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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 moving across a field of stars, as seen by the Very Large Telescope on 4 July 2025
Discovery[1]
Discovery siteATLAS–CHL (W68)
Discovery date1 July 2025
Designations
C/2025 N1
A11pl3Z
Orbital characteristics[7]
Epoch4 July 2025 (JD 2460860.5)
Observation arc53 days (484 obs)
Earliest precovery date22 May 2025
Orbit typeHyperbolic (interstellar)
Perihelion1.358±0.001 AU
Semi-major axis−0.2636±0.0002 AU
Eccentricity6.151±0.008[a]
Max. orbital speed68.3 km/s @ perihelion[2][3][b]
= 58 km/s[3][5][c]
Inclination175.11±0.0006°
(retrograde and inclined 5°)
322.17±0.01°
Argument of
periapsis		127.99±0.01°
Next perihelion29 October 2025 11:12 ± 00:24 UT[6]
Earth MOID0.366 AU
Mars MOID0.018 AU[8]
Jupiter MOID0.246 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)		7.7±0.7[7]

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.[7][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).[11]

The size of 3I/ATLAS's nucleus is uncertain because it is an active comet surrounded by a shell of reflective dust.[10] 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.[10][12] 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.[7][a] When far away from the Sun, the comet's hyperbolic excess velocity () will be 58 km/s (36 mi/s) with respect to the Sun.[3][5][c] 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.[13]

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).[16][17] 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,[18] located 3.50 AU (524 million km; 325 million mi) from Earth and 4.51 AU from the Sun,[14] and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.[11] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center (MPC).[15][17] 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.[17]

Follow-up observations from other observatories, involving both professional and amateur astronomers,[19] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory.[17][20] 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,[15] ZTF observations from 14 to 21 June 2025,[1][21] and ATLAS observations from 25 to 29 June 2025.[11][17][20] 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.[22]

Initial observations of 3I/ATLAS were unclear on whether 3I/ATLAS is an asteroid or a comet.[11][19][21] Various astronomers including Alan Hale reported no cometary features,[22] 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.[1][22] 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.[1][22] 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]

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.[12] 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.[23] 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.[10] 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.[10]

On 6 July, additional observations were published, including Zwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025.[24]

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".[25]

Trajectory

Hyperbolic trajectory of 3I/ATLAS (blue) through the Solar System, with orbits of planets shown (click to start animation)

3I/ATLAS follows an unbound hyperbolic trajectory around the Sun with an extremely high orbital eccentricity of 6.15±0.008.[7][a] This is the highest eccentricity of the three interstellar objects known to date,[19] greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4).[21] 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)[7][a] on 29 October 2025 at 11:12 ± 00:24 UT.[6][g] At perihelion, the comet will be moving at its maximum velocity of 68 km/s (42 mi/s) with respect to the Sun.[20][h] When far away from the Sun, the comet's hyperbolic excess velocity will be 58 km/s (36 mi/s).[3][5][c] 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.[26][13]

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.[27] 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,[28] 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.[7][i]

Closest Approaches (with 3-sigma uncertainties)
Object Date Distance
Mars 2025-10-03 0.195 ± 0.002 AU (29.17 ± 0.30 million km; 18.13 ± 0.19 million mi)[27]
Sun 2025-10-29 1.358 ± 0.003 AU (203.15 ± 0.45 million km; 126.23 ± 0.28 million mi)[6]
Earth 2025-12-19 1.800 ± 0.006 AU (269.28 ± 0.90 million km; 167.32 ± 0.56 million mi)[28]
Jupiter 2026-03-16 0.354 ± 0.009 AU (53.0 ± 1.3 million km; 32.91 ± 0.84 million mi)[7][i]

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.[11] 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.[11] The comet will become reobservable from Earth by early December 2025.[18]

Size and brightness

Observations suggest 3I/ATLAS has an asteroidal absolute magnitude (H) of about 12,[10] which suggests a maximum possible diameter of around 24 km (15 mi) for 3I/ATLAS's nucleus, if it was a dark asteroid.[10] 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).[12] 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.[10] For reference, the maximum estimated diameter of 2I/Borisov's nucleus is between 0.4–0.5 km (0.25–0.31 mi),[9][29] 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[30] and that would place the comet outside the reach of the average observer with 50 mm binoculars.[31] The comet will also be less than 30 degrees from the Sun from 1 October 2025 to 9 November 2025.[32]

