Top Qs
Timeline
Chat
Perspective

3I/ATLAS

Interstellar comet From Wikipedia, the free encyclopedia

3I/ATLAS
Remove ads

3I/ATLAS, also known as C/2025 N1 (ATLAS) and previously as A11pl3Z, is an interstellar comet[1][2] discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station at Río Hurtado, Chile on 1 July 2025. When it was discovered, it was entering the inner Solar System at a distance of 4.5 astronomical units (670 million km; 420 million mi) from the Sun. The comet follows an unbound, hyperbolic trajectory past the Sun with a very fast hyperbolic excess velocity of 58 km/s (36 mi/s) relative to the Sun.[7][c] 3I/ATLAS will not come closer than 1.8 AU (270 million km; 170 million mi) from Earth, so it poses no threat.[16] It is the third interstellar object confirmed passing through the Solar System, after 1I/ʻOumuamua (discovered in October 2017) and 2I/Borisov (discovered in August 2019),[17] hence the prefix "3I".

Quick facts Discovery, Discovery site ...

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 size of 3I/ATLAS's nucleus is uncertain because its light cannot be separated from that of the coma. 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.[18] Images by the Hubble 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).[13] 3I/ATLAS will continue growing a dust coma and a tail as it comes closer to the Sun.[18]

3I/ATLAS will come closest 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.[4] The comet appears to have originated from the Milky Way's thick disk where older stars reside, which means that the comet could be at least 7 billion years old (older than the Solar System) and could have a water-rich composition.[10] Observations so far have found that the comet contains water, in the form of solid ice grains and water vapor.[19][20] Other volatile ices such as carbon dioxide and carbon monoxide are expected to exist in 3I/ATLAS, although these substances have not been detected yet.[18][21]:8–9 Future observations by more sensitive instruments like the James Webb Space Telescope will help determine the composition of 3I/ATLAS.[18]

Remove ads

History

Summarize
Perspective

Discovery

Thumb
3I/ATLAS was discovered in a starry region of the sky. The discovery image by ATLAS is shown in the inset image, which is a zoomed in view of the location where 3I/ATLAS was discovered (red box).

3I/ATLAS was discovered on 1 July 2025[f] by the NASA-funded ATLAS survey telescope at Río Hurtado, Chile (observatory code W68).[24][25][15] At apparent magnitude 18, the newly discovered object was entering the inner Solar System at a speed of 61 km/s (140,000 mph; 220,000 km/h) relative to the Sun,[16] located 3.50 AU (524 million km; 325 million mi) from Earth and 4.51 AU from the Sun,[22] and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.[17] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center (MPC).[23][25] 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.[25]

Follow-up observations from other observatories, involving both professional and amateur astronomers,[26] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory.[25][27] 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,[23] ZTF observations from 14 to 21 June 2025,[3][28] and ATLAS observations from 25 to 29 June 2025.[17][25][27] 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.[29]

Initial observations of 3I/ATLAS were unclear on whether it is an asteroid or a comet.[17][26][28] Various astronomers including Alan Hale reported no cometary features,[29] but observations on 2 July 2025 by the Deep Random Survey (X09) at Chile, Lowell Discovery Telescope (G37) at Arizona, and Canada–France–Hawaii Telescope (T14) at Mauna Kea all showed a marginal coma with a potential tail-like elongation 3 arcseconds in angular length, which indicated the object is a comet.[3][29] 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.[3][29] The MPC also gave 3I/ATLAS the non-periodic comet designation C/2025 N1 (ATLAS).[3] By the time 3I/ATLAS was officially named, the MPC had collected 122 observations of the comet from 31 different observatories.[3]

Further observations

Thumb
Light curve plot showing the measured brightness of 3I/ATLAS from May to August 2025, expressed in apparent magnitude. The brightness of 3I/ATLAS increases over time because it was approaching the Sun during this time interval. The gray shaded region represents the time when 3I/ATLAS was observed by the Transiting Exoplanet Survey Satellite (TESS).

