 Color photo of 3I/ATLAS by theGemini South Observatory on 27 August 2025, showing its fuzzycoma andtail.[1] | |
| Discovery[2] | |
|---|---|
| Discovery site | ATLAS–CHL (W68) |
| Discovery date | 1 July 2025 |
| Designations | |
| C/2025 N1 | |
| A11pl3Z | |
| Orbital characteristics[10] | |
| Epoch | 28 July 2025 (JD 2460884.5) |
| Observation arc | 129 days |
| Earliestprecovery date | 7 May 2025[3] (55 days before discovery) |
| Number of observations | 646 |
| Orbit type | Hyperbolic (interstellar) |
| Perihelion | 1.3561±0.0001 AU |
| Semi-major axis | −0.26396±0.00001 AU |
| Eccentricity | 6.1374±0.0006[a] |
| Max.orbital speed | 68.3 km/s at perihelion[4][5][b] v∞ = 58 km/s[5][7][8][c] |
| Inclination | 175.11±0.00006° (retrograde and inclined 5°) |
| 322.15±0.001° | |
| Argument of periapsis | 128.01±0.0007° |
| Next perihelion | 29 October 2025 11:47 ± 00:01 UT[9][d] |
| EarthMOID | 0.3633 AU |
| MarsMOID | 0.0179 AU[11][e] |
| JupiterMOID | 0.2467 AU |
| Physical characteristics | |
Mean diameter | |
| 16.16±0.01 h[14] | |
| Absolute magnitude (H) | >15.4[12] |
| Comet total magnitude (M1) | 12.3±0.8[10] |
3I/ATLAS, also known asC/2025 N1 (ATLAS) and previously asA11pl3Z, is aninterstellar comet[16][17] discovered by theAsteroid Terrestrial-impact Last Alert System (ATLAS) station. The comet follows an unbound,hyperbolic trajectory past the Sun.[5][c] It will not come closer than 1.8 AU (almost twice the distance of the Sun) from Earth, so it poses no threat.[18] It is the third interstellar object confirmed passing through the Solar System, after1I/ʻOumuamua and2I/Borisov,[19] hence the prefix "3I". It is not expected to brighten enough to become visible in binoculars or to the naked eye. Duringsolar conjunction on 21 October 2025,[f] the comet could be observed by theGOES-19 weather satellite.[20]
3I/ATLAS is an active comet consisting of a solid icynucleus and acoma, which is a cloud ofgas andicy dust escaping from the nucleus. The size of 3I/ATLAS's nucleus is uncertain because its light cannot be separated from that of the coma.[21] The Sun is responsible for the comet's activity because it heats up the comet's nucleus tosublimate its ice into gas, whichoutgasses and lifts up dust from the comet's surface to form its coma.[22] Images by theHubble Space Telescope suggest that the diameter of 3I/ATLAS's nucleus is between 0.32 and 5.6 km (0.2 and 3.5 mi), with the most likely diameter being less than 1 km (0.62 mi).[12] Observations by theJames Webb Space Telescope (JWST) have shown that 3I/ATLAS is unusually rich incarbon dioxide and contains a small amount ofwater ice,water vapor,carbon monoxide, andcarbonyl sulfide.[23] Observations by theVery Large Telescope have also shown that 3I/ATLAS is emittingcyanide gas and atomicnickel vapor at concentrations similar to those seen in Solar System comets.[24]
3I/ATLAS will comeclosest to the Sun on 29 October 2025, at a distance of 1.36 AU (203 million km; 126 million mi) from the Sun, which is between the orbits of Earth and Mars.[10] The comet appears to have originated from either theMilky Way'sthin disk orthick disk;[25] if 3I/ATLAS originated from the thick disk, the comet could be at least 7 billion years old—older than the Solar System.[26][8]
3I/ATLAS was discovered on 1 July 2025[g] by theNASA-fundedATLAS survey telescope atRío Hurtado, Chile (observatory code W68).[29][30][15] Atapparent 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 and4.51 AU from the Sun,[27] and was moving in the sky along the border of theconstellationsSerpens Cauda andSagittarius, near thegalactic plane.[19] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to theInternational Astronomical Union'sMinor Planet Center (MPC).[28][30] These observations initially suggested that the object could be on a highlyeccentric path that might come close to Earth's orbit, which led the MPC to temporarily list the object on theNear-Earth Object Confirmation Page until the orbit could be confirmed.[30]
Follow-up observations from other observatories, involving both professional andamateur astronomers,[31] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with ahyperbolic trajectory.[30][32]Pre-discovery observations of 3I/ATLAS confirmed its interstellar trajectory; these includedZwicky Transient Facility (ZTF, observatory codeI41) observations from 28 to 29 June 2025 that were found within a few hours of the initial report,[28] ZTF observations from 14 to 21 June 2025,[2][33] and ATLAS observations from 25 to 29 June 2025.[19][30][32] 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 theGalactic Center's dense star fields, where the comet would be hard to discern.[34]
Initial observations of 3I/ATLAS were unclear on whether it is anasteroid or acomet.[19][31][33] Various astronomers includingAlan Hale reported no cometary features,[34] but observations on 2 July 2025 by the Deep Random Survey (X09) at Chile,Lowell Discovery Telescope (G37) atArizona, andCanada–France–Hawaii Telescope (T14) atMauna Kea all showed a marginalcoma with a potential tail-like elongation 3arcseconds inangular length, which indicated the object is a comet.[2][34] On 2 July 2025, the MPC announced the discovery of 3I/ATLAS and gave it theinterstellar object designation "3I", signifying it being the third interstellar object confirmed.[2][34] The MPC also gave 3I/ATLAS thenon-periodic comet designation C/2025 N1 (ATLAS).[2] By the time 3I/ATLAS was officially named, the MPC had collected 122 observations of the comet from 31 different observatories.[2]
