Interstellar Comet 3I/ATLAS May Have Turned Blue — Third Fleeting Color Shift Sparks Curiosity

Interstellar comet 3I/ATLAS may be showing a bluish tint after a sudden brightening

Recent spacecraft observations suggest that interstellar comet 3I/ATLAS may be developing a bluish hue after an abrupt, unexpected brightening while it was hidden behind the Sun. If confirmed, this would be the third apparent — and so far temporary — color change for the object since it was first detected in early July.

Fast, ancient visitor: First seen moving toward the Sun at roughly 130,000 mph (210,000 km/h), 3I/ATLAS is the third confirmed interstellar object observed in our Solar System. Some researchers estimate it could be more than 7 billion years old, having been ejected from its home star system in the Milky Way's outskirts and drifted through interstellar space before this encounter.

Hidden brightening and possible blue tint: The comet reached perihelion on Oct. 29 at about 130 million miles (210 million km) from the Sun — roughly 1.4 astronomical units — while largely out of view from Earth. Data from spacecraft that could still monitor the comet indicate it brightened by several orders of magnitude after slipping behind the Sun, a change not fully explained by its closer solar distance. The same analysis reported the comet looked "distinctly bluer than the Sun," a surprising development not seen in earlier observations. The authors suggest the blue color could arise from emission by volatiles such as carbon monoxide (CO) or ammonia (NH3).

Previous color changes: Earlier in its apparition the comet appeared reddish in July, likely from abundant dust, and briefly showed a greenish tint in September, probably from molecules like dicarbon (C2) or cyanogen (CN) in the coma. Each of these color states has been short-lived; the reasons for the transient nature of the hues are not yet understood.

What to expect next: 3I/ATLAS is moving northward and will become progressively easier to observe from the Northern Hemisphere over the coming weeks, though it will not be visible to the naked eye. Viewing will require a decent telescope or strong binoculars. The comet will make its closest approach to Earth on Dec. 19 at about 168 million miles (270 million km), offering improved opportunities for follow-up observations. Two European Space Agency spacecraft may also pass through the comet's extended tail before it departs the inner Solar System.

Unusual characteristics and speculation: Since discovery, the object has shown several curious features, including high carbon dioxide content, strong water outgassing, and a puzzling anti-tail. Scientists also think its surface may have been altered by billions of years of cosmic-ray exposure, which could obscure direct chemical links to its parent system. A small number of commentators have speculated — without supporting evidence — that the object could be artificial. The majority of experts, however, consider 3I/ATLAS consistent with natural cometary behavior.

Note: The recent study reporting the brightening and blue hue has not yet been peer-reviewed, and independent confirmation is pending. Continued ground- and space-based observations over the next weeks will be crucial to determine whether the blue color persists and to better understand the comet's changing activity.