Interstellar Comet 3I/ATLAS

​Interstellar comet 3I/ATLAS is the third confirmed macroscopic object known to have entered the Solar System from interstellar space, after 1I/ʻOumuamua and 2I/Borisov. 3I/ATLAS (also designated C/2025 N1 ATLAS) was discovered on 1 July 2025 by the ATLAS survey telescope in Chile.  It was formally given the “3I” designation by the Minor Planet Center on 2 July 2025, reflecting its confirmed interstellar origin. The object follows an extremely hyperbolic orbit with eccentricity, far above any bound comet, which is one of the key indicators that it is not gravitationally bound to the Sun.  Its velocity at infinity is about 57–58 km/s relative to the Sun, making it significantly faster than 1I/ʻOumuamua (~26 km/s) and 2I/Borisov (~32 km/s). 
 
When it was discovered, 3I/ATLAS was at a heliocentric distance of about 4.5 AU and about 3.5 AU from Earth, in the direction of the constellation Sagittarius.  Its trajectory is only a few degrees from the ecliptic plane but is retrograde, so it travels against the traffic of the major planets. The comet passed perihelion (closest approach to the Sun) on 29 October 2025 at a distance of about 1.36 AU, between the orbits of Earth and Mars.  It never comes close to Earth; its minimum distance is about 1.8 AU, meaning it poses no impact risk.  After traversing the inner Solar System, it will pass near Mars and later Jupiter (closest to Jupiter in early 2026) and then depart on an unbound trajectory back into interstellar space.
 
3I/ATLAS is a clearly active comet with a visible coma rather than a bare asteroid-like body.  Imaging shows that its central condensation is surrounded by a diffuse, dusty coma with a full width at half maximum of about 2 arcseconds, expanding to roughly 10 arcseconds at low surface brightness in early observations. Near‑infrared observations (including by NASA’s SPHEREx mission) indicate a large carbon‑dioxide–rich gas coma extending at least ~3.5×10^5 km from the nucleus.  Within the coma, small ~1 µm dust grains are being ejected at tens of m/s, while larger ~100 µm grains move at a few m/s; modelling suggests dust production rates on the order of several kilograms per second for small grains and tens of kilograms per second for larger grains.  Early photometry implies a relatively blue colour compared to 2I/Borisov, with optical colour indices corresponding to a modest spectral slope (~1–2% per 100 nm). 
 
The strongly hyperbolic orbit and high excess speed rule out any plausible origin within the Solar System, even with planetary scattering, confirming that 3I/ATLAS is interstellar.  Back‑tracking its trajectory indicates that it entered from the general direction of Sagittarius, near the Galactic Centre region, consistent with an origin somewhere in the Milky Way’s thin or thick disk. If it originated in the thick disk, dynamical models suggest that 3I/ATLAS could be older than the Solar System, with an age of order 7 billion years or more, making it a probe of very ancient planetesimal populations.  Because it is both clearly cometary and brighter and more extended than 1I/ʻOumuamua, it provides a rare opportunity to study the composition and activity of icy bodies formed around other stars, and to compare their dust and volatile properties with Solar System comets.
 
3I/ATLAS is observable only with binoculars or, more realistically, a small telescope, mainly in the evening sky in 2025 and the predawn sky into spring 2026; it is not expected to ever be naked‑eye bright. From Earth, 3I/ATLAS was visible after sunset from July to late September 2025, starting in Sagittarius and then moving into Ophiuchus, but at very faint magnitudes (about 17.5 in early July, improving to ~16 by late July).  As it approached its 29 October 2025 perihelion, its angular separation from the Sun dropped below 30°, so by October–early November it was only marginally observable from equatorial regions shortly after sunset. After passing behind the Sun in October, the comet re‑emerged in late October and early November and became visible again just before sunrise, with astrometric observations from Earth resuming on 31 October 2025.  NASA notes that the comet can be followed in the predawn sky until roughly spring 2026, after which it will fade and recede from practical amateur reach.
 
References
 
Bolin, B.T., Belyakov, M., Fremling, C., Graham, M.J., Gray, C.L., et al. (2025, July 5). Interstellar Comet 3I/ATLAS: Discovery and Physical Description. ARXIV.
 
(2025, December 16). 3I/ATLAS. In Wikipedia, https://en.wikipedia.org/wiki/3I/ATLAS
 
(2025, December 2). Comet 3I/ATLAS Facts and FAQS. NASA.