Observations byDavid Jewitt andJane Luu using theNordic Optical Telescope on 2 July 2025 confirmed that 3I/ATLAS was "clearly active" with a diffuse appearance.[35] Miguel R. Alarcón and a team of researchers of the IAC (Instituto de Astrofísica de Canarias) usingTeide Observatory's Two-meter Twin Telescope also found cometary activity on the same date.[36] Multiple different telescopes showed that the comet's coma had a reddish color indicative of dust, similar to that of the previous interstellar comet 2I/Borisov.[37][15][14] A study published by Toni Santana-Ros and colleagues in August 2025 reported that 3I/ATLAS's coma had become redder throughout July 2025, indicating an evolving surface or coma composition as a result of 3I/ATLAS's increasing cometary activity.[14]: 2
On 6 July, additional observations were published, includingZwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025.[38] An even earlier precovery from 21 May 2025, made atWeizmann Astrophysical Observatory (M01), was published on 18 July 2025.[39]
Polarimetric observations by theVery Large Telescope, Nordic Optical Telescope, andRozhen Observatory from July and August 2025 revealed that 3I/ATLAS's coma exhibits an unusually high degree of negativepolarization at smallphase angles—meaning a large percent of the light reflected from 3I/ATLAS's coma have their oscillations oriented along the Sun-comet-observer plane.[40]: 2 The negative polarization of 3I/ATLAS appears similar to those seen intrans-Neptunian objects, and suggests that its coma is made of a mixture of icy and dark material.[40]: 6–7
The newly commissionedVera C. Rubin Observatory has serendipitously imaged 3I/ATLAS during its science validation observations from 21 June to 3 July 2025.[41] These observations showed a slight increase in the comet's coma diameter and provided constraints on the comet's nucleus diameter.[41] The Vera Rubin Observatory would have discovered 3I/ATLAS before the ATLAS survey if it had begun its science validation observations two weeks earlier.[41]: 26 NASA'sTransiting Exoplanet Survey Satellite (TESS) had also observed 3I/ATLAS before it was discovered, with observations from 7 May to 3 June 2025.[3] These observations showed that the comet was already bright and active even when it was roughly 6.4 AU away from the Sun in May 2025, which indicates the comet's activity is likely caused by the sublimation ofvolatile ices other than water.[3]
Water ice in 3I/ATLAS's coma was first reported on 20 July 2025, based onnear-infraredspectroscopic observations by theGemini South andNASA Infrared Telescope Facility on 5 and 14 July 2025.[42]Ultraviolet observations by theSwift Observatory suggested the presence ofwater vapor andhydroxideions in 3I/ATLAS's coma on 30 July 2025 and 1 August 2025.[43] On 21 August 2025, astronomers of NASA'sSPHEREx mission and theCalifornia Institute of Technology reported the detection of water ice and brightcarbon dioxide gas emission in SPHEREx observations from mid-August 2025.[44][13] On 22 August 2025, astronomers atLowell Observatory reported the first tentative detection ofcyanide emission in 3I/ATLAS.[45] Spectroscopic observations by theVery Large Telescope on 21 August 2025 confirmed the presence of cyanide and also detectednickel in 3I/ATLAS's coma.[24][46]
TheHubble Space Telescope took its first images of 3I/ATLAS on 21 July 2025, which revealed its coma in high detail and constrained its nucleus diameter to below 5.6 km (3.5 mi).[47][12] The Hubble images were publicized byNASA and theEuropean Space Agency on 7 August 2025.[16][17] On 6 August 2025, JWST made its first observations of 3I/ATLAS using itsNIRSpec instrument,[48][49] and results were announced by NASA on 25 August 2025.[50] In November 2025, Hubble will performultraviolet spectroscopy on 3I/ATLAS to determine the composition of its gas emissions andsulfur-to-oxygen ratio,[22][51] and the telescope will monitor the comet on its way out of the Solar System.[52] The JWST is scheduled to make its next observations of 3I/ATLAS in December 2025, after the comet's perihelion.[22][53]
The comet was observed by theJames Clerk Maxwell Telescope on 7 September 2025, when 3I/ATLAS was located 2.33 AU from the Sun, and found ahydrogen cyanide production rate of(1.5±0.5)×1025 molecules per second, while a week later, on September 14, the production rate had climbed to(4.5±1.9)×1025 molecules per second (2 kg/s).[54] The cyanide coma on 15 September was about 180,000 kilometers across and was asymmetric, being elongated along the anti-solar direction. On the same date there was also a visible dust tail 50 arcseconds across, corresponding to about 100,000 kilometers.[55] On 14 September, the comet had anapparent magnitude of 14.2.[54]
ExoMars TGO with its Colour and Stereo Surface Imaging System (CaSSIS) imaged the coma of 3I/ATLAS as the comet made it closest approach to Mars.[56] The comet was around50000 times fainter than what the orbiter is used to viewing and imaged using 5-second exposures.[56]
