3I/ATLAS

For other comets discovered by the Asteroid Terrestrial-impact Last Alert System, see Comet 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 6.14±0.02.^[3][a] 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).^[10]

3I/ATLAS

This photo by the Canada–France–Hawaii Telescope shows 3I/ATLAS (center) with a fuzzy and elongated coma on 2 July 2025, indicating it is a comet.[1]
Discovery[2]
Discovery site	ATLAS–CHL (W68)
Discovery date	1 July 2025
Designations
MPC designation	C/2025 N1
Alternative designations	A11pl3Z
Orbital characteristics[3]
Epoch	3 July 2025 (JD 2460859.5)
Observation arc	45 days (362 obs)
Earliest precovery date	22 May 2025
Orbit type	Hyperbolic (interstellar)
Perihelion	1.357±0.002 AU
Semi-major axis	−0.2640±0.0004 AU
Eccentricity	6.139±0.015[a]
Max. orbital speed	68.3 km/s with respect to the Sun @ perihelion[4][5][b] ${\displaystyle v_{\infty }}$ = 58 km/s[6][5][c]
Inclination	175.11±0.0008° (retrograde and inclined 5°)
Longitude of ascending node	322.15±0.02°
Argument of periapsis	128.00±0.02°
Next perihelion	29 October 2025 12:01 ± 00:54 UT[7]
Earth MOID	0.365 AU
Mars MOID	0.02 AU[8]
Jupiter MOID	0.247 AU
Physical characteristics
Mean diameter	~4–5 km (2.5–3.1 mi)[d] <23.6 km (14.7 mi)[e]
Absolute magnitude (H)	≈12
Comet total magnitude (M1)	6.7±0.7

The size of 3I/ATLAS's nucleus is uncertain because it is an active comet surrounded by a shell of reflective dust.^[1] Estimates for the nucleus diameter of 3I/ATLAS range from 0.8 to 24 km (0.5 to 14.9 mi), though a diameter on the lower end of the range is more likely.^[1][11] 3I/ATLAS will come to perihelion on 29 October 2025, at a distance of 1.357 ± 0.002 AU (203.00 ± 0.30 million km; 126.14 ± 0.19 million mi) from the Sun.^[3][a] 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.^[6][5][c]

History

Discovery

3I/ATLAS was discovered on 1 July 2025^[f] by the NASA-funded ATLAS survey telescope at Río Hurtado, Chile (observatory code W68).^[14][15] 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,^[16] located 3.52 AU (527 million km; 327 million mi) from Earth and 4.53 AU from the Sun,^[12] and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.^[10] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center (MPC).^[15][13] 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.^[15]

Follow-up observations from other observatories, involving both professional and amateur astronomers,^[17] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory.^[15][18] Pre-discovery observations of 3I/ATLAS confirmed its interstellar trajectory; these include Zwicky Transient Facility observations from 14–21 June 2025, found by Ye Quanzhi,^[2][19] and ATLAS observations from 25–29 June 2025, found by amateur astronomer Sam Deen.^[10][15][18] Deen has noted additional ATLAS pre-discovery observations from 5–25 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.^[20]

Initial observations of 3I/ATLAS were unclear on whether it is an asteroid or a comet.^[10][17][19] Various astronomers including Alan Hale reported no cometary features on 3I/ATLAS, ^[20] 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.^[2][20] 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.^[2][20] The MPC also gave 3I/ATLAS the non-periodic comet designation C/2025 N1 (ATLAS).^[2] By the time 3I/ATLAS was announced, the MPC had collected 122 observations of the comet from 31 different observatories.^[2]

Further observations

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.^[11] 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.^[21] 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.^[1] 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.^[1]

On 6 July, additional observations were published, including Zwicky Transient Facility (I41) precoveries found by Ye Quanzhi, who found the comet on images taken on several nights between 22 May and 21 June 2025.^[22]

Trajectory

3I/ATLAS follows an unbound hyperbolic trajectory around the Sun with an extremely high orbital eccentricity of 6.14±0.02.^[3][a] This is the highest eccentricity of the three interstellar interlopers known to date,^[17] greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4).^[19] 3I/ATLAS will come closest to the Sun at perihelion on 29 October 2025, at a distance of 1.357 ± 0.002 AU (203.00 ± 0.30 million km; 126.14 ± 0.19 million mi).^[3][a] At perihelion, the comet will move at a maximum velocity of 68 km/s (42 mi/s) with respect to the Sun.^[18][b] When far away from the Sun, the comet's hyperbolic excess velocity (${\displaystyle v_{\infty }}$) was and will be 58 km/s (36 mi/s).^[5][6][c] The comet's trajectory is inclined 175° (retrograde and inclined 5°) with respect to the ecliptic and appears to have originated from the thin galactic disk.^[10]

