3I/ATLAS

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For other comets discovered by the Asteroid Terrestrial-impact Last Alert System, see Comet ATLAS.

3I/ATLAS
Semi-major axis
−0.2638±0.0001 AU
Eccentricity6.145±0.004[a]
Max. orbital speed68.3 km/s @ perihelion[2][3][b]
= 58 km/s
Synodic rotation period
16.79±0.23 h[11]
Absolute
magnitude (H)		12.27±0.03[12]
Comet total
magnitude (M1)		11.1±0.8[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 6.145±0.004.[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).[13]

The size of 3I/ATLAS's nucleus is uncertain because it is an active comet surrounded by a shell of reflective dust.[12] [14] 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.[14][15] 3I/ATLAS will come to

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.[16]

History

Discovery

3I/ATLAS was discovered on 1 July 2025

Serpens Cauda and Sagittarius, near the galactic plane.[13] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center (MPC).[18][20] 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.[20]

Follow-up observations from other observatories, involving both professional and amateur astronomers,[22] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory.[20][23] 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,[18] ZTF observations from 14 to 21 June 2025,[1][24] and ATLAS observations from 25 to 29 June 2025.[13][20][23] 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.[25]

Initial observations of 3I/ATLAS were unclear on whether 3I/ATLAS is an

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.[15] 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.[26] Multi-band observations at the Kottamia Astronomical Observatory 1.88-m telescope, the Palomar 200-inch telescope, and the Astrophysical Research Consortium 3.5-m telescope on 2025 July 2, 3 showed the comet had colors of B-V=0.98±0.23, V-R=0.71±0.09, R-I=0.14±0.10, g-r=0.84±0.05 mag, r-i=0.16±0.03 mag, i-z=-0.02±0.07 mag, and g-i=1.00±0.05 mag and a spectral slope of 16.0±1.9 %/100 nm.[12] 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.[14]

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.[14] More sensitive observations by the 10.4-m Gran Telescopio Canarias from 2 to 5 July were able to determine that the nucleus of 3I/ATLAS has a rotation period of 16.79±0.23 hours, with its brightness varying by 0.2 magnitudes.[11]

On 6 July, additional observations were published, including Zwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025.[27]

Observations of 3I/ATLAS with the Hubble Space Telescope are planned for 21 July 2025.[28] Images from these observations will be immediately made public.[28]

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 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.[29][16]

As 3I/ATLAS approaches perihelion, it will pass at a distance of 0.194 ± 0.001 AU (29.02 ± 0.15 million km; 18.033 ± 0.093 million mi) from Mars on 3 October 2025.[30] After perihelion, it will pass 1.800 ± 0.003 AU (269.28 ± 0.45 million km; 167.32 ± 0.28 million mi) from Earth on 19 December 2025,[31] and then it will pass 0.356 ± 0.005 AU (53.26 ± 0.75 million km; 33.09 ± 0.46 million mi) from Jupiter on 16 March 2026.[7][i]

Closest Approaches (with
3-sigma uncertainties
)
Object Date Distance
Mars 2025-10-03 0.194 ± 0.001 AU (29.02 ± 0.15 million km; 18.033 ± 0.093 million mi)[30]
Sun 2025-10-29 1.357 ± 0.002 AU (203.00 ± 0.30 million km; 126.14 ± 0.19 million mi)[6]
Earth 2025-12-19 1.800 ± 0.003 AU (269.28 ± 0.45 million km; 167.32 ± 0.28 million mi)[31]
Jupiter 2026-03-16 0.356 ± 0.005 AU (53.26 ± 0.75 million km; 33.09 ± 0.46 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.[13] 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.[13] The comet will become reobservable from Earth by early December 2025.[21]

Size and brightness

Observations suggest 3I/ATLAS has an asteroidal absolute magnitude (H) of about 12,[14] which suggests a maximum possible diameter of around 24 km (15 mi) for 3I/ATLAS's nucleus, if it was a dark asteroid.[14] 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).[15] 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.[14] For reference, the maximum estimated diameter of 2I/Borisov's nucleus is between 0.4–0.5 km (0.25–0.31 mi),[9][32] 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, but its actual diameter is likely to be smaller 1.2 km (0.75 mi).[10]

