Scientists: Interstellar Comet 3I/ATLAS Didn’t Repeatedly ‘Change Color’ — It Brightened as Its Gas Coma Dominated the Light

Interstellar visitor 3I/ATLAS: color confusion explained

Comet 3I/ATLAS — only the third confirmed interstellar object to pass through our solar system — has generated intense public and scientific interest since its discovery in July. The object made perihelion (its closest approach to the Sun) on Oct. 30, and images from multiple Sun-facing spacecraft recorded a rapid brightening that is larger than typically seen for comets at similar distances.

In a preprint posted on arXiv, researchers reported that recent observations show the comet appears distinctly bluer than the Sun, while earlier ground-based observations had characterized the dust as red. Several news outlets interpreted this as the comet "changing color" multiple times and speculated about mysterious causes.

According to Qicheng Zhang, a postdoctoral fellow at Lowell Observatory and a co-author of the new study, that reporting mischaracterizes the findings. The team found no evidence that the gas coma itself has changed color repeatedly. Instead, the data indicate the comet’s gas coma became visible and bright enough to dominate the overall light, making the object appear bluer than when reflected dust dominated earlier observations.

Qicheng Zhang (Lowell Observatory): "We don't have any evidence for the gas coma changing colors. Our results show the gas coma is still present and contributing substantially to the overall brightness."

Comets are often described as "dirty snowballs": their solid nuclei contain ices mixed with rock and dust. As a comet approaches the Sun, these ices sublimate into gas and form a halo called a coma. The composition and relative brightness of a comet’s dust and gas determine its observed color; when gas emission becomes dominant, the comet can appear bluer or greenish because many common cometary gases emit light at those wavelengths.

Zhang noted that, technically, 3I/ATLAS effectively "changed color" just once: when the gas coma grew bright enough during the approach to the Sun to alter the comet’s overall appearance. That transition was already evident in amateur images from early September, which showed a blue/green gas coma prior to the spacecraft observations.

The object has also attracted misinformation and extreme speculation, including unfounded claims that it is an alien spacecraft or that government actions are being used to conceal its nature. Scientists emphasize that such claims are unnecessary: 3I/ATLAS offers a rare and valuable opportunity to study material from another star system and learn about conditions beyond our own solar neighborhood.

Numerous ground-based telescopes have imaged 3I/ATLAS, including consumer telescopes as small as six inches. Space-based observations have come from the Hubble Space Telescope, Europe’s ExoMars Trace Gas Orbiter, and China’s Tianwen-1 Mars probe. NASA's Mars Reconnaissance Orbiter was scheduled to image the comet during a close pass by Mars around Oct. 3, but imagery from that flyby has not been released amid a period of reduced NASA operations.

Comet 3I/ATLAS will make its closest approach to Earth on Dec. 19, passing at a distance of roughly 167 million miles (270 million kilometers). Continued observations from both amateur and professional instruments will help clarify the object’s composition and behavior as it departs our neighborhood.