Interstellar comet, with prefix 3I designates the comet as the third confirmed interstellar object passing through the Solar System, after 1I/′Oumuamua and 2I/Borisov.

This comet follows an unbound, hyperbolic Trajectory past the Sun, and will pass by Earth at 1.8 AU, posing no threat.

3I/ATLAS is an active comet consisting of a solid icy nucleus and a coma, which is a cloud of gas and icy dust escaping from the nucleus. The Sun is responsible for the comet′s activity because it heats up the comet′s nucleus to sublimate its ice into gas, which outgasses and lifts up dust from the comet′s surface to form its coma. Images taken by the Hubble Space Telescope and various interplanetary spacecraft suggest that the diameter of 3I/ATLAS's nucleus is less than 1 km (0.62 mi).

Observations by the James Webb Space Telescope (JWST) have shown that 3I/ATLAS is unusually rich in carbon dioxide, and contains a small amount of water ice, water vapor, carbon monoxide, and carbonyl sulfide. Observations by the Very Large Telescope have also shown that 3I/ATLAS is emitting cyanide gas and atomic nickel vapor, at concentrations similar to those seen in Solar System comets.

This comet came to be at the opposite side of the Sun from the Earth, (solar conjunction) on 21 October 2025, made its closest approach to the Sun on 29 October 2025 at a distance of 1.36 AU from the sun, which was between the orbits of Earth and Mars.

The comet appears to have originated from either the Milky Way′s Thin Disk or Thick Disk. If it had originated from the Thick Disk, the comet could be at least 7 billion year old, possibly older than the Solar System itself.

3I/ATLAS was discovered on 01 July 2025] by the NASA-funded ATLAS survey telescope at Río Hurtado, Chile (observatory code W68). At apparent magnitude 18, the newly discovered object was entering the inner Solar System at a speed of 61 km/s (140,000 mph; 220,000 km/h) relative to the Sun, located 3.50 AU (524 million km; 325 million mi) from Earth and 4.51 AU from the Sun, and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.

Comet 3I/ATLAS was given the temporary designation A11pl3' and the discovery observations were submitted to the International Astronomical Union′s Minor Planet Center (MPC). These observations initially suggested that the object could be on a highly eccentric path that might come close to Earth′s orbit, which led the MPC to temporarily list the object on the Near-Earth Object Confirmation Page until the orbit could be confirmed.

Follow-up observations from other observatories, involving both professional and amateur astronomers, began to reveal that the object′s trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory. Pre-discovery observations of 3I/ATLAS confirmed its interstellar trajectory; these included Zwicky Transient Facility (ZTF, observatory code I41) observations from 28 to 29 June 2025 that were found within a few hours of the initial report, ZTF observations from 14 to 21 June 2025,[1][32] and ATLAS observations from 25 to 29 June 2025. Amateur astronomer Sam Deen has noted additional ATLAS pre-discovery observations from 5 to 25 June 2025, and suspected that 3I/ATLAS was not discovered earlier because it was passing in front of the Galactic Center′s dense star fields, where the comet would be hard to discern.

Initial observations of 3I/ATLAS were unclear on whether it is an asteroid or a comet. Various astronomers including Alan Hale reported no cometary features, but observations on 2 July 2025 by the Deep Random Survey (X09) at Chile, Lowell Discovery Telescope (G37) at Arizona, and Canada–France–Hawaii Telescope (T14) at Mauna Kea all showed a marginal coma with a potential tail-like elongation 3 arcseconds in angular length, which indicated the object is a comet. On 2 July 2025, the MPC announced the discovery of 3I/ATLAS and gave it the interstellar object designation "3I", signifying it being the third interstellar object confirmed. The MPC also gave 3I/ATLAS the non-periodic comet designation C/2025 N1 (ATLAS). By the time 3I/ATLAS was officially named, the MPC had collected 122 observations of the comet from 31 different observatories.

3I/ATLAS follows an extremely hyperbolic trajectory past the Sun because it is moving too fast to be bound by the Sun′s gravity. When 3I/ATLAS entered the Solar System, it was moving at a speed of 58 km/s (36 mi/s; 1.02 AU/month) relative to the Sun, which speed is the comet′s hyperbolic excess velocity (v8) and prevents the comet from encircling the Sun making it return again.

As 3I/ATLAS came closer to the Sun and was pulled in by the Sun′s gravity, causing the comet to sped up, and then once it began moving away, the comet slowed down as the Sun′s gravity began pulling back on it. Even with this reverse pulling gravity, the comet will escape the Solar System all the same. 3I/ATLAS is moving far faster than the previous two interstellar objects 1I/′Oumuamua (v8=26 km/s) and 2I/Borisov (v8=32 km/s)

When 3I/ATLAS entered the Solar System, it was closely aligned with the orbital planes of the planets, also know as the ecliptic. Specifically, the comet′s trajectory is tilted 175° (which is retrograde to the orbits of all of the planets). Also, ti was inclined 5° with respect to the ecliptic. The trajectory of 3I/ATLAS will bring the comet close to the planets Venus, Mars, and Jupiter, but not Earth,[58] and hence poses no threat.

As 3I/ATLAS approaches perihelion, it passed by Mars at a distance of 0.194 AU (29.0 million km; 18.0 million mi) on 3 October 2025.After perihelion, it passed 0.65 AU (97 million km; 60 million mi) from Venus on 3 November 2025. It will pass 1.798 AU (269.0 million km; 167.1 million mi) from Earth on 19 December 2025, when it will be inside the asteroid belt and beyond the 4:1 Kirkwood gap. The comet will pass 0.358 AU (53.6 million km; 33.3 million mi) from Jupiter on 16 March 2026.

3I/ATLAS is a faint comet that does not get very close to Earth or the Sun and was not expected to get brighter than about apparent magnitude 11.5 as seen from Earth which is even less bright than Pluto at 13.4 magnitude. Even at its peak brightness, the comet was not visible to an observer on Earth using the naked eye observation (limit 7 magnitude) nor even or smaller binoculars (limit 18 magnitude).