3I/ATLAS is an interstellar comet discovered on 1 July 2025 by the ATLAS survey station, also known as C/2025 N1 (ATLAS). It follows an unbound, hyperbolic path past the Sun and passed Earth at 1.8 AU, posing no threat. The "3I" designation marks it as the third confirmed interstellar object to pass through the Solar System, after 1I/ʻOumuamua and 2I/Borisov.
Discovery and orbit
The comet was first spotted by the NASA-funded ATLAS telescope in Chile while entering the inner Solar System at about 61 km/s. Early data suggested a path that might approach Earth, but follow-up observations from many observatories showed that its trajectory was hyperbolic and therefore interstellar. Pre-discovery images taken by several surveys confirmed this path. Its position near the galactic plane and the dense star fields of the Galactic Center made it hard to detect earlier.
Composition and activity
3I/ATLAS is an active comet with a solid icy nucleus surrounded by a coma of gas and dust. Sunlight heats the nucleus, causing its ice to turn into gas that carries dust away. Hubble and other spacecraft suggest the nucleus is less than 1 km across. The James Webb Space Telescope found the comet unusually rich in carbon dioxide, with smaller amounts of water, carbon monoxide, and methane, while ground telescopes detected cyanide and nickel vapor at levels similar to Solar System comets.
Perihelion and origin
The comet reached its closest point to the Sun on 29 October 2025, at 1.36 AU, between the orbits of Earth and Mars. Its trajectory suggests it came from either the thin or thick disk of the Milky Way. If it originated in the thick disk, it could be at least 7 billion years old, older than the Solar System.