3I/ATLAS follows an extremelyhyperbolic trajectory past the Sun because it is moving too fast to be bound by the Sun's gravity.[18] When 3I/ATLAS entered the Solar System, it was moving at a speed of 58 km/s (36 mi/s; 1.02 AU/month) relative to the Sun—this speed is the comet'shyperbolic excess velocity (v∞).[8][7][c] As 3I/ATLAS comes closer to the Sun and gets pulled in by the Sun's gravity, the comet will speed up,[18][57] and then once it begins moving away, the comet will slow down as the Sun's gravity begins pulling back on it.[5] Nevertheless, the comet will escape the Solar System.[32] 3I/ATLAS is moving far faster than the previous twointerstellar objects1I/ʻOumuamua (v∞=26 km/s) and2I/Borisov (v∞=32 km/s).[7]
The shape of 3I/ATLAS's trajectory is described by a parameter known as theorbital eccentricity.[33] Whereaselliptical orbits have an eccentricity less than 1, hyperbolic orbits have an eccentricity greater than 1. For 3I/ATLAS, its trajectory has an extremely highorbital eccentricity of6.137±0.0006.[10][a] This extremely high eccentricity makes the trajectory of 3I/ATLAS appear relatively straight, rather than curved.[58] 3I/ATLAS has the highest eccentricity of the three interstellar objects known to date,[31] greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4).[33]
3I/ATLAS will come closest to the Sun on 29 October 2025 at 11:47 ± 00:01 UT.[9][h] The comet'sperihelion or closest distance to the Sun is 1.36 AU (203 million km; 126 million mi), which lies between the orbits of Earth andMars.[10][18] At perihelion, the comet will be moving at its maximum speed of 68 km/s (42 mi/s) with respect to the Sun.[32][i]
The trajectory of 3I/ATLAS is coincidentally closely aligned with theorbital planes of the Solar System's planets, or theecliptic.[59][60] Specifically, the comet's trajectory istilted 175° (retrograde and inclined 5°) with respect to the ecliptic.[37][10] The trajectory of 3I/ATLAS will bring the comet close to the planetsVenus, Mars, andJupiter, but not Earth.[59] Since 3I/ATLAS cannot come close to Earth, it poses no threat to Earth.[18][59][58] As 3I/ATLAS approaches perihelion, it passed by Mars at a distance of 0.19 AU (28 million km; 18 million mi) on 3 October 2025.[61] After perihelion, it will pass 0.65 AU (97 million km; 60 million mi) from Venus on 3 November 2025,[62] 1.80 AU (269 million km; 167 million mi) from Earth on 19 December 2025,[63] and then it will pass 0.36 AU (54 million km; 33 million mi) from Jupiter on 16 March 2026.[64][10][j]
| Object | Date | Distance |
|---|---|---|
| Mars | 2025-Oct-03 | 0.1935 ± 0.0002 AU (28.947 ± 0.030 million km; 17.987 ± 0.019 million mi)[61][k] |
| Sun | 2025-Oct-29 | 1.3561 ± 0.0003 AU (202.870 ± 0.045 million km; 126.057 ± 0.028 million mi)[9][d] |
| Venus | 2025-Nov-03 | 0.6491 ± 0.0003 AU (97.104 ± 0.045 million km; 60.338 ± 0.028 million mi)[62] |
| Earth | 2025-Dec-19 | 1.7970 ± 0.0004 AU (268.827 ± 0.060 million km; 167.042 ± 0.037 million mi)[63] |
| Jupiter | 2026-Mar-16 | 0.3594 ± 0.0007 AU (53.77 ± 0.10 million km; 33.408 ± 0.065 million mi)[10][j] |
3I/ATLAS is a faint comet that does not get very close to Earth or the Sun and is not expected to get brighter than aboutapparent magnitude 11.5 as seen from Earth.[65][66] Even at its peak brightness, the comet will not be visible to an observer on Earth using thenaked eye[67] or 70 mmbinoculars.[68] 3I/ATLAS has been imaged byplate-solvingsmart telescopes with anaperture diameter of at least 3 to 4.5 in (7.6 to 11.4 cm) that see objects much fainter than visual observers do.[67] As of 3 October 2025[update], 3I/ATLAS had a total magnitude (nucleus+coma) of 12 which made it about 120 times fainter than cometC/2025 A6 (Lemmon) and about 190 times fainter than cometC/2025 R2 (SWAN) at the time.[66]
From July to late September 2025, 3I/ATLAS was observable from Earth after sunset.[69] During the first half of July 2025, 3I/ATLAS was located in theconstellationSagittarius at an apparent magnitude of 17.5.[69] By the second half of July 2025, 3I/ATLAS had moved to the constellationOphiuchus and had brightened to apparent magnitude 16.[69] During that time, the comet was located in a region of the sky where it was densely filled with stars, which made observations challenging as the comet could overlap background stars.[69] The comet will continue brightening as it approaches the Sun, and throughout August 2025 it moved across the constellations Ophiuchus,Scorpius, andLibra.[69] During September 2025, 3I/ATLAS will remain in the constellation Libra as it brightens to around apparent magnitude 12–13.[69][66] Even under a dark sky, to visually observe a point-like star (much less adiffuse comet) of magnitude 14 would require a telescope with an aperture of at least 200 mm (8 in),[70] but given the diffuse nature of the object, a 300 mm (12 in) telescope might be needed.[l]
As the comet approaches the 29 October 2025 perihelion passage, the comet'ssolar elongation orangular separation from the Sun in the sky continues to decrease, which narrowed down its visibility to only equatorial regions of Earth just after sunset.[69] The comet will be less than 30 degrees from the Sun from 1 October 2025 to 9 November 2025.[71] The reason for 3I/ATLAS's decreasing solar elongation before perihelion is because the comet came tosolar conjunction (on the opposite side of the Sun from Earth) on 21 October 2025,[f] 8-days before perihelion.[19][69] This means 3I/ATLAS will appear behind the Sun[f] from Earth during the comet's perihelion, so the comet will not be observable from Earth during this time.[69][59] The comet passed close to Mars during that month andMars orbiters may be able to observe the comet near perihelion.[19] The faint comet is observable from 18 to 24 October with theGOES-19 weather satellite that can see objects down to magnitude 12.[20]
