3I/ATLAS
3I/ATLAS, also known as C/2025 N1 and previously as A11pl3Z, is an interstellar comet discovered on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System station. The comet follows an unbound, hyperbolic trajectory past the Sun, and passed by Earth at 1.8 AU, posing no threat. The prefix "3I" designates it as the third confirmed interstellar object passing through the Solar System, after 1I/Oumuamua and 2I/Borisov.
3I/ATLAS is an active comet consisting of a solid icy nucleus and a coma, which is a cloud of gas and icy dust escaping from the nucleus. The Sun is responsible for the comet's activity because it heats up the comet's nucleus to sublimate its ice into gas, which outgasses and lifts up dust from the comet's surface to form its coma. Images taken by the Hubble Space Telescope and various interplanetary spacecraft suggest that the diameter of 3I/ATLAS's nucleus is less than. Observations by the James Webb Space Telescope have shown that 3I/ATLAS is unusually rich in carbon dioxide, and contains a small amount of water ice, water vapor, carbon monoxide, and carbonyl sulfide. Observations by the Very Large Telescope have also shown that 3I/ATLAS is emitting cyanide gas and atomic nickel vapor, at concentrations similar to those seen in Solar System comets.
The comet came to solar conjunction on 21 October 2025, and it came closest to the Sun on 29 October 2025, at a distance of from the Sun, which is between the orbits of Earth and Mars. The comet appears to have originated from either the Milky Way's thin disk, or thick disk; if 3I/ATLAS originated from the thick disk, the comet could be at least 7 billion years oldolder than the Solar System.
History
Discovery
3I/ATLAS was discovered on 1 July 2025 by the NASA-funded ATLAS survey telescope at Río Hurtado, Chile. At apparent magnitude 18, the newly discovered object was entering the inner Solar System at a speed of relative to the Sun, located from Earth and from the Sun, and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane. It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center. 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 temporarily list the object on the Near-Earth Object Confirmation Page until the orbit could be confirmed.Follow-up observations from other observatories, involving both professional and amateur astronomers, began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory. Pre-discovery observations of 3I/ATLAS confirmed its interstellar trajectory; these included Zwicky Transient Facility observations from 28 to 29 June 2025 that were found within a few hours of the initial report, ZTF observations from 14 to 21 June 2025, and ATLAS observations from 25 to 29 June 2025. 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.
Initial observations of 3I/ATLAS were unclear on whether it is an asteroid or a comet. Various astronomers including Alan Hale reported no cometary features, but observations on 2 July 2025 by the Deep Random Survey at Chile, Lowell Discovery Telescope at Arizona, and Canada–France–Hawaii Telescope at Mauna Kea all showed a marginal coma with a potential tail-like elongation 3 arcseconds in angular length, which indicated the object is a comet. 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. The MPC also gave 3I/ATLAS the non-periodic comet designation C/2025 N1. By the time 3I/ATLAS was officially named, the MPC had collected 122 observations of the comet from 31 different observatories.
Further observations
Observations by David C. Jewitt and Jane Luu using the Nordic Optical Telescope on 2 July 2025 confirmed that 3I/ATLAS was "clearly active" with a diffuse appearance. Miguel R. Alarcón and a team of researchers of the IAC using Teide Observatory's Two-meter Twin Telescope also found cometary activity on the same date. 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. 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.On 6 July, additional observations were published, including Zwicky Transient Facility precoveries from several nights between 22 May and 21 June 2025. An even earlier precovery from 21 May 2025, made at Weizmann Astrophysical Observatory, was published on 18 July 2025.
Polarimetric observations by the Very Large Telescope, Nordic Optical Telescope, and Rozhen Observatory from July and August 2025 revealed that 3I/ATLAS's coma exhibits an unusually high degree of negative polarization at small phase angles—meaning a large percent of the light reflected from 3I/ATLAS's coma have their oscillations oriented along the Sun-comet-observer plane. The negative polarization of 3I/ATLAS appears similar to those seen in trans-Neptunian objects, and suggests that its coma is made of a mixture of icy and dark material.
