3I/ATLAS — discovered in early July and traveling ~130,000 mph — is the third confirmed interstellar visitor to our solar system. A new preprint finds its gas coma is "distinctly bluer than the Sun," and researchers say there is no evidence the coma suddenly changed color. Amateur photos from early September already showed a bluish-green coma, and more images may appear as NASA or other spacecraft release data. The comet will pass about 167 million miles from Earth on December 19.
3I/ATLAS: Not an Alien Visitor — Its 'Color Change' Was a Misread Brightening
3I/ATLAS — discovered in early July and traveling ~130,000 mph — is the third confirmed interstellar visitor to our solar system. A new preprint finds its gas coma is "distinctly bluer than the Sun," and researchers say there is no evidence the coma suddenly changed color. Amateur photos from early September already showed a bluish-green coma, and more images may appear as NASA or other spacecraft release data. The comet will pass about 167 million miles from Earth on December 19.
11:22 PM, 11/10/2025Science
3I/ATLAS isn’t extraterrestrial — the ‘color change’ was simply a brightening coma
Since its discovery in early July, 3I/ATLAS has captured attention as only the third confirmed interstellar object to visit our solar system and one of the fastest, traveling roughly 130,000 mph. Its closest approach to the Sun so far occurred on October 30, a transit observed by three spacecraft that offered a rare opportunity to study this ancient icy body up close.
What the observations show
Early reports described a rapid brightening of the comet’s gas coma — the cloud of vaporized material released as the object warms near the Sun — and some initial notes characterized the comet as appearing red. A new preprint study, however, finds the coma to be “distinctly bluer than the Sun,” and says there is no evidence the coma underwent a sudden, unexplained color shift.
“We don’t have any evidence for the gas coma changing colors,” Qicheng Zhang of Lowell Observatory told Space.com on November 7. “Our result just showed that the gas coma is likely still around and contributing substantially to the overall brightness.”
In plain terms: as 3I/ATLAS moved closer to the Sun its coma brightened, which altered the overall appearance — a process that began by early September. Amateur astronomers’ photographs from that time already show a bluish-green coma consistent with the new analysis.
Misinformation and scientific debate
Some commentary and media headlines mischaracterized the finding as a sudden color change and fueled online speculation that the object might be artificial or extraterrestrial technology. Study coauthor Zhang expressed frustration that some of this misinformation came from commentators with scientific backgrounds who may have misread or over-interpreted the available papers and data.
The scientific consensus remains that 3I/ATLAS’ properties are best explained by natural cometary processes. While vigorous public debate can be healthy, extraordinary claims require extraordinary evidence — which is not present in this case.
Images and what to expect next
Nasa has delayed releasing some additional images during a period of limited operations related to the U.S. government shutdown, but imagery reportedly taken by the Mars Reconnaissance Orbiter on October 3 may still be published. Observers will get another opportunity to study 3I/ATLAS as it makes its closest approach to Earth on December 19, passing at roughly 167 million miles.
As Zhang noted, this episode could help spark greater public interest in comet science: these objects routinely display unusual and informative behavior that helps astronomers better understand the origins and evolution of the solar system.