After 3I/ATLAS passes perihelion, it will be visible in the sky again just before sunrise in November 2025.[69] The comet will grow dimmer and its solar elongation will increase as it moves away from the Sun.[69] During December 2025, the comet will move through the constellationsVirgo andLeo and its brightness is expected to become dimmer than apparent magnitude 12.[69]
3I/ATLAS is recognized as aninterstellar object because of its extremely hyperbolic path and very high speed relative to the Solar System.[18] 3I/ATLAS did not pass close enough to any of the Solar System's planets to have gained its speed, so it could not have originated from the Solar System.[72]: 7 Tracing the path of 3I/ATLAS in the sky shows that the comet originated frominterstellar space in the direction of the constellation Sagittarius, near theMilky Way'sGalactic Center.[72]: 3 [37]: 5
Unlike the previous two interstellar objects, 3I/ATLAS originated from thesouthern celestial hemisphere in a direction opposite to thesolar apex in the north,[72]: 3 which is the direction of the Sun's movement relative to local stars.[8]: 6 The southern origin of 3I/ATLAS was unexpected because astronomers initially predicted that more interstellar objects should come from the solar apex, and that telescopes should have a more difficult time discovering southern-origin interstellar objects.[8]: 6–7 It is possible that either 3I/ATLAS is a rare discovery, or southern-origin interstellar objects may be more common than initially thought.[8]: 7
The origin of 3I/ATLAS can be deduced by breaking down its hyperbolic excess velocity intoradial (U), transverse (V), and vertical (W) velocity components in thegalactic coordinate system.[8][m] When 3I/ATLAS arrived to the Solar System, it was moving away from the Galactic Center with a velocity ofU=−51.0 km/s with respect to the Sun[n] and was moving upward through thegalactic plane with a velocity ofW=+18.5 km/s with respect to the Sun.[8]: 2 The verticalW velocity of 3I/ATLAS is quite high compared to those of nearby stars and other interstellar objects, which means that the comet follows a tilted orbit around the Milky Way and thus belongs to either thethin disk orthick disk populations.[25] The thick disk mainly consists of older stars whose compositions havelower levels of heavy elements than the Sun.[26][8]: 3
A July 2025 study led by Matthew Hopkins and collaborators estimated with 68 percent confidence that 3I/ATLAS is between 7.6 and 14 billion years old, based on the typical ages of stars in the thick disk.[26][8]: 4 This means that 3I/ATLAS could be older than the Solar System (which is 4.6 billion years old) and may well be the oldest comet yet seen.[26][8]: 4 An independent analysis by Aster Taylor and Darryl Seligman in July 2025 estimated that 3I/ATLAS should be 3 to 11 billion years old, in broad agreement with Hopkins et al.'s estimate.[22][7]
3I/ATLAS cannot be traced back to its original parent star because the comet has been traveling around the Milky Way for billions of years, which is enough time for it to be mixed around with other stars.[22][8]: 2 It is likely that 3I/ATLAS's speed has undergone changes during its journey through interstellar space, viagravity assists from close encounters with stars andnebulae.[16] A September 2025 study by Yiyang Guo and collaborators found that 3I/ATLAS may have passed within 1parsec (3.3light-years) of 25 known stars in the past 10 million years.[25]
Although the parent star of 3I/ATLAS is unknown, the properties and environment of its parent star can be inferred from the comet's composition and dynamical membership in theGalactic disk.[22] If it is a member of the thick disk, the parent star of 3I/ATLAS could be a low-metallicity star with a heavy element abundance of at least 40 percent of the Sun's.[7]: 5 3I/ATLAS is presumed to have formed within aprotoplanetary disk of gas anddust, which surrounded the parent star when it was young.[7]: 1 [8]: 1 Observations by JWST and SPHEREx have shown that 3I/ATLAS is rich incarbon dioxide (CO
2),[21] which suggests that it formed far from its parent star, beyond theCO
2frost line where temperatures are cold enough forCO
2 tocondense into solid.[23] At some point after its formation, 3I/ATLAS was gravitationally flung out of its parent star system, either by a close encounter with agiant planet or a star.[7]: 1 [8]: 1 [22]
3I/ATLAS appears distinctly fuzzier than stars in telescope images, which means that the solid body ornucleus of 3I/ATLAS is surrounded by acoma,[37]: 5 [15]: 3 [41]: 20 a cloud of gas andicy dust ejected from the comet'soutgassing surface.[67][22] The Sun is responsible for the comet's activity because it heats up the comet's nucleus tosublimate itsvolatile ices into gas, which ejects dust from the comet's surface and escapes into space.[67][22] Dust particles in the coma of 3I/ATLAS eventually trail away from the nucleus (with smaller particles blown away bysolar radiation pressure), leading to the formation of a dusttail behind the comet.[41]: 24
