3I/ATLAS's strange behavior challenges what astronomers know about comets

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3I/ATLAS's strange behavior challenges what astronomers know about comets
Ariana Garcia
Thu, October 30, 2025 at 10:15 PM EDT
3 min read



Though pictured here is 'Oumuamua, 3I/ATLAS is the latest interstellar comet lighting up the solar system. (VICTOR HABBICK VISIONS/SCIENCE P/Getty Images/Science Photo Libra)


The interstellar comet 3I/ATLAS has just made its closest pass to the Sun, and it's brighter than scientists expected. Spacecraft monitoring the Sun captured a dramatic surge in brightness as the comet plunged toward perihelion, its nearest point to the star, leaving astronomers puzzled.

"The reason for 3I's rapid brightening, which far exceeds the brightening rate of most Oort cloud comets, remains unclear," wrote Qicheng Zhang of Lowell Observatory and Karl Battams of the U.S. Naval Research Laboratory in a recent preprint.

Ground-based telescopes lost sight of the comet during its close approach, but solar observatories, including SOHO, STEREO-A, and NOAA's GOES-19 kept tracking it. NOAA's CCOR-1 instrument captured a fuzzy cloud, or coma, around the comet about four arcminutes wide. SOHO added color data, revealing that the comet appears distinctly bluer than the Sun-a sign that glowing gas, not dust, dominates its brightness.

The comet's brightness has been skyrocketing. Between September and October, it increased roughly as the 7.5th power of its decreasing distance from the Sun-twice as fast as it brightened earlier this year. Scientists suggest that sudden outgassing of water or other volatiles could be behind the surge.

Its unusual color and activity have surprised experts. Most comets look reddish because sunlight reflects off dust, but 3I/ATLAS is blue, indicating that gases like carbon compounds are lighting up its coma. Harvard astrophysicist Avi Loeb called the color "very surprising" and added that it is now the ninth anomaly observed in this interstellar visitor.



New images interstellar object 3I/ATLAS from various instruments reveal rapid brightening and a color bluer than the Sun. (Credit: Q. Zhang and K. Dattams)


3I is unusual in other ways: it displayed an "anti-tail" in mid-2025, has a nucleus far more massive than previous interstellar comets, and travels faster than both 1I/'Oumuamua and 2I/Borisov. Its gas contains far more nickel than iron, with a nickel-to-cyanide ratio higher than any comet studied before. Loeb has speculated that the object might even harbor a source hotter than the Sun.

Currently hidden behind the Sun from Earth, 3I/ATLAS will make its closest approach to our planet on Dec. 19, 2025. Observations from Hubble, Webb, and ground-based telescopes are expected to reveal unprecedented detail. Florida Congresswoman Anna Paulina Luna is reportedly working to release HiRISE images from Oct. 2, 2025, currently withheld due to the government shutdown.

This is only the third interstellar comet scientists have observed. Its extreme brightening, unusual composition, and intense gas activity offer a rare glimpse into materials and behaviors from outside our solar system, and a reminder that the universe is full of surprises.

This article originally published at 3I/ATLAS's strange behavior challenges what astronomers know about comets.