3I/ATLAS

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

3I/ATLAS, also known as C/2025 N1 (ATLAS) and previously as A11pl3Z, is an interstellar comet discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station at Río Hurtado, Chile on 1 July 2025, when 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 an orbital eccentricity of 6.14 and a very fast hyperbolic excess velocity of 58 km/s (36 mi/s) relative to the Sun.^[4][c] 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).^[15]

3I/ATLAS

3I/ATLAS imaged by the Hubble Space Telescope on 21 July 2025
Discovery[1]
Discovery site	ATLAS–CHL (W68)
Discovery date	1 July 2025
Designations
MPC designation	C/2025 N1
Alternative designations	A11pl3Z
Orbital characteristics[2]
Epoch	15 July 2025 (JD 2460871.5)
Observation arc	67 days
Earliest precovery date	21 May 2025 (41 days before discovery)
Number of observations	1,001
Orbit type	Hyperbolic (interstellar)
Perihelion	1.3569±0.0004 AU
Semi-major axis	−0.26383±0.00006 AU
Eccentricity	6.143±0.003[a]
Max. orbital speed	68.3 km/s @ perihelion[3][4][b] 58 km/s @ ${\displaystyle v_{\infty }}$[4][6][7][c]
Inclination	175.11±0.0002° (retrograde and inclined 5°)
Longitude of ascending node	322.16±0.004°
Argument of periapsis	128.01±0.004°
Next perihelion	29 October 2025 11:35 ± 00:05 UT[8]
Earth MOID	0.3662 AU
Mars MOID	0.0179 AU[9]
Jupiter MOID	0.2466 AU
Physical characteristics
Mean diameter	~4–5 km (2.5–3.1 mi)[d] <11.2 km (7.0 mi)[e]
Synodic rotation period	16.79±0.23 h[13]
Spectral type	B–V = 0.98±0.23[14] V–R = 0.71±0.09 R–I = 0.14±0.10
Absolute magnitude (H)	13.7±0.2[11]
Comet total magnitude (M1)	12.3±0.7[2]

The size of 3I/ATLAS's nucleus is uncertain because it is an active comet surrounded by a coma,^[14][16] a diffuse envelope made of icy dust ejected from the comet's outgassing surface.^[17] The Sun is responsible for the comet's activity because it heats up the comet's nucleus to sublimate its ice into gas, which lifts dust up from the comet's surface and escapes into space.^[18] Estimates for the nucleus diameter of 3I/ATLAS range from 0.8 to 11 km (0.5 to 6.8 mi),^[16][11] though a diameter toward the lower end of the range is more likely.^[19] 3I/ATLAS will continue growing a dust coma and a tail as it comes closer to the Sun, which will complicate future measurements of the comet's nucleus size.^[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.^[2] 3I/ATLAS poses no threat to Earth; it will not come closer than 1.8 AU (270 million km; 170 million mi).^[20] 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.^[7] Observations so far have found that comet is made of water ice and silicate minerals, which are substances commonly found in comets.^[17] Other volatile ices such as carbon dioxide and ammonia are expected to exist in 3I/ATLAS, but have not been detected yet.^[18] Future observations by more sensitive instruments like the James Webb Space Telescope will be able to determine the composition of 3I/ATLAS.^[18]

History

Discovery

3I/ATLAS was discovered on 1 July 2025^[f] by the NASA-funded ATLAS survey telescope at Río Hurtado, Chile (observatory code W68).^[23][24][14] 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,^[20] located 3.50 AU (524 million km; 325 million mi) from Earth and 4.51 AU from the Sun,^[21] and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.^[15] It was given the temporary designation 'A11pl3Z' and the discovery observations were submitted to the International Astronomical Union's Minor Planet Center (MPC).^[22][24] 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.^[24]