As 3I/ATLAS approaches perihelion, it will pass Mars at a distance of 0.194 ± 0.003 AU (29.02 ± 0.45 million km; 18.03 ± 0.28 million mi) on 3 October 2025.^[23] After perihelion, it will pass 1.80 ± 0.01 AU (269.3 ± 1.5 million km; 167.32 ± 0.93 million mi) from Earth on 19 December 2025,^[24] and then it will pass about 0.36 ± 0.02 AU (53.9 ± 3.0 million km; 33.5 ± 1.9 million mi) from Jupiter on 16 March 2026.^[3][g] 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.^[10] 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.^[10] The comet will become reobservable from Earth by early December 2025.^[16]

Size and brightness

Observations suggest 3I/ATLAS has an asteroidal absolute magnitude (H) of about 12,^[1] which suggests a maximum possible diameter of around 24 km (15 mi) for 3I/ATLAS's nucleus, if it was a dark asteroid.^[1] 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).^[11] 3I/ATLAS appears especially bright because the comet had passed opposition on 25 June 2025,^[25] causing its surrounding dust to be brightened by an opposition surge.^{[citation needed]} Nevertheless, 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.^[1] For reference, the maximum estimated diameter of 2I/Borisov's nucleus is between 0.4–0.5 km (0.25–0.31 mi),^[9][26] 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^[27] and that would place the comet outside the reach of the average observer with 50mm binoculars.^[28] The comet will also be less than 30 degrees from the Sun from 2 October 2025 to 10 November 2025.^[25]

Gallery

• 
  Discovery images of 3I/ATLAS from the Asteroid Terrestrial-impact Last Alert System on 1 July 2025
• 
  Comet 3I/ATLAS, formerly known as A11pl3Z, imaged on 2 July 2025 remotely at Río Hurtado, Chile
• 
  Hyperbolic trajectory of 3I/ATLAS (white) through the Solar System, with orbits of planets shown. Positions of objects shown as of 3 July 2025
• 
  3I/ATLAS imaged by ESO’s Very Large Telescope on 3 July 2025

Notes

1. The JPL SBDB lists a 1-sigma uncertainty for the eccentricity (which covers 68% of the possibilities). A 3-sigma uncertainty would be 3 times larger and would cover 99% of the possibilities.
2. The escape velocity from the Solar System depends mostly on how close you are to the Sun. Mars at 1.5 AU from the Sun has an orbital speed of only 24 km/s. The escape velocity from the Solar System at Mercury's orbit at 0.4 AU from the Sun is about 68 km/s, which is 3I/ATLAS's velocity at 1.35 AU from the Sun. The escape velocity from the surface of the Sun is 618 km/s.
3. Formula for the hyperbolic excess velocity: ${\displaystyle {\sqrt {-GM/a}}}$, where ${\displaystyle G}$ is the gravitational constant, ${\displaystyle M}$ the mass of the Sun, and ${\displaystyle a}$ the comet's semi-major axis. Calculation:
4. The 4–5 km (2.5–3.1 mi) diameter is based on the possibility that it is an order of magnitude larger than 2I/Borisov, which is around 0.4–0.5 km (0.25–0.31 mi) in diameter.^[9]
5. The 23.6 km (14.7 mi) diameter is based on the assumption that the object is asteroid-like in composition+appearance with an abs mag (H) of ~12 and a dark albedo of 0.05. Asteroid 1392 Pierre has a similar abs mag (H) and albedo. However, we should treat it as an upper limit, as a coma is present around the object, which certainly indicates the nucleus is significantly smaller.^[1]
6. In the Minor Planet Center discovery announcement, the discovery observation time (marked with an asterisk "*") is "2025 07 01.218880,"^[2] which translates to 1 July 2025 05:15:11 UT.^[12] While earlier observations were later found, this was the first that was reported to the Minor Planet Center, received on 1 July 2025 at 07:48 UT.^[13]
7. At the close approach to Jupiter on 16 March 2026, the 3-sigma uncertainty in the object's position is 2,300,000 km (0.015 AU).