3I/ATLAS completes inbound leg of its Solar System passage on October 29
Interstellar object 3I/ATLAS (C/2025 N1 ATLAS) reached its perihelion distance of about 1.36 AU from the Sun on October 29, 2025, marking the midpoint of its one-time passage through the Solar System. The hyperbolic object will be unobservable from Earth during the event but will later pass within 97 million km (60 million miles) of Venus and 54 million km (34 million miles) of Jupiter on its way out of the system.
Comet 3I/ATLAS on August 27, 2025. Credit: Gemini South
Interstellar object 3I/ATLAS (C/2025 N1 ATLAS) reached its closest point to the Sun (perihelion) at approximately 11:47 UTC on October 29, 2025, at a heliocentric distance of about 1.356 AU (203 million km or 126 million miles).
This object follows a strongly hyperbolic orbit (eccentricity about 6.1), confirming it is not gravitationally bound to the Sun and identifying it as the third confirmed interstellar visitor, after 1I/ʻOumuamua (2017) and 2I/Borisov (2019).
3I/ATLAS was discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS). Its rapid motion and non-periodic trajectory revealed its interstellar nature soon after discovery. Optical and spectroscopic observations from multiple facilities have shown a diffuse coma and dust tail, indicating that 3I/ATLAS behaves as an active comet rather than an inert fragment.
At perihelion, 3I/ATLAS was located between the orbits of Earth and Mars and was in solar conjunction as seen from Earth. Its solar elongation was small (less than about 15 degrees), making it effectively unobservable by ground-based telescopes until late November. Solar irradiance at that distance reached roughly 770 watts/m2, driving its maximum thermal stress and activity.
The object has now begun` its outbound leg through the Solar System. On November 3, 2025, it will pass within about 97 million km (60 million miles) of Venus, then move outward to reach its closest approach to Earth (about 267 million km/166 million miles) on December 19, 2025, and later pass within about 54 million km (34 million miles) of Jupiter on March 16, 2026.
Its geometry will also allow brief observation opportunities for space missions such as ESA’s JUICE, which will be positioned along a favorable line of sight in early November.
Researchers expect the comet’s activity to increase near perihelion as solar heating peaks. Comparable interstellar bodies have shown markedly different behavior: 2I/Borisov displayed strong, continuous outgassing in 2019, while 1I/ʻOumuamua remained inert. Because 3I/ATLAS is hidden behind the Sun during this critical period, any change in brightness or possible fragmentation will be measurable only after it re-emerges.
Spectroscopic studies before perihelion revealed nickel-vapour emission lines, a feature uncommon among comets. These findings have been interpreted by some researchers as signs of extensive surface processing or compositional evolution during prolonged interstellar exposure.
The comet’s retrograde inclination (about 175 degrees) means it travels almost opposite the planetary orbital direction, entering the Solar System from above the ecliptic and departing below it.
In a recent essay titled The Acid Test of 3I/ATLAS at Perihelion, astrophysicist Avi Loeb referred to the October 29 perihelion as the acid test for understanding the object’s true nature. Loeb proposed that if 3I/ATLAS is a weakly bound natural comet, it could fragment under solar heating, but if it were of artificial origin, it might maneuver or release secondary bodies.
Once 3I/ATLAS re-emerges from solar conjunction in late November 2025, observatories will resume imaging and spectroscopy to determine whether the perihelion heating altered its structure or activity. The event marks a rare observational window to study an interstellar object under intense solar conditions before it leaves the Solar System permanently.
References:
1 IS COMET 3I/ATLAS REALLY A SPACESHIP? – Space Weather – October 29, 2025
2 The Acid Test of 3I/ATLAS at Perihelion – Avi Loeb (Medium) – October 28, 2025
I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.
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