1 July 2025

2 July 2025

3 July 2025

  • 3I/ATLAS imaged by ESO's Very Large Telescope
    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 [33]
    VLT FORS2 ESO image of 3I/ATLAS in motion through 13 minutes - visible as a line [33]
  • Visible and near-infrared color composite photo of 3I/ATLAS by the Gemini North telescope
    Visible and near-infrared color composite photo of 3I/ATLAS by the Gemini North telescope
  • 3I/ATLAS's movement across a field of stars, as seen by Gemini North. The colorful appearance of the comet's trail is due to the telescope changing light filters while observing the comet.
    3I/ATLAS's movement across a field of stars, as seen by Gemini North. The colorful appearance of the comet's trail is due to the telescope changing light filters while observing the comet.

Notes

  1. ^ a b c d e The JPL SBDB lists a 1-sigma uncertainty for the eccentricity/perihelion (which covers 68% of the possibilities). A 3-sigma uncertainty would be 3 times larger and would cover 99% of the possibilities.
  2. ^ Formula for the perihelion velocity:[4] , where is the gravitational constant, the mass of the Sun, the comet's perihelion distance, and its semi-major axis. Calculation: [1]
  3. ^ a b c Formula for the hyperbolic excess velocity: , where is the gravitational constant, the mass of the Sun, and the comet's semi-major axis. Calculation: [2]
  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 and appearance with an abs mag (H) of ~12 and a dark albedo of 0.05. However, it should be treated as an upper limit, as a coma is present around the object, which certainly indicates the nucleus is significantly smaller.[10]
  6. ^ In the Minor Planet Center discovery announcement, the discovery observation time (marked with an asterisk "*") is "2025 07 01.218880,"[1] which translates to 1 July 2025 05:15:11 UT.[14] 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.[15]
  7. ^ An n-body integration shows 3I/ATLAS comes to perihelion about 11 minutes later than the JPL SBDB epoch 4 July 2025 solution. (changing 11:01 to 11:12.)
  8. ^ 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.
  9. ^ a b At the close approach to Jupiter on 16 March 2026, the 3-sigma uncertainty in the object's position is ±1.3 million km (0.0087 AU).