Follow-up observations from other observatories, involving both professional and amateur astronomers,^[25] began to reveal that the object's trajectory would not come near Earth, but instead could be interstellar with a hyperbolic trajectory.^[24][26] 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,^[22] ZTF observations from 14 to 21 June 2025,^[1][27] and ATLAS observations from 25 to 29 June 2025.^[15][24][26] 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.^[28]

Initial observations of 3I/ATLAS were unclear on whether 3I/ATLAS is an asteroid or a comet.^[15][25][27] Various astronomers including Alan Hale reported no cometary features,^[28] but observations on 2 July 2025 by the Deep Random Survey (X09) at Chile, Lowell Discovery Telescope (G37) at Arizona, and Canada–France–Hawaii Telescope (T14) at Mauna Kea showed a marginal coma and a short tail 3 arcseconds in angular length, which indicated the object is a comet.^[1][28] On 2 July 2025, the MPC announced the discovery of 3I/ATLAS and gave it the interstellar object designation "3I", signifying it being the third interstellar object confirmed.^[1][28] The MPC also gave 3I/ATLAS the non-periodic comet designation C/2025 N1 (ATLAS).^[1] By the time 3I/ATLAS was announced, the MPC had collected 122 observations of the comet from 31 different observatories.^[1]

Further observations

Observations by David Jewitt and Jane Luu using the Nordic Optical Telescope on 2 July 2025 confirmed that 3I/ATLAS was "clearly active" with a diffuse tail.^[19] Miguel R. Alarcón and a team of researchers of the IAC (Instituto de Astrofísica de Canarias) using Teide Observatory's Two-meter Twin Telescope also found cometary activity on the same date, with a tail at least 25,000 km (16,000 mi) long.^[29] Multi-band observations at the Kottamia Astronomical Observatory 1.88-m telescope, the Palomar 200-inch telescope, and the Astrophysical Research Consortium 3.5-m telescope on 2025 July 2, 3 showed the comet had colors of B-V=0.98±0.23, V-R=0.71±0.09, R-I=0.14±0.10, g-r=0.84±0.05 mag, r-i=0.16±0.03 mag, i-z=-0.02±0.07 mag, and g-i=1.00±0.05 mag and a spectral slope of 16.0±1.9 %/100 nm.^[14] Faulkes Telescope North measurements of 3I/ATLAS's brightness through different light filters showed that the comet's coma had a reddish color indicative of dust, similar to that of the previous interstellar comet 2I/Borisov.^[16]

On 6 July, additional observations were published, including Zwicky Transient Facility (I41) precoveries from several nights between 22 May and 21 June 2025.^[30] An even earlier precovery from 21 May 2025, made at Weizmann Astrophysical Observatory (M01), was published on 18 July 2025.^[31]

The newly-commissioned Vera C. Rubin Observatory has serendipitously imaged 3I/ATLAS during its science validation observations from 21 June to 3 July 2025.^[11] These observations showed a slight increase in the comet's coma diameter and provided constraints on the comet's nucleus diameter.^[11] The Vera Rubin Observatory would have discovered 3I/ATLAS before the ATLAS survey if it had begun its science validation observations two weeks earlier.^[11]: 26

The Hubble Space Telescope took images of 3I/ATLAS on 21 July 2025.^[32] 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][33] and the telescope will monitor the comet on its way out of the Solar System.^[34] 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][35] 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]

Trajectory

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

Tilted view of 3I/ATLAS's trajectory through the Solar System, with orbits 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.^[20] When 3I/ATLAS entered the Solar System, it was moving at a speed of 58 km/s (36 mi/s) relative to the Sun—this speed is the comet's hyperbolic excess velocity (v_∞).^[7][6][c] As 3I/ATLAS comes closer to the Sun and gets pulled in by the Sun's gravity, the comet will speed up,^[20][36] and then once it begins moving away, the comet will slow down as the Sun's gravity begins pulling back on it.^[4] Nevertheless, the comet will escape the Solar System.^[26] 3I/ATLAS moves far faster compared to the previous two interstellar objects 1I/ʻOumuamua (v_∞=26 km/s) and 2I/Borisov (v_∞=32 km/s).^[6]

