3I/ATLAS

3I/ATLAS, also known as C/2025 N1 (ATLAS) and previously as A11pl3Z, is an interstellar comet discovered on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station. The comet follows an unbound, hyperbolic trajectory past the Sun, and passed by Earth at 1.8 AU, posing no threat. The prefix "3I" designates it as the third confirmed interstellar object passing through the Solar System, after 1I/ʻOumuamua and 2I/Borisov.

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.

The comet came to solar conjunction on 21 October 2025, and it came closest to the Sun on 29 October 2025, at a distance of 1.36 AU (203 million km; 126 million mi) from the Sun, which is 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 3I/ATLAS originated from the thick disk, the comet could be at least 7 billion years old—older than the Solar System.

History

Discovery

3I/ATLAS was discovered on 1 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. It was given the temporary designation 'A11pl3Z' 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, 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.

Further observations

Observations by David C. Jewitt and Jane Luu using the Nordic Optical Telescope on 2 July 2025 confirmed that 3I/ATLAS was "clearly active" with a diffuse appearance. Miguel R. Alarcón and a team of researchers of the IAC (Instituto de Astrofísica de Canarias) using Teide Observatory's Two-meter Twin Telescope also found cometary activity on the same date. Multiple different telescopes showed that the comet's coma had a reddish color indicative of dust, similar to that of the previous interstellar comet 2I/Borisov. A study published by Toni Santana-Ros and colleagues in August 2025 reported that 3I/ATLAS's coma had become redder throughout July 2025, indicating an evolving surface or coma composition as a result of 3I/ATLAS's increasing cometary activity.

On 6 July, additional observations were published, including Zwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025. An even earlier precovery from 21 May 2025, made at Weizmann Astrophysical Observatory (M01), was published on 18 July 2025.

Polarimetric observations by the Very Large Telescope, Nordic Optical Telescope, and Rozhen Observatory from July and August 2025 revealed that 3I/ATLAS's coma exhibits an unusually high degree of negative polarization at small phase angles—meaning a large percent of the light reflected from 3I/ATLAS's coma have their oscillations oriented along the Sun-comet-observer plane. The negative polarization of 3I/ATLAS appears similar to those seen in trans-Neptunian objects, and suggests that its coma is made of a mixture of icy and dark material.

The newly commissioned Vera C. Rubin Observatory has serendipitously imaged 3I/ATLAS during its science validation observations from 21 June to 3 July 2025. These observations showed a slight increase in the comet's coma diameter and provided constraints on the comet's nucleus diameter. The Vera Rubin Observatory would have discovered 3I/ATLAS before the ATLAS survey if it had begun its science validation observations two weeks earlier.. Rubin continued observing 3I/ATLAS until 20 July 2025, when the comet was no longer observable from Chile while the Sun was below the horizon; the observatory's 3200-megapixel LSST Camera collected nearly 100 exposures across this period, capturing the comet's nightly evolution as it approached the Sun.NASA's Transiting Exoplanet Survey Satellite (TESS) had also observed 3I/ATLAS before it was discovered, with observations from 7 May to 3 June 2025. These observations showed that the comet was already bright and active even when it was roughly 6.4 AU away from the Sun in May 2025, which indicates the comet's activity is likely caused by the sublimation of volatile ices other than water.

Water ice in 3I/ATLAS's coma was first reported on 20 July 2025, based on near-infrared spectroscopic observations by the Gemini South and NASA Infrared Telescope Facility on 5 and 14 July 2025. Ultraviolet observations by the Swift Observatory suggested the presence of water vapor and hydroxide ions in 3I/ATLAS's coma on 30 July 2025 and 1 August 2025. On 21 August 2025, astronomers of NASA's SPHEREx mission and the California Institute of Technology reported the detection of water ice and bright carbon dioxide gas emission in SPHEREx observations from mid-August 2025. On 22 August 2025, astronomers at Lowell Observatory reported the first tentative detection of cyanide emission in 3I/ATLAS. Spectroscopic observations by the Very Large Telescope on 21 August 2025 confirmed the presence of cyanide and also detected nickel in 3I/ATLAS's coma.

The Hubble Space Telescope took its first images of 3I/ATLAS on 21 July 2025, which revealed its coma in high detail and constrained its nucleus diameter to below 5.6 km (3.5 mi). The Hubble images were publicized by NASA and the European Space Agency on 7 August 2025. On 6 August 2025, JWST made its first observations of 3I/ATLAS using its NIRSpec instrument, and results were announced by NASA on 25 August 2025. In November 2025, Hubble performed ultraviolet spectroscopy on 3I/ATLAS to determine the composition of its gas emissions and sulfur-to-oxygen ratio, and the telescope will monitor the comet on its way out of the Solar System. The JWST is scheduled to make its next observations of 3I/ATLAS in December 2025, after the comet's perihelion.

The comet was observed by the James Clerk Maxwell Telescope on 7 September 2025, when 3I/ATLAS was located 2.33 AU from the Sun, and found a hydrogen cyanide production rate of (1.5±0.5)×10²⁵ molecules per second, while a week later, on September 14, the production rate had climbed to (4.5±1.9)×10²⁵ molecules per second (2 kg/s). The cyanide coma on 15 September was about 180,000 kilometers across and was asymmetric, being elongated along the anti-solar direction. On the same date there was also a visible dust tail 50 arcseconds across, corresponding to about 100,000 kilometers. On 14 September, the comet had an apparent magnitude of 14.2.