As 3I/ATLAS comes closer to the Sun, it will become more active as its volatile ices will heat up more and begin sublimating faster;[37]: 10 this could potentially lead to outbursts, where the comet suddenly ejects a large amount of dust and brightens,[22][74] or even fragmentation events, where the comet's nucleus breaks into pieces like what happened with the previous interstellar comet 2I/Borisov.[22][59] As of August 2025[update], observations have not detected any outbursts in 3I/ATLAS; its brightness and activity level appear to be steady.[75] Observations by NASA'sTransiting Exoplanet Survey Satellite (TESS) show that 3I/ATLAS may have been exhibiting cometary activity as early as 7 May 2025 (two months before its discovery), when it was roughly 6.4 AU away from the Sun.[3]
The coma of 3I/ATLAS appears slightlyelliptical and spans up to 26,400 by 24,700 km (16,400 by 15,300 mi) in diameter (about twice thediameter of Earth),[o] according to high-resolution images from the 10.4-meter (34 ft)Gran Telescopio Canarias on 2 July 2025.[72]: 2 Observations by NASA's SPHEREx mission from mid-August 2025 showed that 3I/ATLAS has a more extensive carbon dioxide gas coma that spans at least 348,000 km (216,000 mi) in radius, although it is only visible innear-infrared.[13] Because the comet is far away from Earth, its coma appears small in the sky; observations from July 2025 have measured anangular diameter of about 2arcseconds for the most visible part of the coma (the coma'sfull width at half maximum)[37]: 5 [76]: 2 [15]: 1 and 10 arcseconds for the full extent of the coma.[72]: 2 As 3I/ATLAS approaches the Sun and becomes more active, the size and density of its coma will continue to grow. Pre-discovery observations by theVera C. Rubin Observatory showed that the diameter of 3I/ATLAS's coma had grown from 13,040 km (8,100 mi) on 21 June 2025 to 18,760 km (11,660 mi) on 2 July 2025.[41]: 28
Spectroscopic and imaging observations indicate that the coma of 3I/ATLAS has a reddish color and is mainly made of relatively large dust grains that are severalmicrometers (μm) in radius.[42]: 6 [15]: 4 The reddish color of 3I/ATLAS is similar to the colors ofD-type asteroids, Solar System comets, and the interstellar comet 2I/Borisov, and is likely caused byirradiatedorganic compounds such astholins in the comet's coma.[77] Within the coma of 3I/ATLAS, small dust grains with1 μm radii are ejected from the nucleus at fast speeds of22 m/s, whereas large dust grains with100 μm radii are ejected at slower speeds of about2 m/s.[12]: 7 Based on the shape and brightness of 3I/ATLAS's coma inHubble Space Telescope images from July 2025, it is estimated that the comet was ejecting 6 kg (13 lb) of small dust particles per second and 60 kg (130 lb) of large dust particles per second during that month.[12]: 7, 10 The dust ejection rate of 3I/ATLAS is similar to that of 2I/Borisov when it was approaching the Solar System,[15]: 4 but it is lower than the typical rates for distantJupiter-family comets.[14]: 3
During July and August 2025, the coma of 3I/ATLAS appeared to be elongated westward in the sky—in a direction toward the Sun and toward the comet's direction of motion rather than away.[76]: 4 [41]: 22, 24 [23] This Sun-facing feature is not a tail (contrary to initial reports),[14][12]: 5 but is rather a dust plume that is being emitted from the heated, sunlit surface of 3I/ATLAS's nucleus, where ice sublimation occurs faster and thus ejects more dust.[12]: 5 [16] The Sun-facing elongation of 3I/ATLAS's coma resembles those of other distant comets likeC/2014 UN271 (Bernardinelli–Bernstein), which have been known to preferentially eject dust from the sunlit side of their surfaces.[41]: 24
By late August 2025, the coma of 3I/ATLAS no longer appeared elongated toward the Sun and the comet had developed an anti-solar tail.[78][79] However, the Sun-facing plume is still present, as telescope images from 26 August 2025 showed that the inner coma of 3I/ATLAS (within 5 arcseconds from the nucleus) appearedfan-shaped and slightly brighter on the Sun-facing side.[79]
Hubble images from July 2025 first showed that 3I/ATLAS had a very faint and broad tail pointing eastward, in a direction away from the Sun.[12]: 7 A tail pointing away from the Sun is a common cometary feature that is formed when small dust particles are blown away by solar radiation pressure.[12]: 5 The broad appearance of this tail suggests that small dust particles were ejected from 3I/ATLAS's surface at high speeds.[12]: 7 The anti-solar tail of 3I/ATLAS was thought to be pointed directly away from Earth in July 2025, which would have made itforeshortened and mostly hidden behind 3I/ATLAS's coma.[76]: 2 [80]: 2 By the end of August 2025, observations by the 8.2-meter Gemini South telescope showed that the anti-solar tail of 3I/ATLAS had become more visible and had grown to 30 arcseconds in angular length or roughly 56,000 km (35,000 mi).[1][78][p] The tail of 3I/ATLAS is expected to become more obvious as the comet approaches perihelion with changing viewing geometry and increasing cometary activity.[76]: 2 As of 15 September, 3I/ATLAS has a visible dust tail length of 50 arcseconds (~100,000 km).[55]
![Images of 3I/ATLAS by the Very Large Telescope from July and August 2025, showing changes in its coma shape over time. The comet's Sun-facing plume (pointing right) is visible in all images. During August 2025 (bottom row), 3I/ATLAS begins to develop an anti-solar tail pointing left.[40]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a3I-ATLAS_coma_Gray_et_al._2025_Fig_2.png)
