3I/ATLAS

3I/ATLAS, also known as C/2025 N1 (ATLAS) and previously as A11pl3Z, is an interstellar comet discovered while inbound by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station at Río Hurtado, Chile, on 1 July 2025 when it was 4.5 AU (670 million km; 420 million mi) from the Sun and moving at a relative speed of 61 km/s (38 mi/s). It follows an unbound, hyperbolic trajectory around the Sun with an orbital eccentricity of 6.15±0.008. It is the third interstellar object confirmed passing through the Solar System, after 1I/ʻOumuamua (discovered 19 October 2017) and 2I/Borisov (discovered 29 August 2019).

The size of 3I/ATLAS's nucleus is uncertain because it is an active comet surrounded by a shell of reflective dust. Estimates for the nucleus diameter of 3I/ATLAS range from 0.8 to 24 km (0.5 to 14.9 mi), though a diameter toward the lower end of the range is more likely. 3I/ATLAS will come to perihelion on 29 October 2025, at a distance of 1.358 ± 0.001 AU (203.15 ± 0.15 million km; 126.234 ± 0.093 million mi) from the Sun. When far away from the Sun, the comet's hyperbolic excess velocity (${\displaystyle v_{\infty }}$) will be 58 km/s (36 mi/s) with respect to the Sun. The comet's velocity suggests it originated in the thick galactic disk which contains many older stars, and therefore the comet may be water-rich and could be more than 7 billion years old.

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 list the object in the Near-Earth Object Confirmation Page.

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 3I/ATLAS 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 showed a marginal coma and a short tail 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 announced, the MPC had collected 122 observations of the comet from 31 different observatories.

Observations by David Jewitt and Jane Luu using the Nordic Optical Telescope on 2 July 2025 confirmed that 3I/ATLAS was "clearly active" with a diffuse tail. 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, with a tail at least 25,000 km (16,000 mi) long. Faulkes Telescope North measurements of 3I/ATLAS's brightness through different light filters showed that the comet's coma had a reddish color indicative of dust, similar to that of the previous interstellar comet 2I/Borisov. Immediate observations from various telescopes were unable to determine a rotation period and instead found that the brightness of 3I/ATLAS apparently shows little variation (less than 0.2 magnitudes), which may be due to the comet's dust coma obscuring its rotating nucleus.

On 6 July, additional observations were published, including Zwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025.

Observations with the Hubble Space Telescope are planned for 21 July 2025, with a zero month proprietary period "so the observations can be of maximum benefit to the community".

3I/ATLAS follows an unbound hyperbolic trajectory around the Sun with an extremely high orbital eccentricity of 6.15±0.008. This is the highest eccentricity of the three interstellar objects known to date, greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4). 3I/ATLAS will come closest to the Sun at perihelion at a distance of 1.358 ± 0.001 AU (203.15 ± 0.15 million km; 126.234 ± 0.093 million mi) on 29 October 2025 at 11:12 ± 00:24 UT. At perihelion, the comet will be moving at its maximum velocity of 68 km/s (42 mi/s) with respect to the Sun. When far away from the Sun, the comet's hyperbolic excess velocity will be 58 km/s (36 mi/s). The comet's trajectory is inclined 175° (retrograde and inclined 5°) with respect to the ecliptic and appears to have originated from the thick galactic disk.

As 3I/ATLAS approaches perihelion, it will pass at a distance of 0.195 ± 0.002 AU (29.17 ± 0.30 million km; 18.13 ± 0.19 million mi) from Mars on 3 October 2025. After perihelion, it will pass 1.800 ± 0.006 AU (269.28 ± 0.90 million km; 167.32 ± 0.56 million mi) from Earth on 19 December 2025, and then it will pass 0.354 ± 0.009 AU (53.0 ± 1.3 million km; 32.91 ± 0.84 million mi) from Jupiter on 16 March 2026.

During the comet's close approach to Mars, it may reach an apparent magnitude of 11 from the planet, which means Mars orbiters may be able to observe it. On the other hand, from Earth, 3I/ATLAS will not be observable at perihelion because Earth and the comet will be on opposite sides of the Sun by that time. The comet will become reobservable from Earth by early December 2025.

Observations suggest 3I/ATLAS has an asteroidal absolute magnitude (H) of about 12, which suggests a maximum possible diameter of around 24 km (15 mi) for 3I/ATLAS's nucleus, if it was a dark asteroid. However, because 3I/ATLAS is an active comet surrounded by a coma or a shell of reflective dust, the actual size of its nucleus is expected to be significantly smaller as it would be properly calculated from a combined nucleus and coma absolute magnitude (M1). 3I/ATLAS appears to be weakly active compared to the other interstellar comet 2I/Borisov, and is thus suspected to have a nucleus diameter likely an order of magnitude (ten times) larger than that of 2I/Borisov's. For reference, the maximum estimated diameter of 2I/Borisov's nucleus is between 0.4–0.5 km (0.25–0.31 mi), so the maximum diameter of 3I/ATLAS's nucleus could be up to 4–5 km (2.5–3.1 mi) if the order of magnitude argument holds true.

The comet is not expected to get brighter than about apparent magnitude 11.5 and that would place the comet outside the reach of the average observer with 50 mm binoculars. The comet will also be less than 30 degrees from the Sun from 1 October 2025 to 9 November 2025.

• 
  Discovery image of 3I/ATLAS from the Asteroid Terrestrial-impact Last Alert System

• 
  Comet 3I/ATLAS, formerly known as A11pl3Z, imaged remotely at Río Hurtado, Chile
• 
  3I/ATLAS with a fuzzy and elongated coma, imaged by the Canada–France–Hawaii Telescope
• 
  3I/ATLAS imaged at the Deep Random Survey in Chile

• 
  3I/ATLAS imaged by ESO's Very Large Telescope
• 
  VLT FORS2 ESO image of 3I-ATLAS in motion through 13 minutes - visible as a line

• Comet 3I/ATLAS Frequently Asked Questions, NASA
• Interactive orbit animation, by David Rankin, Catalina Sky Survey
• Star Chart, by Gianluca Masi, Virtual Telescope Project
• Merrifield, Mike (10 July 2025). "NEWS: An Interstellar Object in our Solar System". Sixty Symbols. Brady Haran.