The comet is not expected to get brighter than about apparent magnitude 11.5[33] and that would place the comet outside the reach of the average observer with 50 mm binoculars.[34] The comet will also be less than 30 degrees from the Sun from 1 October 2025 to 9 November 2025.[35]

1 July 2025

2 July 2025

3 July 2025

  • Imaged by ESO's Very Large Telescope
    Imaged by
    ESO's Very Large Telescope
  • VLT FORS2 ESO image of 3I/ATLAS in motion through 13 minutes - visible as a line[36]
    VLT FORS2 ESO image of 3I/ATLAS in motion through 13 minutes - visible as a line[36]
  • Visible and near-infrared color composite photo by the Gemini North telescope
    Visible and near-infrared color composite photo by the Gemini North telescope
  • 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.
    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. ^
    3-sigma
    uncertainty would be 3 times larger and would cover 99.7% 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 absolute magnitude ~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 indicates the nucleus is significantly smaller, likely with a diameter smaller than 1.2 km (0.75 mi).[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.[17] 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.[18]
  7. ^ An n-body integration shows 3I/ATLAS comes to perihelion about 11 minutes later than the JPL SBDB epoch 11 July 2025 solution. (changing 11:19 to 11:30.)
  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 ±690 thousand km (0.0046 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. 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 17 July 2025. Retrieved 18 July 2025. (when rdot = 0; 3-sigma uncertainty is ±260 thousand km (0.0017 AU).)
  7. ^ a b c d e f g h Farnocchia, Davide. "JPL Small-Body Database Lookup: C/2025 N1 (ATLAS)" (2025-07-18 last obs.). Jet Propulsion Laboratory. Archived from the original on 3 July 2025. Retrieved 18 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. ^
    S2CID 203837079
    .
  10. ^
    S2CID 203837079
    .
  11. ^
    arXiv:2507.12922 [astro-ph.EP
    ].
  12. ^
    doi:10.1093/mnrasl/slaf078
    .
  13. ^ 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.
  14. ^
    arXiv:2507.02757 [astro-ph.EP
    ].
  15. ^
    Bibcode:2025ATel17263....1J
    . Retrieved 3 July 2025.
  16. ^
    arXiv:2507.05318 [astro-ph.EP
    ]., Preprint, submitted to ApJ Letters
  17. ^ 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.
  18. ^ a b c "NEOCP observation log A11pl3Z". 1 July 2025. Archived from the original on 3 July 2025. Retrieved 2 July 2025.
  19. ^ "NASA Discovers Interstellar Comet Moving Through Solar System". NASA. 2 July 2025. Archived from the original on 3 July 2025. Retrieved 2 July 2025.
  20. ^ 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.
  21. ^ a b "Comet 3I/ATLAS". NASA. 3 July 2025. Retrieved 3 July 2025.
  22. ^ 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.
  23. ^ 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.
  24. ^ 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.
  25. ^ 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.
  26. Bibcode:2025ATel17264....1A
    . Retrieved 3 July 2025.
  27. ^ "MPEC 2025-N51 : Comet 3I/ATLAS". Minor Planet Electronic Circulars. Minor Planet Center. 6 July 2025. Retrieved 6 July 2025.
  28. ^ a b 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.
  29. ^ 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.
  30. ^ 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 18 July 2025. (Mars approach 3-sigma uncertainty is ±150 thousand km (0.0010 AU).)
  31. ^ a b "3I/ATLAS geocentric distance and uncertainty on 19 December 2025". JPL Horizons. Archived from the original on 3 July 2025. Retrieved 18 July 2025. (Earth approach 3-sigma uncertainty is ±450 thousand km (0.0030 AU). For comparison, JPL #1 had an Earth approach 3-sigma uncertainty of ±18 million km.)
  32. arXiv:2003.14064 [astro-ph.EP
    ].
  33. ^ Yoshida, Seiichi (4 July 2025). "3I/2025 N1". Retrieved 6 July 2025.
  34. ^ Zarenski, Ed (2004). "Limiting Magnitude in Binoculars" (PDF). Cloudy Nights. Archived (PDF) from the original on 21 July 2011. Retrieved 6 July 2025.
  35. ^ "Solar elongation (S-O-T) from 20 June 2025 to 31 December 2025". JPL Horizons. Retrieved 5 July 2025.
  36. ^ 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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