75 kilometers. As solar radiation heated the surface, outgassing intensified and built a carbon dioxide coma stretching roughly 700,000 kilometers around the center. “As 3I/ATLAS warms up, gas escaping from its interior carries ice and dust grains into the surrounding cloud, or coma. Those grains hold clues to the conditions where the comet was born,” said Dr. Silvia Protopapa, lead author of the Southwest Research Institute study.

Collaboration across 50 institutions and infrared spectroscopy

Ground-based telescopes in Hawaii and Spain logged daily structural changes in the coma, while the James Webb Space Telescope used its Near-Infrared Spectrograph (NIRSpec) across three sessions between August 2025 and April 2026 to map how the chemical compounds disperse. A separate analysis published in the journal Nature on June 22, 2026, led by astrochemist Martin Cordiner of NASA‘s Goddard Space Flight Center, also documented unusual ratios of carbon and deuterium (heavy hydrogen). The overarching monitoring effort brings together more than 50 scientific institutions across 20 countries to feed data into models of chemical evolution in extrasolar planetary disks.

Passage inside the Martian orbit and departure toward deep space

After swinging past its closest solar threshold at 246,000 km/h (68.3 km/s), the comet headed back toward deep space, maintaining a velocity above 40 km/s as it exits the Sun’s gravitational sphere of influence. The route carried the visitor just inside the orbit of Mars in late 2025, before growing distance ended visibility for optical telescopes in mid-2026.

Chemical contrasts with predecessors 1I/Oumuamua and 2I/Borisov

The physical behavior of 3I/ATLAS differs from the two previously cataloged interstellar objects, illustrating the wide chemical diversity of extrasolar environments. High-resolution instruments recorded distinct molecular emissions and a scarcity of the fine silicate dust commonly found in local comets:

• 1I/Oumuamua: Discovered in 2017, the first interstellar object showed no detectable cometary tail, whereas 3I/ATLAS displays an active gas coma and measurable surface erosion.
• 2I/Borisov: Spotted in 2019, the second visitor was rich in carbon monoxide, while 3I/ATLAS holds a substantially higher proportion of carbon dioxide (CO₂) and methanol.
• Sublimation and brightness: The conversion rate of ice into gas climbed by 20% for every million kilometers of solar approach, pushing the comet’s peak brightness to magnitude 10.