Mysterious interstellar visitor 3I/ATLAS appears to change colour for third time with sudden brightening

Interstellar Comet 3I/ATLAS Shows Mysterious Blue Tint After Sudden Brightening (Image: NASA/ESA)

A mysterious glow has returned to the skies as interstellar comet 3I/ATLAS appears to be developing a faint blue hue after an unexpected brightening behind the Sun. The phenomenon, captured by spacecraft data, has puzzled astronomers once again.

Why is 3I/ATLAS turning blue?
According to a recent unpublished study, researchers noticed a striking blue tint in the comet’s coma after it re-emerged from behind the Sun in late October. The team suggested that gases like carbon monoxide or ammonia may be leaking from its icy surface, giving it a cooler tone. The same study revealed that the comet had brightened by several orders of magnitude while hidden from Earth, a change not fully explained by its closeness to the Sun.

This marks the third time that scientists have observed a colour shift in 3I/ATLAS. It first glowed red in July, likely caused by dust reflecting sunlight, and later appeared green in September, possibly due to molecules such as dicarbon or cyanide. Each colour change, however, faded quickly, leaving experts uncertain about what drives these transformations.

This image, captured by astrophotographers Michael Jäger and Gerald Rhemann on 7 September, showed 3I/ATLAS emitting a greenish glow. (Image: Michael Jäger/Gerald Rhemann)

What makes 3I/ATLAS unique among interstellar visitors?
3I/ATLAS is the third-known interstellar object to enter our solar system. It was discovered on 1 July 2025 by the ATLAS telescope in Chile, travelling at over 210,000 kilometres per hour. Scientists believe it was expelled from its original star system more than seven billion years ago and has been drifting across the Milky Way ever since.

The comet made a close approach to Mars in early October before reaching its nearest point to the Sun on 29 October, at a distance of around 210 million kilometres. For several weeks, it remained invisible to Earth-based telescopes, although a few orbiting spacecraft continued to track it. It is now reappearing in the morning sky as it moves northward.

Initial photographs of 3I/ATLAS from the International Gemini Observatory depicted the comet as a massive rainbow streak across the night sky. However, the effect resulted from the filters and timelapse method used in capturing the image. (Image: International Gemini Observatory/NOIRLab/NSF/AURA/K. Meech (IfA/U. Hawaii)Image Processing: Jen Miller & Mahdi Zamani (NSF NOIRLab))

When and where can the comet be seen next?
Stargazers in the Northern Hemisphere may soon catch a glimpse of the comet using telescopes or powerful binoculars. It will not be visible to the naked eye but will reach its closest distance to Earth on 19 December, coming within 270 million kilometres.

Two European Space Agency spacecraft are expected to cross the comet’s tail later this year, offering new insights into its structure and composition. Scientists are eager to learn whether the blue tint will remain as 3I/ATLAS continues its journey out of the solar system.

Could 3I/ATLAS be something other than a comet?
Some scientists have suggested that 3I/ATLAS may be an artificial object or of alien technology, given its features: extremely high levels of water leakage, abundant carbon dioxide, and a strange anti-tail. Yet, there has not been any verification for this suggestion. Most experts agree that the object behaves as a typical comet would, though it carries materials shaped by billions of years of cosmic radiation.

As observations continue, astronomers hope the returning visibility of 3I/ATLAS will help solve the mystery of its shifting colours and reveal more about its interstellar origins.