The newly commissioned Vera C. Rubin Observatory has serendipitously imaged 3I/ATLAS during its science validation observations from 21 June to 3 July 2025. These observations showed a slight increase in the comet's coma diameter and provided constraints on the comet's nucleus diameter. The Vera Rubin Observatory would have discovered 3I/ATLAS before the ATLAS survey if it had begun its science validation observations two weeks earlier. NASA's Transiting Exoplanet Survey Satellite had also observed 3I/ATLAS before it was discovered, with observations from 7 May to 3 June 2025. 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 of volatile ices other than water.
Water ice in 3I/ATLAS's coma was first reported on 20 July 2025, based on near-infrared spectroscopic observations by the Gemini South and NASA Infrared Telescope Facility on 5 and 14 July 2025. Ultraviolet observations by the Swift Observatory suggested the presence of water vapor and hydroxide ions in 3I/ATLAS's coma on 30 July 2025 and 1 August 2025. On 21 August 2025, astronomers of NASA's SPHEREx mission and the California Institute of Technology reported the detection of water ice and bright carbon dioxide gas emission in SPHEREx observations from mid-August 2025. On 22 August 2025, astronomers at Lowell Observatory reported the first tentative detection of cyanide emission in 3I/ATLAS. Spectroscopic observations by the Very Large Telescope on 21 August 2025 confirmed the presence of cyanide and also detected nickel in 3I/ATLAS's coma.
The Hubble 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. The Hubble images were publicized by NASA and the European Space Agency on 7 August 2025. On 6 August 2025, JWST made its first observations of 3I/ATLAS using its NIRSpec instrument, and results were announced by NASA on 25 August 2025. In November 2025, Hubble performed ultraviolet spectroscopy on 3I/ATLAS to determine the composition of its gas emissions and sulfur-to-oxygen ratio, and the telescope will monitor the comet on its way out of the Solar System. The JWST is scheduled to make its next observations of 3I/ATLAS in December 2025, after the comet's perihelion.
The comet was observed by the James Clerk Maxwell Telescope on 7 September 2025, when 3I/ATLAS was located 2.33 AU from the Sun, and found a hydrogen cyanide production rate of molecules per second, while a week later, on September 14, the production rate had climbed to molecules per second. 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. On 14 September, the comet had an apparent magnitude of 14.2.
The ExoMars TGO with its Colour and Stereo Surface Imaging System imaged the coma of 3I/ATLAS as the comet made its closest approach to Mars. The comet was around 50,000 times fainter than what the orbiter is used to viewing and imaged using 5-second exposures.
Trajectory
3I/ATLAS follows an extremely hyperbolic trajectory past the Sun because it is moving too fast to be bound by the Sun's gravity. When 3I/ATLAS entered the Solar System, it was moving at a speed of relative to the Sun this speed is the comet's hyperbolic excess velocity. As 3I/ATLAS came closer to the Sun and was pulled in by the Sun's gravity, the comet sped up, and then once it began moving away, the comet slowed down as the Sun's gravity began pulling back on it. The comet will escape the Solar System nevertheless. 3I/ATLAS is moving far faster than the previous two interstellar objects 1I/Oumuamua and 2I/Borisov.The shape of 3I/ATLAS's trajectory is described by a parameter known as the orbital eccentricity. Whereas elliptical orbits have an eccentricity less than 1, hyperbolic orbits have an eccentricity greater than 1. For 3I/ATLAS, its trajectory has an extremely high orbital eccentricity of. This extremely high eccentricity makes the trajectory of 3I/ATLAS appear relatively straight, rather than curved. 3I/ATLAS has the highest eccentricity of the three interstellar objects known to date, greater than 1I/Oumuamua's and 2I/Borisov's.
