A mysterious visitor from beyond our solar system, the interstellar object 3I/ATLAS, may be shifting colours — for a third time. Recent observations suggest this icy traveler has developed a faint blue-hue after passing behind the Sun, marking yet another step in a sequence of fascinating changes.
A Journey from the Stars
3I/ATLAS is the third known interstellar object to enter our solar system, and likely an extremely ancient one — it may have been cast out of its home star system more than 7 billion years ago and spent that time drifting through interstellar space. It was first detected speeding inwards at more than 130,000 mph (210,000 km/h) back in early July.
In late October it made its closest approach to the Sun (known as perihelion), coming within about 210 million km — roughly 1.4 times the Earth-Sun distance.
Colour Changes: From Red to Green to Blue?
When it was first observed, 3I/ATLAS displayed a reddish colour, likely due to dust being released from its surface and scattering sunlight. By September, it seemed to shift to a greenish hue — possibly tied to the presence of molecules like dicarbon or cyanide in its coma.
Now, new data suggest it may be turning blue. Researchers analyzing spacecraft observations found that after it passed behind the Sun (and thus became hidden from Earth-based observers) the object brightened rapidly, and in that brightness spike it appeared “distinctly bluer than the Sun.”
This new blue tint might be caused by gases such as carbon monoxide or ammonia leaking from the comet’s nucleus — materials that absorb and reflect sunlight differently than dust or other common cometary substances.
Why the Colour Changes Matter
These shifts in colour are more than just eye-catching — they provide clues about the object’s composition, surface processes, and history. Comets from our solar system often brighten or shed dust as they approach the Sun, but for an interstellar object like 3I/ATLAS the stakes are higher: it’s a sample of material from beyond our own stellar neighbourhood.
The fact that its colour appears to have changed three times suggests a dynamic series of processes: dust shedding, gas release, perhaps even surface evolution under solar heating. Each change opens a window into what the object is made of and how it behaves in a solar environment.
What We Don’t Know Yet
It’s important to note that the blue colour change is tentative. The study reporting it hasn’t yet been peer-reviewed and other observations have yet to confirm the blue appearance. Likewise, the earlier red and green phases were short-lived and somewhat ambiguous.
We also don’t fully understand why the colour shifts happened when they did, nor whether the blue hue — if real — will persist as the object moves away from the Sun. Past shifts did not stick permanently.
What’s Next for 3I/ATLAS?
Over the coming weeks and months, 3I/ATLAS will become increasingly visible to Earth-based telescopes again as it travels northward in the night sky. While it won’t become visible to the naked eye, stargazers with a decent telescope or strong binoculars may get a view.
It’s also due to make its closest approach to our planet on December 19, coming within roughly 270 million km (about 1.8 times the Earth-Sun distance).
If it continues to brighten or remains blue, astronomers will have a rare chance to study the tail and composition of an interstellar visitor up close — possibly even with spacecraft instruments. The more data we gather, the better we’ll understand how such objects form, evolve, and carry clues about other star systems.
In Summary
3I/ATLAS is a remarkable interstellar comet, having journeyed billions of years through space before entering our solar system. It’s exhibited at least two distinct colour changes — from red to green — and now likely a third change to blue, marking unusual activity. While the findings are still preliminary, they underscore how little we know about such wanderers and how much they can teach us. As it moves back out toward the stars, astronomers will be keeping a close watch.
Source: Live Science – “’Interstellar visitor’ 3I/ATLAS may have just changed color — for the third time” (4 November 2025).