High-resolution images by the Hubble Space Telescope from July 2025 indicate that the diameter of 3I/ATLAS's nucleus is between 0.32 and 5.6 km (0.2 and 3.5 mi).[12][16][17] There is a large uncertainty in the estimated diameter of 3I/ATLAS's nucleus because it is surrounded by a coma of reflective dust, which makes the nucleus appear brighter and larger than it actually is.[12]: 4, 12 For example, early studies from July 2025 gave diameter estimates that were as high as 10 to 20 km (6.2 to 12.4 mi), although astronomers were well aware that the nucleus of 3I/ATLAS should be much smaller.[41][37]: 6 While the exact brightness of the coma is unknown, Hubble images show that the coma's brightness must account for a large fraction of the nucleus's apparent brightness, so the actual diameter of 3I/ATLAS's nucleus should be at the lower end of the estimated range.[12]: 4, 12 The estimated dust loss rate of 3I/ATLAS suggests that its nucleus is likely less than a kilometer in diameter, like 2I/Borisov.[12]: 11 Although observations by theSwift Observatory suggested that 3I/ATLAS could have a larger diameter of 4.94 km (3.07 mi) based on the comet's water vapor emission area, this diameter is likely an overestimate because most of 3I/ATLAS's water vapor is believed to come from its coma rather than its nucleus.[43]: 5–6
Near-infraredspectroscopy by JWST in August 2025 has shown that the coma of 3I/ATLAS is unusually rich incarbon dioxide (CO
2) gas, with small amounts ofwater ice,water vapor,carbon monoxide (CO) gas, andcarbonyl sulfide (OCS) gas.[23] The JWST has also revealed the presence of "heavy"CO
2 containing theisotopecarbon-13 (13C) in 3I/ATLAS's coma, although partial obscuration by dust and other gases makes it difficult to quantify the abundance of13C relative to the more common isotope12C.[23] Spectroscopic observations by theSPHERExspace observatory agree that 3I/ATLAS contains high amounts ofCO
2 in its coma, although SPHEREx did not have the sensitivity to detect water vapor and CO.[13][21]
From the August 2025 JWST observations, it is estimated that 3I/ATLAS's nucleus was emitting 129 ± 1 kg (284.4 ± 2.2 lb) ofCO
2 per second,[q] 6.6 ± 0.2 kg (14.6 ± 0.4 lb) of water per second,[r] 14.0 ± 0.9 kg (30.9 ± 2.0 lb) of CO per second,[s] and 0.43 ± 0.09 kg (0.95 ± 0.20 lb) of OCS per second.[t][23] Analysis ofabsorption features in JWST's spectrum of 3I/ATLAS suggests that the water ice in the comet's coma consists of fine grains smaller than 1 micrometer in size.[23] The water ice in 3I/ATLAS is likelyamorphous, although crystalline water ice cannot be ruled out.[23] A tentative detection of water vapor andhydroxyl radicals (OH) by theSwift Observatory on 31 July and 1 August 2025 suggested that the water ice grains in 3I/ATLAS's coma are sublimating beyond the nucleus,[43] likely at a distance between 4,000 and 10,000 km (2,500 and 6,200 mi) from the nucleus.[23] The water ice grains were most likely ejected from the comet's nucleus by outgassing ofvolatile substances such asCO
2 and CO, rather than water ice sublimation.[43]
Ultraviolet spectroscopy by telescopes on Earth have additionally detectedcyanide (CN) gas[45] and atomicnickel (Ni I) vapor in the coma of 3I/ATLAS.[24]: 1 The emission of atomic nickel is not unusual; it has been seen in various comets, including the interstellar comet 2I/Borisov.[81] However, noiron (Fe I) vapor was detected in 3I/ATLAS, which is unusual because nickel and iron are typically found in roughly equal amounts when outgassing from comets.[81][24] Nickel emission in 3I/ATLAS was first detected by the Very Large Telescope (VLT) on 20 July 2025, and cyanide emission was first detected by the VLT later on 14 August 2025.[24]: 5 Throughout July to August 2025, the VLT observed a "rapid and steady" increase in the concentration of nickel and cyanide in 3I/ATLAS's coma, due to the comet's increasing activity during its approach to the Sun (from 4.4 to 2.85 AU).[24]: 7 It is estimated that 3I/ATLAS was emitting 4.6 ± 0.7 g (0.16 ± 0.02 oz) of nickel per second[u] and 17.6 ± 2.0 g (0.62 ± 0.07 oz) of cyanide per second by the time the VLT observations ended on 21 August 2025.[v][24]: 1 Although it is unusual that nickel emission was detected before cyanide, the concentrations of nickel and cyanide in 3I/ATLAS are generally similar to those seen in other Solar System comets observed at similar distances from the Sun.[24]: 9 It is uncertain how nickel vapor is released from 3I/ATLAS;Rohan Rahatgaonkar and colleagues who analyzed the VLT observations proposed that there are several possible processes involving thechemical decomposition of nickel-containingorganic compounds (e.g.nickel tetracarbonyl) viaspace weathering.[46][24]: 12 The amount of nickel observed in 3I/ATLAS is still consistent with earlier predictions that it should have a metal-poor composition.[24]: 12
During July to August 2025, the VLT did not detect any signs of atomicoxygen[O I],dicarbon (C
2),tricarbon (C
3), andamino radicals (NH
2) in the coma of 3I/ATLAS.[24]: 12 Likewise, observations by theMDM Observatory did not detectC
2 andC
3 in the coma of 3I/ATLAS during August 2025, which suggests that the comet is highly depleted incarbon chain compounds.[82] The upper limit to the comet'sC