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 appearance.[30] 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.[31] 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.[32][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, including Zwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025.[33] An even earlier precovery from 21 May 2025, made at Weizmann Astrophysical Observatory (M01), was published on 18 July 2025.[34]

The newly commissioned Vera C. Rubin Observatory has serendipitously imaged 3I/ATLAS during its science validation observations from 21 June to 3 July 2025.[35] These observations showed a slight increase in the comet's coma diameter and provided constraints on the comet's nucleus diameter.[35] The Vera Rubin Observatory would have discovered 3I/ATLAS before the ATLAS survey if it had begun its science validation observations two weeks earlier.[35]:26 NASA's Transiting Exoplanet Survey Satellite (TESS) had also observed 3I/ATLAS before it was discovered, with observations from 7 May to 3 June 2025.[5] 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.[5]

Water ice was first detected in 3I/ATLAS's coma on 5 and 14 July 2025, via near-infrared spectroscopic observations by the Gemini South and NASA Infrared Telescope Facility.[19] Ultraviolet observations by the Swift Observatory detected water vapor and hydroxide ions in 3I/ATLAS's coma on 30 July 2025 and 1 August 2025.[20]

The Hubble Space Telescope took images of 3I/ATLAS on 21 July 2025, which revealed its coma in high detail and constrained the diameter of its nucleus to below 5.6 km (3.5 mi).[36][13] NASA and the European Space Agency announced the Hubble images and measurements in a press release published on 7 August 2025.[1][2] In November 2025, Hubble will perform ultraviolet spectroscopy on 3I/ATLAS to determine the composition of its gas emissions and sulfur-to-oxygen ratio,[18][37] and the telescope will monitor the comet on its way out of the Solar System.[38] Furthermore, the James Webb Space Telescope (JWST) is scheduled to observe 3I/ATLAS in August and December 2025, before and after the comet's perihelion, respectively.[18][39][40] Infrared spectroscopy by the JWST will be able to detect certain compounds in 3I/ATLAS, such as water, carbon monoxide, carbon dioxide and ammonia.[18] The JWST made its first observations of 3I/ATLAS using its NIRSpec instrument on 6 August 2025,[41] although the data will not be released to the public until 3 months after.[39]

Remove ads

Trajectory

Summarize
Perspective
Thumb
Top view of 3I/ATLAS's trajectory (blue) through the Solar System, with orbits and positions of planets shown (click to start animation)
Tilted view of 3I/ATLAS's trajectory through the Solar System, with orbits and positions of planets shown (click to start animation)

3I/ATLAS follows an extremely hyperbolic trajectory past the Sun because it is moving too fast to be bound by the Sun's gravity.[16] 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's hyperbolic excess velocity (v).[10][9][c] As 3I/ATLAS comes closer to the Sun and gets pulled in by the Sun's gravity, the comet will speed up,[16][42] and then once it begins moving away, the comet will slow down as the Sun's gravity begins pulling back on it.[7] Nevertheless, the comet will escape the Solar System.[27] 3I/ATLAS is moving far faster than the previous two interstellar objects 1I/ʻOumuamua (v=26 km/s) and 2I/Borisov (v=32 km/s).[9]

The shape of 3I/ATLAS's trajectory is described by a parameter known as the orbital eccentricity.[28] 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 6.141±0.002.[4][a] This extremely high eccentricity makes the trajectory of 3I/ATLAS appear relatively straight, rather than curved.[43] 3I/ATLAS has the highest eccentricity of the three interstellar objects known to date,[26] greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4).[28]

3I/ATLAS will come closest to the Sun on 29 October 2025 at 11:44 ± 00:02 UT.[11][g] The comet's perihelion or closest distance to the Sun is 1.36 AU (203 million km; 126 million mi), which lies between the orbits of Earth and Mars.[4][16] At perihelion, the comet will be moving at its maximum speed of 68 km/s (42 mi/s) with respect to the Sun.[27][h]

The trajectory of 3I/ATLAS is coincidentally closely aligned with the orbital planes of the Solar System's planets, or the ecliptic.[44][45] Specifically, the comet's trajectory is tilted 175° (retrograde and inclined 5°) with respect to the ecliptic.[32][4] The trajectory of 3I/ATLAS will bring the comet close to the planets Venus, Mars, and Jupiter, but not Earth.[44] Since 3I/ATLAS cannot come close to Earth, it poses no threat to Earth.[16][44][43] As 3I/ATLAS approaches perihelion, it will pass by Mars at a distance of 0.19 AU (28 million km; 18 million mi) on 3 October 2025.[46] After perihelion, it will pass 0.65 AU (97 million km; 60 million mi) from Venus on 3 November 2025,[47] 1.80 AU (269 million km; 167 million mi) from Earth on 19 December 2025,[48] and then it will pass 0.36 AU (54 million km; 33 million mi) from Jupiter on 16 March 2026.[49][4][i]