References

  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. ^ Deen, Sam (2 July 2025). "Groups.io MPML: Re: What is going on with A11pl3Z? #40734".
  3. ^ a b c d "3I/ATLAS: vinf 1600 + perihelion + vinf 2450 (stepsize 425 years)". JPL Horizons. Archived from the original on 4 July 2025. Retrieved 14 July 2025.
  4. ^ Lissauer, Jack J.; de Pater, Imke (2013). Fundamental Planetary Sciences: Physics, Chemistry, and Habitability. New York: Cambridge University Press. pp. 29–31. ISBN 9781108411981.
  5. ^ a b c Deen, Sam (2 July 2025). "Groups.io MPML: Re: What is going on with A11pl3Z? #40719".
  6. ^ a b c "Perihelion on 29 Oct 2025". JPL Horizons. Archived from the original on 7 July 2025. Retrieved 14 July 2025. (when rdot = 0; 3-sigma uncertainty is ±470 thousand km (0.0031 AU).)
  7. ^ a b c d e f g h Farnocchia, Davide. "JPL Small-Body Database Lookup: C/2025 N1 (ATLAS)" (2025-07-14 last obs.). Jet Propulsion Laboratory. Archived from the original on 3 July 2025. Retrieved 14 July 2025.
  8. ^ ""Pseudo-MPEC" for 3I = C/2025 N1 (ATLAS)". projectpluto.com. Find_Orb. Retrieved 15 July 2025. Enter 3I in the "enter an object name" field, then click "compute orbit and ephemeris".
  9. ^ a b Jewitt, David; Luu, Jane (6 October 2019). "Initial Characterization of interstellar comet 2I/2019 Q4 (Borisov)". The Astrophysical Journal. 886 (2): L29. arXiv:1910.02547. Bibcode:2019ApJ...886L..29J. doi:10.3847/2041-8213/ab530b. S2CID 203837079.
  10. ^ a b c d e f g h Seligman, Darryl Z.; Micheli, Marco; Farnocchia, Davide; Denneau, Larry; Noonan, John W.; Santana-Ros, Toni; et al. (3 July 2025). "Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS". arXiv:2507.02757 [astro-ph.EP].
  11. ^ a b c d e f Dickinson, David (2 July 2025). "Inbound: Astronomers Discover Third Interstellar Object". Universe Today. Archived from the original on 2 July 2025. Retrieved 2 July 2025.
  12. ^ a b c Jewitt, David; Luu, Jane (3 July 2025). "Interstellar Interloper C/2025 N1 is Active". The Astronomer's Telegram (17263). Retrieved 3 July 2025.
  13. ^ a b Hopkins, Matthew J.; Dorsey, Rosemary C.; Forbes, John C.; Bannister, Michele T.; Lintott, Chris J.; Leicester, Brayden (9 July 2025). "From a Different Star: 3I/ATLAS in the context of the Ōtautahi-Oxford interstellar object population model". arXiv:2507.05318 [astro-ph.EP]., Preprint, submitted to ApJ Letters
  14. ^ a b "Horizons Batch for 3I/ATLAS (C/2025 N1) from 2025-Jul-01.218880 to 2025-Jul-01.264248". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 11 July 2025.
  15. ^ a b c "NEOCP observation log A11pl3Z". 1 July 2025. Archived from the original on 3 July 2025. Retrieved 2 July 2025.
  16. ^ "NASA Discovers Interstellar Comet Moving Through Solar System". NASA. 2 July 2025. Archived from the original on 3 July 2025. Retrieved 2 July 2025.
  17. ^ 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.
  18. ^ a b "Comet 3I/ATLAS". NASA. 3 July 2025. Retrieved 3 July 2025.
  19. ^ 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.
  20. ^ 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.
  21. ^ 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.
  22. ^ 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.
  23. ^ Alarcon, Miguel R.; Serra-Ricart, Miquel; Licandro, Javier; Guerra Arencibia, Sergio; Ruiz Cejudo, Ignacio; Trujillo, Ignacio (3 July 2025). "Deep g'-band Imaging of Interstellar Comet 3I/ATLAS from the Two-meter Twin Telescope (TTT)". The Astronomer's Telegram (17264). Retrieved 3 July 2025.
  24. ^ "MPEC 2025-N51 : Comet 3I/ATLAS". Minor Planet Electronic Circulars. Minor Planet Center. 6 July 2025. Retrieved 6 July 2025.
  25. ^ Jewitt, David; Agarwal, Jessica; Li, Jing; Kim, Yoonyoung; Mutchler, Max (7 July 2025). "17830 - The Next Interstellar Interloper". Space Telescope Science Institute. Archived from the original on 1 April 2025. Retrieved 10 July 2025.
  26. ^ Baker, Harry (10 July 2025). "3I/ATLAS: Everything you need to know about the new 'interstellar visitor' shooting through the solar system". livescience.com. Retrieved 10 July 2025.
  27. ^ a b "JPL Horizons, Observer Table for C/2025 N1 (ATLAS) from Mars (body center) on 3 Oct 2025". Jet Propulsion Laboratory. Archived from the original on 10 July 2025. Retrieved 14 July 2025. (Mars approach 3-sigma uncertainty is ±260 thousand km (0.0017 AU).)
  28. ^ a b "3I/ATLAS geocentric distance and uncertainty on 19 December 2025". JPL Horizons. Archived from the original on 3 July 2025. Retrieved 14 July 2025. (Earth approach 3-sigma uncertainty is ±850 thousand km (0.0057 AU).)
  29. ^ Hui, Man-To; Ye, Quan-Zhi; Föhring, Dora; Hung, Denise; Tholen, David J. (2020). "Physical characterisation of interstellar comet 2I/2019 Q4 (Borisov)". arXiv:2003.14064 [astro-ph.EP].
  30. ^ Yoshida, Seiichi (4 July 2025). "3I/2025 N1". Retrieved 6 July 2025.
  31. ^ Zarenski, Ed (2004). "Limiting Magnitude in Binoculars" (PDF). Cloudy Nights. Archived (PDF) from the original on 21 July 2011. Retrieved 6 July 2025.
  32. ^ "Solar elongation (S-O-T) from 20 June 2025 to 31 December 2025". JPL Horizons. Retrieved 5 July 2025.
  33. ^ Olivier Hainaut, ESO (8 July 2025). "Sequence of VLT images of 3I/ATLAS, a new interstellar object". eso.org. European Southern Observatory. Retrieved 9 July 2025.
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