3I/ATLAS passed closest to the Sun on 29 October 2025 at 11:44 UT. The comet's perihelion or closest distance to the Sun was, which lies between the orbits of Earth and Mars. At perihelion, the comet was moving at its maximum speed of with respect to the Sun.
The trajectory of 3I/ATLAS is coincidentally closely aligned with the orbital planes of the Solar System's planets, or the ecliptic. Specifically, the comet's trajectory is tilted 175° with respect to the ecliptic. The trajectory of 3I/ATLAS will bring the comet close to the planets Venus, Mars, and Jupiter, but not Earth, and hence poses no threat. As 3I/ATLAS approaches perihelion, it passed by Mars at a distance of on 3 October 2025. After perihelion, it passed from Venus on 3 November 2025. It passed from Earth on 19 December 2025, when it was inside the asteroid belt and beyond the 4:1 Kirkwood gap. The comet will pass from Jupiter on 16 March 2026. The comet will reach the inner Oort cloud around the year 2189, and will reach the outer edge of the Oort cloud in about 8000 years.
| Object | Date | Distance |
| Mars | 2025-Oct-03 04:38 UT | |
| Mercury | 2025-Oct-08 02:43 UT | |
| Sun | 2025-Oct-29 11:44 UT | |
| Venus | 2025-Nov-03 05:56 UT | |
| Earth | 2025-Dec-19 06:01 UT | |
| Jupiter | 2026-Mar-16 12:22 UT | |
| Eupheme | 2026-Mar-17 10:41 UT |
Observability
3I/ATLAS is a faint comet that does not get very close to Earth or the Sun and was not expected to get brighter than about apparent magnitude 11.5 as seen from Earth. Even at its peak brightness, the comet was not visible to an observer on Earth using the naked eye or smaller binoculars. 3I/ATLAS has been imaged by plate-solving smart telescopes with an aperture diameter of at least that see objects much fainter than visual observers do. During the Mars approach on 3 October 2025, 3I/ATLAS had a total magnitude of 12 which made it about 120 times fainter than comet C/2025 A6 (Lemmon) and about 190 times fainter than comet C/2025 R2 (SWAN) at the time. By mid-November, the comet was on the borderline to be able to be detected in tripod-mounted 20x80 binoculars., 3I/ATLAS is around magnitude 13 and located in Cancer.From July to late September 2025, 3I/ATLAS was observable from Earth after sunset. During the first half of July 2025, 3I/ATLAS was located in the constellation Sagittarius at an apparent magnitude of 17.5. By the second half of July 2025, 3I/ATLAS had moved to the constellation Ophiuchus and had brightened to apparent magnitude 16. 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. The comet continued brightening as it approached the Sun, and throughout August 2025 it moved across the constellations Ophiuchus, Scorpius, and Libra. During September 2025, 3I/ATLAS remained in the constellation Libra as it brightens to around apparent magnitude 12–13. Even under a dark sky, to visually observe a point-like star of magnitude 14 would require a telescope with an aperture of at least, but given the diffuse nature of the object, a telescope might be needed.
As the comet approached the 29 October 2025 perihelion passage, the comet's solar elongation or angular separation from the Sun in the sky continued to decrease, which narrowed down its visibility to only equatorial regions of Earth just after sunset. The comet was less than 30 degrees from the Sun from 1 October 2025 to 9 November 2025. The reason for 3I/ATLAS's decreasing solar elongation before perihelion is because the comet came to solar conjunction on 21 October 2025, 8 days before perihelion. This means 3I/ATLAS appeared behind the Sun from Earth during the comet's perihelion, so the comet was not observable from Earth during this time. The comet passed close to Mars during that month and Mars orbiters may be able to observe the comet near perihelion. The faint comet was observable from 18 to 24 October with the GOES-19 weather satellite that can see objects down to magnitude 12. During the same period it was also observed by Solar and Heliospheric Observatory and the Polarimeter to Unify the Corona and Heliosphere satellites.