More information Object, Date ...
Remove ads

Observability

Summarize
Perspective

3I/ATLAS is a faint comet that is not expected to get brighter than about apparent magnitude 11.5 from Earth.[50] Even at its peak brightness, the comet will not be visible to an observer on Earth using their naked eye[51] or to an average observer with a pair of 50 mm binoculars.[52] 3I/ATLAS has been imaged by plate-solving smart telescopes with an aperture diameter of at least 3 to 4.5 in (7.6 to 11.4 cm).[51]

From July to September 2025, 3I/ATLAS is observable from Earth after sunset.[53] During the first half of July 2025, 3I/ATLAS was located in the constellation Sagittarius at an apparent magnitude of 17.5.[53] By the second half of July 2025, 3I/ATLAS had moved to the constellation Ophiuchus and had brightened to apparent magnitude 16.[53] 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.[53] The comet will continue brightening as it approaches the Sun, and throughout August 2025 it will move across the constellations Ophiuchus, Scorpius, and Libra.[53] During September 2025, 3I/ATLAS will remain in the constellation Libra as it brightens from apparent magnitude 14 to 11.7.[53]

As the comet approaches perihelion by October 2025, the comet's solar elongation or angular separation from the Sun in the sky will decrease, which will narrow down its visibility to only equatorial regions of Earth just after sunset.[53] The comet will be less than 30 degrees from the Sun from 1 October 2025 to 9 November 2025.[54] The reason for 3I/ATLAS's decreasing solar elongation before perihelion is because the comet will come to perihelion on the opposite side of the Sun from Earth—in other words, it will enter solar conjunction.[17][53] This means 3I/ATLAS will appear behind the Sun[k] from Earth during the comet's perihelion, so the comet will not be observable from Earth during this time.[53][44] On the other hand, the comet will pass close to Mars during that month and may reach an apparent magnitude of 11 from the planet, which means Mars orbiters may be able to observe the comet near perihelion.[17]

After 3I/ATLAS passes perihelion, it will be visible in the sky again just before sunrise in November 2025.[53] The comet will grow dimmer and its solar elongation will increase as it moves away from the Sun.[53] 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.[53]

Thumb
The apparent path of 3I/ATLAS in Earth's sky from 2024 to 2026. The comet's positions over 10-day intervals are marked with red points and are labeled with their respective dates in yellow. The path begins in the constellation Sagittarius on the left and ends in Gemini on the right. The apparent loops at the path's ends are caused by parallax due to Earth's orbital motion around the Sun.
Remove ads

Origin and age

Summarize
Perspective
Thumb
Thumb
Diagrams illustrating the orbits of the Sun (yellow) and 3I/ATLAS (red) within the Milky Way galaxy. 3I/ATLAS has a tilted orbit that brings it farther above and below the galactic plane than the Sun, as shown by the side view (bottom image).

3I/ATLAS is recognized as an interstellar object because of its extremely hyperbolic path and very high speed relative to the Solar System.[16] 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.[55]:7 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.[55]:3[32]:5

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,[55]:3 which is the direction of the Sun's movement relative to local stars.[10]: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.[10]: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.[10]:7

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.[10][l] When 3I/ATLAS arrived to the Solar System, it was moving away from the Galactic Center with a velocity of U=−51.0 km/s with respect to the Sun[m] and was moving upward through the galactic plane with a velocity of W=+18.5 km/s with respect to the Sun.[10]:2 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 the thick disk population, which mainly consists of older stars whose compositions have lower levels of heavy elements than the Sun.[57][10]:3

A July 2025 study led by Matthew Hopkins and collaborators estimated with 68% confidence that 3I/ATLAS is between 7.6 and 14 billion years old, based on the typical ages of stars in the thick disk.[57][10]: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 seen as of yet.[57][10]: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.[18][9]

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.[18][10]:2 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.[1] Regardless, the properties and environment of its parent star can be inferred from the comet's composition and origin in the thick disk.[18] As a member of the thick disk, the parent star of 3I/ATLAS was likely a low-metallicity star with a heavy element abundance of at least 40% of the Sun's.[9]:5 3I/ATLAS is presumed to have formed within a protoplanetary disk of gas and dust, which surrounded the parent star when it was young.[9]:1[10]:1 3I/ATLAS has been observed to have a water-rich composition, which suggests that it formed far from the parent star, beyond the snow line where temperatures are cold enough for water to condense into solid.[19]:5 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.[9]:1[10]:1[18]

