Interstellar comet 3I/ATLAS, identified in July 2025, has shown a bluish hue in recent observations, marking its third color change since discovery. The phenomenon occurred after perihelion, the point of closest approach to the Sun, on October 29, at 130 million kilometers. The change, detected by researchers using data from spacecraft, suggests the release of gases such as carbon monoxide or ammonia. The comet, which travels at 130,000 km/h, is one of the oldest interstellar objects ever observed.
The color change surprised scientists, who noticed an intense glow after the comet reappeared from behind the Sun. The blue hue contrasts with the initial red, seen in July, and the temporary green, in September. The composition of 3I/ATLAS, with a high presence of carbon dioxide and water, may explain the variations.
- Origin: Ejected from a stellar system 7 billion years ago.
- Trajectory: It passed close to Mars in October and will approach Earth in December.
- Observation: Visible with telescopes in the Northern Hemisphere until the end of the year.
The comet’s visibility will increase in the coming weeks, but requires equipment such as telescopes or long-range binoculars.
New hue defies analysis
The bluish color, observed by researchers in spacecraft data, indicates the release of volatile gases from the comet’s icy nucleus. The change, not yet confirmed by scientific reviews, is unusual and requires further study.
The intense brightness recorded before perihelion is not explained solely by solar proximity. The chemical composition of the comet, rich in elements such as carbon monoxide, may be linked to the phenomenon.
Visual change history
In July, 3I/ATLAS displayed a red hue due to dust on its surface. In September, a green hue suggested the presence of dicarbon or cyanide. The new blue color, seen in October, is the third change recorded.
Scientists monitor these changes with images from the Hubble Space Telescope and other equipment. The instability of colors may be linked to the comet’s interaction with solar heat.
Detailed analysis will be possible with future observations, especially in December, when the comet will be closest to Earth.
Trajectory and future observations
3I/ATLAS will reach its closest point to Earth on December 19, at 168 million kilometers. Two ESA spacecraft could cross its tail before it leaves the solar system.
Visibility for amateur astronomers will increase in the Northern Hemisphere, but the comet will not be seen with the naked eye. Medium-sized telescopesare necessary to observe it.
Researchers plan to use data from ground-based and space-based observatories to study the comet’s chemical composition.
Unique visitor characteristics
The comet has anomalous features, such as high carbon dioxide and water emissions, as well as an inverted tail. Its icy crust, altered by billions of years of cosmic radiation, makes it difficult to trace its stellar origin.
Some speculate that 3I/ATLAS could be an alien technology, but the hypothesis lacks evidence. Most scientists say it behaves like a typical comet.
Origin and scientific impact
3I/ATLAS, probably originating on the outskirts of the Milky Way, is one of the few known interstellar objects. Its passage offers a unique opportunity to study materials from other stellar systems.
The scientific community awaits more data to confirm the color changes and understand their composition. Observations until the end of 2025 will be crucial in unraveling the mysteries of this cosmic visitor.