After 3I/ATLAS passed perihelion, it became visible in the sky again just before sunrise in November 2025. Further observations and spectrometry was conducted by Physical Research Laboratory scientists from the 1.2m infra-red telescope on Mount Abu in mid-November 2025. The comet grew dimmer and its solar elongation increased as it moved away from the Sun. During December 2025, the comet will move through the constellations Virgo and Leo and its brightness is expected to become dimmer than apparent magnitude 12.
On 11 December 2025, the comet passed less than two degrees from the half Moon. By 11 December 2025, the comet went more than 90 degrees from the Sun and became observable at midnight. But opposition, 177.6 degrees, when the comet will be up all night, will not occur until 22 January 2026, when the comet will be much fainter.
Origin and age
3I/ATLAS is recognized as an interstellar object because of its extremely hyperbolic path and very high speed relative to the Solar System. 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. Tracing the path of 3I/ATLAS in the sky shows that the comet originated from interstellar space in the direction of the constellation Sagittarius, near the Milky Way's Galactic Center.Unlike the previous two interstellar objects, 3I/ATLAS originated from the southern celestial hemisphere in a direction opposite to the solar apex in the north, which is the direction of the Sun's movement relative to local stars. 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. It is possible that either 3I/ATLAS is a rare discovery, or southern-origin interstellar objects may be more common than initially thought.
The origin of 3I/ATLAS can be deduced by breaking down its hyperbolic excess velocity into radial (U), transverse (V), and vertical (W) velocity components in the galactic coordinate system. When 3I/ATLAS arrived to the Solar System, it was moving away from the Galactic Center with a velocity of U with respect to the Sun and was moving upward through the galactic plane with a velocity of W+ with respect to the Sun. The vertical W 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 the thin disk or thick disk populations. The thick disk mainly consists of older stars whose compositions have lower levels of heavy elements than the Sun.
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. This means that 3I/ATLAS could be older than the Solar System and may well be the oldest comet yet seen. 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.
Parent star and formation
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. It is likely that 3I/ATLAS's speed has undergone changes during its journey through interstellar space, via gravity assists from close encounters with stars and nebulae. A September 2025 study by Yiyang Guo and collaborators found that 3I/ATLAS may have passed within of 25 known stars in the past 10 million years.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 the Galactic disk. 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. 3I/ATLAS is presumed to have formed within a protoplanetary disk of gas and dust, which surrounded the parent star when it was young. Observations by JWST and SPHEREx have shown that 3I/ATLAS is rich in carbon dioxide, which suggests that it formed far from its parent star, beyond the frost line where temperatures are cold enough for to condense into solid. At some point after its formation, 3I/ATLAS was gravitationally flung out of its parent star system, either by a close encounter with a giant planet or a star.
Physical characteristics
Cometary activity
Coma
3I/ATLAS appears distinctly fuzzier than stars in telescope images, which means that the solid body or nucleus of 3I/ATLAS is surrounded by a coma, a cloud of gas and icy dust ejected from the comet's outgassing surface. The Sun is responsible for the comet's activity because it heats up the comet's nucleus to sublimate its volatile ices into gas, which ejects dust from the comet's surface and escapes into space. Dust particles in the coma of 3I/ATLAS eventually trail away from the nucleus, leading to the formation of a dust tail behind the comet.As 3I/ATLAS came closer to the Sun, it became more active as its volatile ices heated up more and began sublimating faster; this could potentially have led to outbursts, where the comet suddenly ejects a large amount of dust and brightens, or even fragmentation events, where the comet's nucleus breaks into pieces like what happened with the previous interstellar comet 2I/Borisov., observations had not detected any outbursts in 3I/ATLAS; its brightness and activity level appear to be steady. Observations by NASA's Transiting Exoplanet Survey Satellite show that 3I/ATLAS may have been exhibiting cometary activity as early as 7 May 2025, when it was roughly 6.4 AU away from the Sun.