The shape of 3I/ATLAS's trajectory is described by a parameter known as the orbital eccentricity.^[27] 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 6.143±0.003.^[2][a] This extremely high eccentricity makes the trajectory of 3I/ATLAS appear relatively straight, rather than curved.^[37] 3I/ATLAS has the highest eccentricity of the three interstellar objects known to date,^[25] greater than 1I/ʻOumuamua's (e=1.2) and 2I/Borisov's (e=3.4).^[27]

3I/ATLAS will come closest to the Sun on 29 October 2025 at 11:35 ± 00:05 UT.^[8][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.^[2][a][20] At perihelion, the comet will be moving at its maximum speed of 68 km/s (42 mi/s) with respect to the Sun.^[26][h]

The trajectory of 3I/ATLAS is coincidentally closely aligned with the orbital planes of the Solar System's planets, or the ecliptic.^[38][39] Specifically, the comet's trajectory is tilted 175° (retrograde and inclined 5°) with respect to the ecliptic.^[16][2] The trajectory of 3I/ATLAS will bring the comet close to the planets Venus, Mars, and Jupiter, but not Earth.^[38] Since 3I/ATLAS cannot come close to Earth, it poses no threat to Earth.^[20][38][37] 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.^[40] After perihelion, it will pass 0.65 AU (97 million km; 60 million mi) from Venus on 3 November 2025,^[41] 1.80 AU (269 million km; 167 million mi) from Earth on 19 December 2025,^[42] and then it will pass 0.36 AU (54 million km; 33 million mi) from Jupiter on 16 March 2026.^[2][i]

Closest approaches (with 3-sigma uncertainties)

Object	Date	Distance
Mars	2025-10-03	0.194 ± 0.0007 AU (29.02 ± 0.10 million km; 18.033 ± 0.065 million mi)[40]
Sun	2025-10-29	1.357 ± 0.001 AU (203.00 ± 0.15 million km; 126.141 ± 0.093 million mi)[8]
Venus	2025-11-03	0.650 ± 0.001 AU (97.24 ± 0.15 million km; 60.421 ± 0.093 million mi)[41]
Earth	2025-12-19	1.798 ± 0.002 AU (268.98 ± 0.30 million km; 167.13 ± 0.19 million mi)[42]
Jupiter	2026-03-16	0.357 ± 0.003 AU (53.41 ± 0.45 million km; 33.19 ± 0.28 million mi)[2][i]

Observability

3I/ATLAS is a faint comet that is not expected to get brighter than about apparent magnitude 11.5 from Earth.^[43] Even at its peak brightness, the comet will not be visible to an observer on Earth using their naked eye^[44] or an average observer with a pair of 50 mm binoculars.^[45] 3I/ATLAS has been imaged by self-stacking smart telescopes with an aperture (diameter) of at least 3 to 4.5 in (7.6 to 11.4 cm).^[44]

From July to September 2025, 3I/ATLAS is observable from Earth after sunset.^[46] During the first half of July 2025, 3I/ATLAS was located in the constellation Sagittarius at an apparent magnitude of 17.5.^[46] By the second half of July 2025, 3I/ATLAS had moved to the constellation Ophiuchus and had brightened to apparent magnitude 16.^[46] 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.^[46] The comet will continue brightening as it approaches the Sun, and throughout August 2025 it will move across the constellations Ophiuchus, Scorpius, and Libra.^[46] During September 2025, 3I/ATLAS will remain in the constellation Libra as it brightens from apparent magnitude 14 to 11.7.^[46]

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.^[46] The comet's solar elongation will be less than 30 degrees from 1 October 2025 to 9 November 2025.^[47] 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.^[15][46] This means 3I/ATLAS will appear behind the Sun from Earth during the comet's perihelion, so the comet will not be observable from Earth during this time.^[46][38] 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.^[15]