2-to-CN ratio suggests that 3I/ATLAS is one of the most carbon chain-depleted comets ever known.[82]
![The near-infrared spectrum of 3I/ATLAS as measured by the JWST's NIRSpec instrument on 6 August 2025. The spectrum plots the brightness of 3I/ATLAS (vertical axis) over wavelength of light (horizontal axis). The gases that make up 3I/ATLAS (H 2O, CO 2, and CO) can be seen as emission peaks, which are labeled with their respective names.[23]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a3I-ATLAS_JWST_NIRSpec_Cordiner_et_al._Fig_1.png)
![Ultraviolet spectrum of 3I/ATLAS over time, as measured by the Very large Telescope from 4 July (below) to 21 August 2025 (top). The left and middle panels show emission lines caused by atomic nickel (Ni I), and the right panel shows weak emission lines caused by cyanide (CN) gas. These emission lines grew more obvious over time because 3I/ATLAS was becoming more active during its approach to the Sun.[24]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a3I-ATLAS_ultraviolet_spectrum_Rahatgaonkar_et_al._2025_Fig_1.png)
![Plot showing nickel emission rates of known comets (vertical axis) versus distance from the Sun (horizontal axis). 3I/ATLAS (blue) is not the only interstellar comet known to emit nickel, as 2I/Borisov (green) is shown here as well. The nickel emission rate in 3I/ATLAS has been observed to increase at closer distances from the Sun.[24]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a3I_comet_nickel_production_rates_Rahatgaonkar_et_al._2025_Fig_2.png)
Within the coma of 3I/ATLAS, themixing ratio ofCO
2 relative to water is8.0±1.0—this is one of the highestCO
2 abundances seen in a comet, especially at its distance from the Sun (3.32 AU) when it was first observed by JWST.[23] Whereas Solar System comets generally follow a trend of increasingCO
2-to-water mixing ratio with distance from the Sun (due toCO
2 sublimating more than water at closer distances to the Sun),[21] only 3I/ATLAS and the unusual Oort cloud cometC/2016 R2 (PanSTARRS) are known to break this trend with exceptionally highCO
2-to-water mixing ratios.[23] On the other hand, the coma of 3I/ATLAS has a more typical CO-to-water mixing ratio of1.4±0.2, resembling other Solar System comets including the interstellar comet 2I/Borisov.[23]
The reason for the high abundance ofCO
2 in 3I/ATLAS's coma is uncertain; this could either mean that the comet has aCO
2-rich nucleus or there is some mechanism limiting the sublimation of water from its nucleus.[23] A team of NASA researchers led byMartin A. Cordiner have hypothesized that if 3I/ATLAS has aCO
2-rich nucleus, the comet may have either been exposed to high amounts ofradiation or may have formed close to theCO
2 frost line in its parent star's protoplanetary disk.[23][83] Alternatively, if 3I/ATLAS contains more water than suggested by JWST, 3I/ATLAS could have an unusually thick, insulating crust hiding water ice underneath.[23][83] Further observations by the JWST, especially when 3I/ATLAS becomes more active, will be needed to determine which scenario is more likely.[23]
The nucleus of 3I/ATLAS is expected to be irregularly shaped like other comets, which should cause periodic variations in 3I/ATLAS's brightness as it rotates.[41]: 25 However, because the nucleus of 3I/ATLAS is partially obscured by a dusty coma, the nucleus's brightness variations are reduced, which makes it difficult to measure itsrotation period using telescopic observations.[37]: 8 [80]: 4 [41]: 5–6 Some early studies from July 2025 were unable to determine a rotation period for 3I/ATLAS's nucleus,[37][3][41]: 5 but one study led by Raul de la Fuente Marcos and colleagues reported a rotation period of16.79±0.23 hours, using observations by the Gran Telescopio Canarias from 2 to 5 July.[72] Another study published by Toni Santana-Ros and colleagues in August 2025 reported a similar (albeit shorter) rotation period of16.16±0.01 hours, using observations by multiple different telescopes from 2 to 29 July.[14] Santana-Ros and colleagues noted that the brightness variations of 3I/ATLAS had decreased from 0.3 to 0.2magnitudes throughout July 2025, which likely indicates that 3I/ATLAS's nucleus became more obscured as it became more active.[14]: 3 In August 2025, Viriginio Oldani and other Italian astronomers performed an analysis of 3I/ATLAS's coma shape in Hubble Space Telescope images and suggested that the rotational north pole of 3I/ATLAS's nucleus may be pointed toward the directions of eitherRA 154°,Dec +25° or RA 334°, Dec –25°.[84]
A July 2025 study led by Atsuhiro Yaginuma and collaborators found that launching aspace probe from Earth to perform afly by of 3I/ATLAS is not feasible, as any post-discovery launch (after 1 July 2025) would require an extremely highdelta-v (Δv) at least24 km/s, which is beyond the capability of anypropulsion system available at this time. If 3I/ATLAS had been discovered before 1 July 2025, a space probe launched from Earth on that date would have required Δv~7 km/s to visit the comet. It would have been more feasible to visit 3I/ATLAS using a space probe departing from Mars, which requires substantially less Δv. For example, putative space probes departing from Mars between July and September 2025 would require Δv~5 km/s to fly by 3I/ATLAS in early October 2025.[85]