The coma of 3I/ATLAS appears slightly elliptical and spans up to in diameter, according to high-resolution images from the Gran Telescopio Canarias on 2 July 2025. 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 in radius, although it is only visible in near-infrared. Because the comet is far away from Earth, its coma appears small in the sky; observations from July 2025 have measured an angular diameter of about 2 arcseconds for the most visible part of the coma and 10 arcseconds for the full extent of the coma. As 3I/ATLAS approaches the Sun and becomes more active, the size and density of its coma continued to grow. Pre-discovery observations by the Vera C. Rubin Observatory showed that the diameter of 3I/ATLAS's coma had grown from on 21 June 2025 to on 2 July 2025.
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 several micrometers in radius. The reddish color of 3I/ATLAS is similar to the colors of D-type asteroids, Solar System comets, and the interstellar comet 2I/Borisov, and is likely caused by irradiated organic compounds such as tholins in the comet's coma. Within the coma of 3I/ATLAS, small dust grains with radii are ejected from the nucleus at fast speeds of, whereas large dust grains with radii are ejected at slower speeds of about. Based on the shape and brightness of 3I/ATLAS's coma in Hubble Space Telescope images from July 2025, it is estimated that the comet was ejecting of small dust particles per second and of large dust particles per second during that month. The dust ejection rate of 3I/ATLAS is similar to that of 2I/Borisov when it was approaching the Solar System, but it is lower than the typical rates for distant Jupiter-family comets.
Sun-facing plume
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. This Sun-facing feature is not a tail, 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. The Sun-facing elongation of 3I/ATLAS's coma resembles those of other distant comets like (Bernardinelli–Bernstein), which have been known to preferentially eject dust from the sunlit side of their surfaces.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. However, the Sun-facing plume is still present, as telescope images from 26 August 2025 showed that the inner coma of 3I/ATLAS appeared fan-shaped and slightly brighter on the Sun-facing side.
Tail
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. 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. The broad appearance of this tail suggests that small dust particles were ejected from 3I/ATLAS's surface at high speeds. The anti-solar tail of 3I/ATLAS was thought to be pointed directly away from Earth in July 2025, which would have made it foreshortened and mostly hidden behind 3I/ATLAS's coma. 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. The tail of 3I/ATLAS is expected to become more obvious as the comet approaches perihelion with changing viewing geometry and increasing cometary activity. As of 15 September, 3I/ATLAS has a visible dust tail length of 50 arcseconds. Post-perihelion, the tail of the comet was recovered by Michael Jäger on 8 November 2025.Size and mass
High-resolution images by the Hubble Space Telescope from July 2025 indicate that the diameter of 3I/ATLAS's nucleus is between. 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. For example, early studies from July 2025 gave diameter estimates that were as high as, although astronomers were well aware that the nucleus of 3I/ATLAS should be much smaller. 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. The estimated dust loss rate of 3I/ATLAS suggests that its nucleus is likely less than a kilometer in diameter, like 2I/Borisov.Observations by various interplanetary spacecraft have shown that 3I/ATLAS experienced significant non-gravitational acceleration that peaked up to AU/day2 one month before perihelion. If this non-gravitational acceleration is purely caused by outgassing, it would suggest that 3I/ATLAS's nucleus has a mass of roughly. If 3I/ATLAS has a typical comet nucleus density of, then this mass would correspond to a nucleus diameter between, which is in agreement with estimates from Hubble imaging.