After 3I/ATLAS passes perihelion, it will be visible in the sky again just before sunrise in November 2025.^[46] The comet will grow dimmer and its solar elongation will increase as it moves away from the Sun.^[46] 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.^[46]

Origin and age

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.^[20] 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.^[13]: 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.^{[13]: 3 [16]: 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,^[13]: 3 which is the direction of the Sun's movement relative to local stars.^[7]: 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.^[7]: 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.^[7]: 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.^[7][j] 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^[k] and was moving upward through the galactic plane with a velocity of W=+18.5 km/s with respect to the Sun.^[7]: 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.^[49][7]: 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.^[49][7]: 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.^[49][7]: 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][6]

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][7]: 2 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.^[6]: 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.^{[6]: 1 [7]: 1} The water-ice rich composition of 3I/ATLAS suggests that it formed far from the parent star, beyond the snow line at which water ice could condense into solid.^[17]: 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.^{[6]: 1 [7]: 1 [18]}

Physical characteristics

Size

Observations by the Vera C. Rubin Observatory suggest 3I/ATLAS has an asteroidal absolute magnitude (H) of 13.7±0.2, which suggests a maximum possible diameter of 11.2 km (7.0 mi) for 3I/ATLAS's nucleus, if it was a dark asteroid.^[11] However, because 3I/ATLAS is an active comet surrounded by a coma or a shell of reflective dust, the actual size of its nucleus is expected to be significantly smaller as it would be properly calculated from a combined nucleus and coma absolute magnitude (M1).^[11][19] 3I/ATLAS appears to be weakly active compared to the other interstellar comet 2I/Borisov, and is thus suspected to have a nucleus diameter likely an order of magnitude (ten times) larger than that of 2I/Borisov's.^[16] For reference, the maximum estimated diameter of 2I/Borisov's nucleus is between 0.4–0.5 km (0.25–0.31 mi),^[10][50] so the maximum diameter of 3I/ATLAS's nucleus could be up to 4–5 km (2.5–3.1 mi) if the order of magnitude argument holds true, but its actual diameter is likely to be smaller 1.2 km (0.75 mi).^[12]

Composition

Optical and near-infrared spectroscopy by the Gemini South and NASA Infrared Telescope Facility on 5 and 14 July 2025 revealed that 3I/ATLAS's coma is composed of water ice and silicate grains that are 10 μm in radius.^[17] 3I/ATLAS has been predicted to have a water-rich composition because it originated from a low-metallicity star from the thick disk.^[7][18] Other volatile ices such as carbon dioxide and ammonia are expected to exist in 3I/ATLAS, but have not been detected yet.^[18] Spectroscopic observations by the Very Large Telescope on 4–5 July 2025 did not find any clear signs of gases containing hydroxides (OH) and cyanides (CN), which place upper limit outgassing rates of 8.2×10²⁶ mol/s and 5.6×10²³ mol/s, respectively.^[51]: 3 3I/ATLAS was likely too far from the Sun to begin emitting these gases at the time these observations were made.^[51]: 3 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

Initial observations from various telescopes were unable to determine a rotation period and instead found that the brightness of 3I/ATLAS apparently shows little variation (less than 0.2 magnitudes), which may be due to the comet's dust coma obscuring its rotating nucleus.^[16] More sensitive observations by the 10.4-m Gran Telescopio Canarias from 2 to 5 July were able to determine that the nucleus of 3I/ATLAS has a rotation period of 16.79±0.23 hours, with its brightness varying by 0.2 magnitudes.^[13]

Exploration

Launching a space probe from Earth to perform a fly by of 3I/ATLAS is not feasible, as any post-discovery (1 July 2025) launch would require an extremely high delta-v (Δv) at least 24 km/s, which is beyond the capability of any propulsion system available at the time.^[52] 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.^[52] 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.^[52] For example, 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.^[52] It may be possible to repurpose spacecraft orbiting Mars to observe 3I/ATLAS during its close encounter with Mars in October 2025.^[52]