Between 1 and 7 October 2025, two ESA's Mars orbiters,Trace Gas Orbiter (TGO) andMars Express, observed 3I/ATLAS during its closest approach to the planet. TGO was able to capture the comet with its camera, but it couldn't be seen on photos by Mars Express, likely because of much lower exposure time.[86] NASA'sMars Reconnaissance Orbiter also observed the comet, but data wasn't released due to NASA shutdown.[87] TheJuno spacecraft orbiting Jupiter may be able to observe 3I/ATLAS when it passes close to Jupiter in March 2026,[64] but it is unlikely thatJuno could be redirected to 3I/ATLAS because the spacecraft is low on fuel and has issues with its engine.[88] Astrophysicist Marshall Eubanks has calculated that thePsyche spacecraft passed about 0.302 AU (45.2 million km; 28.1 million mi) from 3I/ATLAS on 4 September 2025, while theJupiter Icy Moons Explorer (Juice) will pass within 0.428 AU (64.0 million km; 39.8 million mi) from 3I/ATLAS on 4 November 2025.[59][89] However, commanding these spacecraft to observe 3I/ATLAS would be difficult and could introduce risks to their primary missions.[59] Despite this,Juice will attempt to observe 3I/ATLAS in November 2025 using its cameras, spectrometers, and a particle sensor. Due to the challenging thermal conditions duringJuice's travel through the inner Solar System, the data from these observations are not expected to reach Earth before February 2026.[89][90][91]
During the period 30 October - 6 November 2025, it is predicted thatEuropa Clipper will potentially be immersed within theion tail of 3I/ATLAS, providing the opportunity to detect the signatures of an interstellar comet's ion tail. Characteristic changes to the solar wind are also expected to be observed; a magnetic draping structure associated with the comet may be identifiable. It is further predicted that spacecraftHera will possibly be immersed within the ion tail of 3I/ATLAS during the period 25 October - 1 November 2025.[92]
On 16 July 2025, astrophysicistAvi Loeb and other researchers of theInitiative for Interstellar Studies published a paper onarXiv speculating that 3I/ATLAS could be anextraterrestrial spacecraft because they believed the object had "anomalous" characteristics,[93] such as having an apparently large size,[94] lack of identifiable chemicals, and an apparently improbable trajectory aligned with the Solar System'secliptic plane.[95] Other astronomers, includingChris Lintott, immediately criticized Loeb's speculation; the science news websiteLive Science reported that "the overwhelming consensus is that it is a comet," with many researchers "disappointed with the new paper and pointed out that it distracts from the work of other scientists."[95] Several astrophysicists, includingScott Manley andDavid Kipping, have pointed out flaws in Loeb's statistical analysis of 3I/ATLAS's trajectory.[96][97]Darryl Seligman, who led the first published study on 3I/ATLAS, stated that "there have been numerous telescopic observations of 3I/ATLAS demonstrating that it's displaying classical signatures of cometary activity."[95] Seligman further elaborated that chemicals in 3I/ATLAS may not be detectable yet since the object was still far away from the Sun.[95] JWST has since confirmed that 3I/ATLAS is outgassingCO
2, water, and CO.[23][21]On 31 July to 1 August, theSwift Observatory found tentative evidence for water vapor and OH. Cyanide gas (CN) and atomic nickel vapor were detected by the VLT on 14 August 2025 and 20 July 2025 respectively.[24]: 1 These substances are commonly found in comets.[81]
Loeb haspreviously suggested that1I/ʻOumuamua and other interstellar objects including the possibly interstellarmeteorCNEOS 2014-01-08 could be extraterrestrial spacecraft, for which he was also criticized by many researchers.[95][98] While Loeb has written in his blog that "the most likely outcome will be that 3I/ATLAS is a completely natural interstellar object, probably a comet," he defended his hypothesis as an "interesting exercise in its own right, and is fun to explore, irrespective of its likely validity."[95] AstronomerSamantha Lawler highlighted that "while it is important to remain open-minded about any 'testable prediction', the new paper [by Loeb et al.] pushes this sentiment to the limit."[95] Lawler further said thatextraordinary claims require extraordinary evidence, while "the evidence presented [by Loeb et al.] is absolutely not extraordinary."[95]
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![VLT FORS2 ESO image of 3I/ATLAS in motion through 13 minutes - visible as a line[99]](/image.pl?url=http%3a%2f%2fen.wikipedia.org%2f%2fupload.wikimedia.org%2fwikipedia%2fcommons%2fthumb%2ff%2ff6%2fSequence_of_VLT_images_of_3I-ATLAS%252C_a_new_interstellar_object_%2528potw2527b%2529.tiff%2flossy-page1-418px-Sequence_of_VLT_images_of_3I-ATLAS%252C_a_new_interstellar_object_%2528potw2527b%2529.tiff.jpg&f=jpg&w=240)
![3I/ATLAS imaged by the Hubble Space Telescope on 21 July 2025.[16][17] This is a false color image, where a blue tint is applied to make fainter details more visible.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a3I-ATLAS_Hubble_heic2509b.jpg)
![Color photo of 3I/ATLAS by Gemini South Observatory. The stars are colored streaks due to the tracking of the comet.[1]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGrowing_Tail_of_Interstellar_Comet_3I-ATLAS_(noirlab2525a).jpg)
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