Composition and gas emissions
Near-infrared spectroscopy by JWST in August 2025 has shown that the coma of 3I/ATLAS is unusually rich in carbon dioxide gas, with small amounts of water ice, water vapor, carbon monoxide gas, and carbonyl sulfide gas. The JWST has also revealed the presence of "heavy" containing the isotope carbon-13 in 3I/ATLAS's coma, although partial obscuration by dust and other gases makes it difficult to quantify the abundance of 13C relative to the more common isotope 12C. Spectroscopic observations by the SPHEREx space observatory agree that 3I/ATLAS contains high amounts of in its coma, although SPHEREx did not have the sensitivity to detect water vapor and CO.From the August 2025 JWST observations, it is estimated that 3I/ATLAS's nucleus was emitting of per second, of water per second, of CO per second, and of OCS per second. Analysis of absorption 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. The water ice in 3I/ATLAS is likely amorphous, although crystalline water ice cannot be ruled out. A tentative detection of water vapor and hydroxyl radicals by the Swift Observatory on 31 July and 1 August 2025 suggested that the water ice grains in 3I/ATLAS's coma are sublimating beyond the nucleus, likely at a distance between from the nucleus. The water ice grains were most likely ejected from the comet's nucleus by outgassing of volatile substances such as and CO, rather than water ice sublimation.
Ultraviolet spectroscopy by telescopes on Earth have additionally detected cyanide gas and atomic nickel vapor in the coma of 3I/ATLAS. The emission of atomic nickel is not unusual; it has been seen in various comets, including the interstellar comet 2I/Borisov. However, no iron vapor was detected in 3I/ATLAS, which is unusual because nickel and iron are typically found in roughly equal amounts when outgassing from comets. Nickel emission in 3I/ATLAS was first detected by the Very Large Telescope on 20 July 2025, and cyanide emission was first detected by the VLT later on 14 August 2025. 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. It is estimated that 3I/ATLAS was emitting of nickel per second and of cyanide per second by the time the VLT observations ended on 21 August 2025. 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. 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 the chemical decomposition of nickel-containing organic compounds via space weathering. The amount of nickel observed in 3I/ATLAS is still consistent with earlier predictions that it should have a metal-poor composition.
During July to August 2025, the VLT did not detect any signs of atomic oxygen Forbidden mechanism|, dicarbon, tricarbon, and amino radicals in the coma of 3I/ATLAS. Likewise, observations by the MDM Observatory did not detect and in the coma of 3I/ATLAS during August 2025, which suggests that the comet is highly depleted in carbon chain compounds. The upper limit to the comet's -to-CN ratio suggests that 3I/ATLAS is one of the most carbon chain-depleted comets ever known.
Carbon dioxide abundance
Within the coma of 3I/ATLAS, the mixing ratio of relative to water is —this is one of the highest abundances seen in a comet, especially at its distance from the Sun when it was first observed by JWST. Whereas Solar System comets generally follow a trend of increasing -to-water mixing ratio with distance from the Sun, only 3I/ATLAS and the unusual Oort cloud comet C/2016 R2 (PanSTARRS) are known to break this trend with exceptionally high -to-water mixing ratios. On the other hand, the coma of 3I/ATLAS has a more typical CO-to-water mixing ratio of, resembling other Solar System comets including the interstellar comet 2I/Borisov.The reason for the high abundance of in 3I/ATLAS's coma is uncertain; this could either mean that the comet has a -rich nucleus or there is some mechanism limiting the sublimation of water from its nucleus. A team of NASA researchers led by Martin A. Cordiner have hypothesized that if 3I/ATLAS has a -rich nucleus, the comet may have either been exposed to high amounts of radiation or may have formed close to the frost line in its parent star's protoplanetary disk. Alternatively, if 3I/ATLAS contains more water than suggested by JWST, 3I/ATLAS could have an unusually thick, insulating crust hiding water ice underneath. Further observations by the JWST, especially when 3I/ATLAS becomes more active, will be needed to determine which scenario is more likely.
