3I/ATLAS Changed Color 3x Since Discovery, Changes Again as it Approaches Earth

3I/ATLAS will make its closest approach to Earth on December 19th, 2025, passing at a distance of 268.9 million kilometers. According to Harvard astrophysicist Dr. Avi Loeb, the timing is ideal for observation as December 19th coincides with a new moon, eliminating moonlight interference for ground-based telescopes worldwide.

Multiple space agencies have directed their instruments toward this rare visitor from another star system. The European Space Agency’s XMM-Newton spacecraft observed 3I/ATLAS for approximately 20 hours on December 3rd, while Japan’s XRISM spacecraft has also captured X-ray data. These observations allow scientists to detect gases in the object’s coma that other instruments cannot easily identify, according to ESA reports. The X-ray emissions result from interactions between the solar wind and the gas surrounding the object.

Astronomers have documented three distinct color shifts since 3I/ATLAS was discovered in July 2025. According to geophysicist Stefan Burns, the object initially displayed a deep red color indicating dust and organic compounds. It then shifted to green in late September as it crossed the water ice activation zone at 2.5 astronomical units from the Sun. Recent observations from December 10th now show 3I/ATLAS taking on a striking gold color, marking its third color transformation. Burns noted that another comet, C/2025 K1 ATLAS, underwent a similar shift to gold before fragmenting, though this does not necessarily predict the same outcome for the interstellar visitor.

One of the most unusual features of 3I/ATLAS is its anti-tail, a jet of material pointing directly toward the Sun rather than away from it. According to Dr. Loeb’s analysis, the latest images from December 14th and 15th show this anti-tail extending half a million kilometers from the nucleus, larger than the distance between Earth and the Moon. No comet has ever displayed an anti-tail of this magnitude. Dr. Loeb calculates the material must be traveling at least 130 meters per second toward the Sun, which contradicts expectations that solar radiation pressure would push such material in the opposite direction.

A new paper from researchers at the Teide Observatory in the Canary Islands, referenced by Dr. Loeb, detected a periodic wobble in the anti-tail jet with a period of 7.74 hours. The geometry of this wobble suggests the jet base is aligned within 8 degrees of the rotation poles, an alignment Dr. Loeb describes as the object’s 14th documented anomaly with a probability of occurring by chance at roughly 1 in 40,000.

Meanwhile, NASA announced it lost contact with its MAVEN spacecraft orbiting Mars on December 6th. According to NASA’s December 15th update, tracking data suggests MAVEN was rotating unexpectedly when it emerged from behind Mars. MAVEN had previously observed 3I/ATLAS during its October 2nd flyby within 30 million kilometers of Mars. While there is no confirmed connection between the spacecraft anomaly and the interstellar object, the timing has drawn attention from observers following both developments. NASA’s Mars Odyssey and Mars Reconnaissance Orbiter remain operational.

After its closest Earth approach, 3I/ATLAS will continue toward Jupiter, with a flyby expected in spring 2026. The object currently shines at magnitude 13 to 15, requiring medium-sized telescopes for observation. Scientists expect it to continue dimming as it moves away from the Sun, making the coming weeks the final opportunity to study this interstellar visitor before it exits our solar system permanently.

Sources

3I/ATLAS is Green After Perihelion. (n.d.). Medium. https://medium.com/@avi-loeb/3i-atlas-is-green-after-perihelion-234efe6fedbb

Newsmax. (2025, December 12). Is the govt. hiding something? Maven Spacecraft Goes Dark Amidst Comet Close Encounter: Dr. Avi Loeb [Video]. YouTube. https://www.youtube.com/watch?v=csIDSjv1LFo

Serra-Ricart, M., Licandro, J., & Alarcon. (2025). Pre-perihelion detection of a wobbling high-latitude jet in the interstellar comet 3I/ATLAS. Astronomy and Astrophysics. https://doi.org/10.1051/0004-6361/202558072

Stefan Burns. (2025, December 15). 3I/ATLAS is now Glowing GOLD as Earth’s Closest Approach Countdown Begins [Video]. YouTube. https://www.youtube.com/watch?v=Uer_Pdev3go

The Anti-Tail of 3I/ATLAS is Larger Than the Distance to the Moon. (n.d.). Medium. https://avi-loeb.medium.com/the-anti-tail-of-3i-atlas-is-larger-than-the-distance-to-the-moon-0862b138bf62