Scientists have observed that the interstellar comet 3I/ATLAS has turned blue, marking the third color change since its discovery. 3I/ATLAS is the third interstellar visitor recorded by humanity. The comet entered the solar system at a speed of 58 kilometers per second (210,000 km/h) and is believed to have wandered the outer regions of the Milky Way for more than 7 billion years.
Discovered in July, 3I/ATLAS passed near Mars in early October and then went behind the Sun, making it unobservable from Earth for several weeks. It is now becoming visible again.
When first observed, the comet appeared red, likely due to dense dust shedding from its surface. In September, it glowed green, indicating the presence of gases such as cyanide or diatomic carbon (C₂) in its atmosphere. Recent observations show a bluish tint, possibly caused by leaking carbon monoxide or ammonia, though these findings have not yet been peer-reviewed.
The comet reached its closest point to the Sun (130 million miles / 210 million km) on October 29, experiencing a sudden flare and brightness increase. Scientists are still unable to explain this dramatic reaction, as proximity to the Sun alone should not trigger such a strong effect.
In the coming weeks, 3I/ATLAS will be visible through telescopes in the Northern Hemisphere, though it will not be visible to the naked eye. The comet will reach its closest approach to Earth (270 million km) on December 19.
The European Space Agency (ESA) plans to have two spacecraft pass through the comet's tail, providing a unique opportunity to directly measure its composition.
3I/ATLAS has exhibited several unusual features: a high carbon dioxide content, unexpected water vapor emissions, and a backward-extending "anti-tail." These anomalies have led some researchers to speculate about a possible artificial origin. However, most scientists maintain that 3I/ATLAS behaves like a normal comet.