Remove ads

Physical characteristics

Summarize
Perspective

Cometary activity

Coma

Thumb
Left panel: 3I/ATLAS imaged by the Very Large Telescope on 4 July 2025, showing the comet's fuzzy appearance relative to background stars around it. The comet appears elongated toward the Sun.
Right panel: The fuzzy appearance of 3I/ATLAS can be seen in its radial brightness profile, which plots the surface brightness in relation to radius. 3I/ATLAS's surface brightness extends to a greater radius compared to a background star of the same peak brightness, so the comet must have a 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,[32]:5[15]:3[35]:20 a cloud of gas and icy dust ejected from the comet's outgassing surface.[51][18] 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.[51][18] Dust particles in the coma of 3I/ATLAS eventually trail away from the nucleus (with smaller particles blown away by solar radiation pressure), leading to the formation of a dust tail behind the comet.[35]: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;[32]:10 this could potentially lead to outbursts, where the comet suddenly ejects a large amount of dust and brightens,[18][58] or even fragmentation events, where the comet's nucleus breaks into pieces like what happened with the previous interstellar comet 2I/Borisov.[18][44] Observations by NASA's Transiting 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.[5]

The coma of 3I/ATLAS appears slightly elliptical and spans up to 26,400 by 24,700 km (16,400 by 15,300 mi) in diameter (about twice the diameter of Earth),[n] according to high-resolution images from the 10.4-meter (34 ft) Gran Telescopio Canarias on 2 July 2025.[55]:2 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 (the coma's full width at half maximum)[32]:5[59]:2[15]:1 and 10 arcseconds for the full extent of the coma.[55]: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 the Vera 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.[35]:28

Spectroscopic and imaging observations indicate that the coma of 3I/ATLAS is mainly made of relatively large dust grains that are several micrometers (μm) in radius.[19]:6[15]:4 Small dust grains with 1 μm radii are ejected at faster speeds of 22 m/s, whereas large dust grains with 100 μm radii are ejected at slower speeds of about 2 m/s.[13]:7 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 6 kg (13 lb) of small dust particles per second and 60 kg (130 lb) of large dust particles per second during that month.[13]: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 distant Jupiter-family comets.[14]:3

Sun-facing plume and tail

Thumb
3I/ATLAS imaged by the Hubble Space Telescope on 21 July 2025. The image is contour mapped to highlight faint features, such as the anti-sun facing tail on the left (east).

During July 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.[59]:4[35]:22,24 This sun-facing feature is not a tail (contrary to initial reports[14][13]: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.[13]:5[1] The sun-facing elongation of 3I/ATLAS's coma resembles those of other distant comets like C/2014 UN271 (Bernardinelli–Bernstein), which have been known to preferentially eject dust from the sunlit side of their surfaces.[35]:24

Hubble images from July 2025 show that 3I/ATLAS appears to have a very faint and broad tail pointing eastward, in a direction away from the Sun.[13]: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.[13]:5 The broad appearance of this putative tail suggests that small dust particles were ejected from 3I/ATLAS's surface at high speeds.[13]:7 An anti-sun facing tail of 3I/ATLAS is expected to be pointed directly away from Earth, which would make it appear foreshortened and mostly hidden behind 3I/ATLAS's coma.[59]:2[21]:2 The tail of 3I/ATLAS is expected to become more obvious as the comet approaches perihelion with changing viewing geometry and increasing cometary activity.[59]:2

Size

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).[13][1][2] 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.[13]: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.[35][32]: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.[13]: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.[13]:11 Although observations by the Swift 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.[20]:5–6

Composition

Thumb
Thermal evolution model plotting the subsurface temperature of 3I/ATLAS (0–40 cm depth) over time and distance from the Sun. Horizontal black dashed lines indicate temperature thresholds for the sublimation of volatiles (CO, CO2, NH3, and H2O) and vertical purple dotted lines indicate the time of closest approach to a planet. The blue-to-red color gradient indicates the solar elongation of 3I/ATLAS.[60]

In visible wavelengths of light, the coma of 3I/ATLAS appears reddish with a spectrum lacking absorption features, similar to the spectra of D-type asteroids, active Solar System comets, and the interstellar comet 2I/Borisov.[32][19] These objects are known to contain reddish, complex organic compounds produced by extensive exposure to cosmic radiation.[60]:1,3 Since 3I/ATLAS's coma shares a similar spectrum as these objects, it is plausible that it also shares a similar composition, although there would likely be differences due to 3I/ATLAS's different history.[60]:1,3 Spectroscopic observations in ultraviolet and near-infrared wavelengths have shown that the coma of 3I/ATLAS contains water in the form of solid ice grains, water vapor, and hydroxide (OH) ions.[20] 3I/ATLAS has been predicted to have a water-rich composition because it originated from a low-metallicity star from the thick disk.[10][18]