Rotation
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. 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 its rotation period using telescopic observations. Some early studies from July 2025 were unable to determine a rotation period for 3I/ATLAS's nucleus, but one study led by Raul de la Fuente Marcos and colleagues reported a rotation period of hours, using observations by the Gran Telescopio Canarias from 2 to 5 July. Another study published by Toni Santana-Ros and colleagues in August 2025 reported a similar rotation period of hours, using observations by multiple different telescopes from 2 to 29 July. Santana-Ros and colleagues noted that the brightness variations of 3I/ATLAS had decreased from 0.3 to 0.2 magnitudes throughout July 2025, which likely indicates that 3I/ATLAS's nucleus became more obscured as it became more active. In August 2025, Virginio 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 either RA 154°, Dec +25° or RA 334°, Dec –25°. A December 2025 analysis by Miquel Serra-Ricart and colleagues found that 3I/ATLAS exhibited a wobbling jet of gas and dust in images from August 2025, which they attribute to the rotation of the comet's nucleus. Serra-Ricart and colleagues determined a rotation period of hours based on the wobbling rate of 3I/ATLAS jet.Exploration
A July 2025 study led by Atsuhiro Yaginuma and collaborators found that launching a space probe from Earth to perform a fly by of 3I/ATLAS is not feasible, as any post-discovery launch would require an extremely high delta-v at least, which is beyond the capability of any propulsion 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~ 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~ to fly by 3I/ATLAS in early October 2025.Between 1 and 7 October 2025, two of ESA's Mars orbiters, Trace Gas Orbiter and Mars 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. NASA's Mars Reconnaissance Orbiter also observed the comet, but data wasn't released due to NASA being shut down. The CNSA's Mars orbiter Tianwen-1 also managed to image 3I/ATLAS.
The Juno spacecraft orbiting Jupiter may be able to observe 3I/ATLAS when it passes close to Jupiter in March 2026, but it is unlikely that Juno could be redirected to 3I/ATLAS because the spacecraft is low on fuel and has issues with its engine. Astrophysicist Marshall Eubanks has calculated that the Psyche spacecraft passed about from 3I/ATLAS on 4 September 2025, while the Jupiter Icy Moons Explorer passed within from 3I/ATLAS on 4 November 2025. However, commanding these spacecraft to observe 3I/ATLAS would be difficult and could introduce risks to their primary missions. Despite this, Juice attempted to observe 3I/ATLAS in November 2025 using its cameras, spectrometers, and a particle sensor. Due to the challenging thermal conditions during Juice
From 30 October to 6 November 2025, it was predicted that Europa Clipper would potentially be immersed within the ion 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 was further predicted that spacecraft Hera would possibly be immersed within the ion tail of 3I/ATLAS from 25 October to 1 November 2025.
Alien spacecraft speculation
On 16 July 2025, astrophysicist Avi Loeb and other researchers of the Initiative for Interstellar Studies published a paper on arXiv speculating that 3I/ATLAS could be an extraterrestrial spacecraft because they believed the object had "anomalous" characteristics, such as having an apparently large size, lack of identifiable chemicals, and an apparently improbable trajectory aligned with the Solar System's ecliptic plane. Other astronomers, including Chris Lintott, immediately criticized Loeb's speculation; the science news website Live 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". Several astrophysicists, including Scott Manley and David Kipping, have pointed out flaws in Loeb's statistical analysis of 3I/ATLAS's trajectory. 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." Seligman further elaborated that chemicals in 3I/ATLAS may not be detectable yet since the object was still far away from the Sun. JWST has since confirmed that 3I/ATLAS is outgassing, water, and CO.On 31 July to 1 August, the Swift Observatory found tentative evidence for water vapor and OH. Cyanide gas and atomic nickel vapor were detected by the VLT on 14 August 2025 and 20 July 2025 respectively. These substances are commonly found in comets.
Loeb has previously suggested that 1I/Oumuamua and other interstellar objects including the possibly interstellar meteor CNEOS 2014-01-08 could be extraterrestrial spacecraft, for which he was also criticized by many researchers. 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". Astronomer Samantha Lawler highlighted that "while it is important to remain open-minded about any 'testable prediction', the new paper pushes this sentiment to the limit." Lawler further said that extraordinary claims require extraordinary evidence, while "the evidence presented is absolutely not extraordinary."