Alien spacecraft controversy

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 an extraterrestrial spacecraft because they believed the object had "anomalous" characteristics,^[53] such as having an apparently large size,^[12] lack of identifiable chemicals, and a trajectory aligned with the Solar System's ecliptic plane.^[54] 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."^[54] 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."^[54] Seligman further elaborated that chemicals in 3I/ATLAS may not be detectable yet since the object was still far away from the Sun.^[54] Since then, observations have reported evidence of 3I/ATLAS containing water ice,^[17] which is a substance commonly found in comets.^[17][7]

Loeb has previously claimed that other unusual objects like 1I/ʻOumuamua could be extraterrestrial spacecraft, which too have garnered criticism from many researchers.^[54] 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."^[54] 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."^[54] Lawler further cited extraordinary claims require extraordinary evidence, and that "the evidence presented [by Loeb et al.] is absolutely not extraordinary."^[54]

Gallery

1 July 2025

• 
  Discovery image from the Asteroid Terrestrial-impact Last Alert System

2 July 2025

• 
  Imaged remotely at Río Hurtado, Chile
• 
  With a fuzzy and elongated coma, imaged by the Canada–France–Hawaii Telescope
• 
  At the Deep Random Survey in Chile

3 July 2025

• 
  Visible and near-infrared color composite photo by the Gemini North telescope
• 
  Movement across a field of stars, as seen by Gemini North. The colorful appearance of the comet's trail is due to the telescope changing light filters while observing the comet.

4 July 2025

• 
  3I/ATLAS moving across a field of stars, as seen by the ESO's Very Large Telescope
• 
  Very Large Telescope image of 3I/ATLAS in detail
• 
  VLT FORS2 ESO image of 3I/ATLAS in motion through 13 minutes - visible as a line^[55]

21 July 2025

• 
  3I/ATLAS moving across a field of stars, as seen by the Hubble Space Telescope
• 
  Additional Hubble Space Telescope images of 3I/ATLAS

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:^[5] ${\displaystyle {\sqrt {-GM(2/q-1/a)}}}$, where ${\displaystyle G}$ is the gravitational constant, ${\displaystyle M}$ the mass of the Sun, ${\displaystyle q}$ the comet's perihelion distance, and ${\displaystyle a}$ its semi-major axis. Calculation:
3. Formula for the hyperbolic excess velocity: ${\displaystyle {\sqrt {-GM/a}}}$, where ${\displaystyle G}$ is the gravitational constant, ${\displaystyle M}$ the mass of the Sun, and ${\displaystyle a}$ the comet's semi-major axis. Calculation:
4. The 4–5 km (2.5–3.1 mi) diameter is based on the possibility that it is an order of magnitude larger than 2I/Borisov, which is around 0.4–0.5 km (0.25–0.31 mi) in diameter.^[10]
5. The 11.2 km (7.0 mi) diameter is based on the assumption that the object is asteroid-like in composition and appearance with an absolute magnitude of 13.7 and a dark albedo of 0.05.^[11] However, it should be treated as an upper limit, as a coma is present around the object, which indicates the nucleus is significantly smaller, likely with a diameter smaller than 1.2 km (0.75 mi).^[11][12]
6. In the Minor Planet Center discovery announcement, the discovery observation time (marked with an asterisk "*") is "2025 07 01.218880,"^[1] which translates to 1 July 2025 05:15:11 UT.^[21] 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.^[22]
7. An n-body integration shows 3I/ATLAS comes to perihelion about 11 minutes later than the JPL SBDB epoch 15 July 2025 solution. (changing 11:24 to 11:35.)
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 ±440 thousand km (0.0029 AU).
10. 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.^{[7]: 2 [48]}
11. 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.^[48]