Water vapor and OH emission in 3I/ATLAS was first detected by the Swift Observatory on 31 July and 1 August 2025, when the comet was 3.5 AU from the Sun.[20] Before this, early spectroscopic observations from 4–5 July 2025 did not detect OH in 3I/ATLAS, probably because the comet was too far from the Sun (~4.4 AU) to begin emitting detectable amounts of OH at the time.[61]:3 It is estimated that the comet was emitting water at a rate of 40 kg/s (88 lb/s) over an area of at least 19.5 km2 (7.5 sq mi) during the Swift observations.[20]:5 At distances beyond 2.5 AU from the Sun, temperatures are too cold for water ice to sublimate efficiently inside the comet nucleus, so it is unusual for 3I/ATLAS's coma to contain water vapor and OH when it was 3.5 AU from the Sun.[20]:6 Only a few comets, such as C/1980 E1 (Bowell) and C/2009 P1 (Garradd), are known to emit water vapor and OH at such large distances.[20]:6 Rather than being outgassed directly from the nucleus, the water vapor and OH are believed to be produced by the sublimation of water ice grains in 3I/ATLAS's coma, as has been hypothesized for the two aforementioned comets.[20]:6 The water ice grains in 3I/ATLAS's coma were presumably ejected from the nucleus via outgassing of volatiles such as carbon dioxide (CO2) and carbon monoxide (CO).[20]:6

Because the comet exhibits activity relatively far from the Sun, volatile ices such as CO2 and CO are expected to exist in 3I/ATLAS, although none of these substances have been detected yet.[18][5][21]:8–9 Early spectroscopic observations from 3–5 July 2025 found no evidence of gas emission containing cyanides (CN), dicarbon (C2), tricarbon (C3), oxygen, and CO+ cations—the observed lack of these substances is expected because 3I/ATLAS was too far from the Sun (~4.4 AU) to begin emitting them at the time.[61]:3[60]:4 Several hypotheses for 3I/ATLAS's apparent lack of volatiles have been proposed: an August 2025 study led by Thomas Puzia and colleagues suggested that 3I/ATLAS's volatile outgassing could be suppressed by a thick layer of irradiated organic compounds on its surface,[60]:6 while another August 2025 study led by Zexi Xing and colleagues suggested that 3I/ATLAS could be carbon-poor and more water-rich because of its low-metallicity origin.[20]:6 Future observations by more sensitive instruments like the James Webb Space Telescope will be able to determine the composition of 3I/ATLAS more conclusively, especially when the comet comes closer to the Sun and becomes more active.[18]

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.[35]:25 However, because the nucleus of 3I/ATLAS is partially obscured by a coma, the nucleus's brightness variations are reduced, which makes it difficult to measure its rotation period using telescopic observations.[32]:8[21]:4[35]:5–6 Some early studies from July 2025 were unable to determine a rotation period for 3I/ATLAS's nucleus,[32][5][35]:5 but one study led by Raul de la Fuente Marcos and colleagues reported a rotation period of 16.79±0.23 hours, using observations by the Gran Telescopio Canarias from 2 to 5 July.[55] Another study published by Toni Santana-Ros and colleagues in August 2025 reported a similar (albeit shorter) rotation period of 16.16±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 0.2 magnitudes throughout July 2025, which likely indicates that 3I/ATLAS's nucleus became more obscured as it became more active.[14]:3

Remove ads

Exploration

Summarize
Perspective

Launching a space probe from Earth to perform a fly by of 3I/ATLAS is not feasible, as any post-discovery launch (after 1 July 2025) would require an extremely high delta-vv) at least 24 km/s, which is beyond the capability of any propulsion system available at the time.[62] 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.[62] A July 2025 study led by Atsuhiro Yaginuma and collaborators found that it would be more feasible to visit 3I/ATLAS using a space probe launched from Mars, which requires substantially less Δv.[62] For example, putative space probes launched from Mars between July and September 2025 would require Δv~5 km/s to fly by 3I/ATLAS in early October 2025.[62]

