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3I/ATLAS

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For other comets discovered by the Asteroid Terrestrial-impact Last Alert System, see Comet ATLAS.
3I/ATLAS
3I/ATLAS imaged at the Deep Random Survey in Chile on 2 July 2025
Discovery[1]
Discovery siteATLAS–CHL (W68)
Discovery date1 July 2025
Designations
C/2025 N1
A11pl3Z
Orbital characteristics[2]
Epoch1 July 2025 (JD 2460857.5)
Observation arc18 days
Earliest precovery date14 June 2025
Orbit typeInterstellar (hyperbolic)
Perihelion1.358±0.022 AU
Semi-major axis−0.2637±0.004 AU
Eccentricity6.15±0.17
Max. orbital speed68.3 km/s with respect to the Sun @ perihelion
Inclination175.112±0.008°
322.15±0.18°
Argument of
periapsis		127.97±0.22°
Next perihelion29 October 2025
Earth MOID0.37 AU
Mars MOID0.01 AU
Jupiter MOID0.25 AU
Comet total
magnitude (M1)		6.7±0.6[2]

3I/ATLAS, also known as C/2025 N1 (ATLAS) (previously A11pl3Z), is an interstellar comet discovered by the Asteroid Terrestrial-impact Last Alert System at Río Hurtado, Chile on 1 July 2025 when the comet was 4.5 AU from the Sun.[1][3] It follows an unbound, hyperbolic trajectory around the Sun with an orbital eccentricity of 6.15±0.17.[2] It is the third interstellar object confirmed, after 1I/ʻOumuamua and 2I/Borisov.[3]

It will come to perihelion on 29 October 2025, at a distance of 1.36±0.02 AU from the Sun[2] and near the orbit of Mars, having passed beforehand at a minimum distance from it of 0.195 AU on 3 October 2025.[4] At perihelion, it will have a velocity of 68 km/s (42 mi/s) with respect to the Sun. (For comparison Mars at 1.5 AU from the Sun has an orbital speed of 24 km/s.) Around 19 December 2025 it will pass 1.8±0.1 AU from Earth.[5] It will pass about 0.35±0.18 AU from Jupiter around 16 March 2026.[2] The object is expected to have an interstellar velocity () of around 58 km/s (36 mi/s).[6][a]

Preliminary observations gave 3I/ATLAS an asteroidal absolute magnitude (H) of 14.8.[7] This value can be an inaccurate representation, however, as 3I/ATLAS displays signs of cometary activity or even being in outburst,[8] which introduces errors to the absolute magnitude and inflates size estimates.

Discovery

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Comet 3I/ATLAS, formerly known as A11pl3Z, imaged on 2 July 2025 remotely at Rio Hurtado, Chile

3I/ATLAS was discovered on 1 July 2025 by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope at Río Hurtado, Chile.[9][10] At apparent magnitude 18, the newly-discovered object was slowly moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.[3] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center (MPC).[10][11] 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.[10]

Follow-up observations from other observatories, involving both professional and amateur astronomers,[12] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory.[10][13] 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,[14][1] and ATLAS observations from 25–29 June 2025, found by amateur astronomer Sam Deen.[13][10][3] 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 Galactic Center's dense star fields.[15]

Initial observations of 3I/ATLAS were unclear on whether it is an asteroid or a comet.[12][14][3] 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.[1][15] On the other hand, various astronomers including Alan Hale reported no cometary features on 3I/ATLAS.[15] On 2 July 2025 21:31 UT, the MPC announced the discovery of 3I/ATLAS and gave it the interstellar object designation "3I", signifying it being the third interstellar object confirmed.[1][15] The MPC also gave 3I/ATLAS the non-periodic comet designation C/2025 N1 (ATLAS).[1] By the time 3I/ATLAS was announced, the MPC had collected 122 observations of the comet from 31 different observatories.[1]

Trajectory

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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 follows an unbound hyperbolic trajectory around the Sun with an extremely high orbital eccentricity of 6.15±0.17.[2] This is the highest eccentricity observed of any interstellar interloper,[12] greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4).[14] 3I/ATLAS will come closest to the Sun at perihelion on 29 October 2025, at a distance of 1.36 ± 0.02 AU (203.5 ± 3.0 million km; 126.4 ± 1.9 million mi).[2] At perihelion, the comet will move at a velocity of 68 km/s (42 mi/s) with respect to the Sun.[13] When far away from the Sun, the comet's hyperbolic excess velocity () would be 58 km/s (36 mi/s).[6] The comet's trajectory is inclined 175° with respect to the ecliptic and appears to have originated from the thin galactic disk.[3]

See also

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  • 1I/ʻOumuamua – the first interstellar interloper discovered
  • 2I/Borisov — the second interstellar interloper and the first interstellar comet discovered

Notes

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  1. ^ Formula for the hyperbolic excess velocity: , where is the Gravitational constant, the mass of the Sun, and the comet's semi-major axis. Calculation: [1]

References

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  1. ^ a b c d e f g "MPEC 2025-N12 : 3I/ATLAS = C/2025 N1 (ATLAS)". Minor Planet Electronic Circulars. Minor Planet Center. 2 July 2025. Retrieved 2 July 2025.
  2. ^ a b c d e f g "JPL Small-Body Database Lookup: C/2025 N1 (ATLAS)" (2025-07-02 last obs.). Jet Propulsion Laboratory. Retrieved 2 July 2025.
  3. ^ a b c d e f Dickinson, David (2 July 2025). "Inbound: Astronomers Discover Third Interstellar Object". Universe Today. Retrieved 2 July 2025.
  4. ^ "JPL Horizons, Observer Table for C/2025 N1 (ATLAS) from Mars (body center)" (2025-07-02 last obs.). Jet Propulsion Laboratory. Retrieved 2 July 2025.
  5. ^ "3I geocentric distance and uncertainty around 19 December 2025". JPL Horizons. Retrieved 2 July 2025.
  6. ^ a b "Groups.io MPML: Re: What is going on with A11pl3Z?". 2 July 2025.
  7. ^ "Archive: The NEO Confirmation Page". 2 July 2025. A11pl3Z
  8. ^ Rankin, David (2 July 2025). "Bluesky". Bluesky. Retrieved 2 July 2025.
  9. ^ "NASA Discovers Interstellar Comet Moving Through Solar System". NASA. 2 July 2025. Retrieved 2 July 2025.
  10. ^ a b c d e Chang, Kenneth (2 July 2025). "It Came From Outside Our Solar System, and It Looks Like a Comet". The New York Times. Archived from the original on 3 July 2025. Retrieved 3 July 2025.
  11. ^ "NEOCP observation log A11pl3Z". 1 July 2025. Retrieved 2 July 2025.
  12. ^ a b c Trigo-Rodríguez, Josep M. (2 July 2025). "Descubierto un tercer objeto interestelar cruzando a gran velocidad el sistema solar". The Conversation (in Spanish). Retrieved 2 July 2025.
  13. ^ a b c Whitt, Kelly Kizer (2 July 2025). "It's official! An interstellar object is visiting our solar system". EarthSky. Retrieved 2 July 2025.
  14. ^ a b c Luntz, Stephen (2 July 2025). "We May Have Our Third Interstellar Visitor And It's Nothing Like The Previous Two". IFLScience. Retrieved 2 July 2025.
  15. ^ a b c d Green, Daniel W. E. (2 July 2025). "COMET C/2025 N1 (ATLAS) = 3I/ATLAS". Central Bureau Electronic Telegram (5578). Central Bureau for Astronomical Telegrams. Retrieved 2 July 2025.
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