Spacecraft orbiting Mars may be able to observe 3I/ATLAS during its close encounter with Mars in October 2025.[17][44][62] The Juno spacecraft orbiting Jupiter may be able to observe 3I/ATLAS when it passes close to Jupiter in March 2026,[49] 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.[63] Astrophysicist Marshall Eubanks has calculated that the Psyche spacecraft will pass about 0.4 AU (60 million km; 37 million mi) of 3I/ATLAS, while the Jupiter Icy Moons Explorer (Juice) will pass within 0.33 AU (49 million km; 31 million mi) of 3I/ATLAS.[44] However, commanding these spacecraft to observe 3I/ATLAS will be difficult and could introduce risks to their primary missions.[44]

Remove ads

Alien spacecraft speculation

Summarize
Perspective

On 16 July 2025, astrophysicist Avi Loeb of Harvard University and other researchers of the Initiative for Interstellar Studies published a paper on arXiv speculating that 3I/ATLAS could be a "hostile" extraterrestrial spacecraft because they believed the object had "anomalous" characteristics,[64] such as having an apparently large size,[65] lack of identifiable chemicals, and a trajectory aligned with the Solar System's ecliptic plane.[66] Other astronomers, including Chris Lintott of University of Oxford, 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."[66] 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."[66] Seligman further elaborated that chemicals in 3I/ATLAS may not be detectable yet since the object was still far away from the Sun.[66] Since then, observations have reported evidence of 3I/ATLAS containing water,[19][20] which is a substance commonly found in comets.[19][10]

Loeb has previously claimed that 1I/ʻOumuamua and other interstellar objects including the reputedly interstellar meteor CNEOS 2014-01-08 could be extraterrestrial spacecraft, which too have garnered criticism from many researchers.[66][67] 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."[66] Astronomer Samantha Lawler of University of Regina 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."[66] Lawler further cited extraordinary claims require extraordinary evidence, and that "the evidence presented [by Loeb et al.] is absolutely not extraordinary."[66]

Remove ads

1 July 2025

2 July 2025

3 July 2025

4 July 2025

21 July 2025

Remove ads

Notes

  1. The JPL SBDB lists a 1-sigma uncertainty for the eccentricity/perihelion (which covers 68% of the possibilities). A 3-sigma uncertainty would be 3 times larger and would cover 99.7% of the possibilities.
  2. Formula for the perihelion velocity:[8] , where is the gravitational constant, the mass of the Sun, the comet's perihelion distance, and its semi-major axis. Calculation:
  3. Formula for the hyperbolic excess velocity: , where is the gravitational constant, the mass of the Sun, and the comet's semi-major axis. Calculation:
  4. The JPL SBDB shows the time of perihelion passage (tp) is known with a 1-sigma uncertainty of ±2 minutes (which is a 3-sigma uncertainty of ±6 minutes). Math: 0.0012 days * 24 hours * 60 minutes = ≈2 minutes
  5. Mars-MOID is on 3I's outbound node, so Mars crosses this point in March 2027.
  6. In the Minor Planet Center discovery announcement, the discovery observation time (marked with an asterisk "*") is "2025 07 01.218880,"[3] which translates to 1 July 2025 05:15:11 UT.[22] 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.[23]
  7. An n-body integration shows 3I/ATLAS comes to perihelion about 11 minutes later than the JPL SBDB epoch 19 July 2025 solution. (changing 11:33 to 11:44.)
  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 1.36 AU from the Sun. The escape velocity from the surface of the Sun is 618 km/s.
  9. At the close approach to Jupiter on 16 March 2026, the 3-sigma uncertainty in the object's position is ±240 thousand km (0.0016 AU). 3I/ATLAS will be 5.10 AU from the Sun and Jupiter will be 5.24 AU from the Sun.
  10. On 3 October 2025, 3I/ATLAS will be 1.66 AU from the Sun and Mars will be 1.54 AU from the Sun.
  11. The minimum angular separation between the Sun and 3I/ATLAS is 2.59 degrees on 21 October 2025.
  12. In the galactic coordinate system, U positive toward the direction of the Galactic Center, V positive toward the direction of galactic rotation, and W positive toward the direction of the North Galactic Pole.[10]:2[56]
  13. The Sun is also moving away from the Galactic Center and thus has a negative U velocity component, although it is slower than that of 3I/ATLAS.[56]
  14. Earth's mean diameter (averaged between poles and equator) is roughly 12,756 km (7,926 mi) (see mean radius of Earth). Multiplying that by 2 gives 25,512 km (15,852 mi).
Remove ads

References

Loading related searches...

Wikiwand - on

Seamless Wikipedia browsing. On steroids.

Remove ads