Interstellar Comet 3I/ATLAS Brightens and Turns Green — Gemini Images Suggest New Activity

New observations from the Gemini North telescope show the interstellar comet 3I/ATLAS has become brighter and acquired a faint green tint as it heats up following a late-October close pass by the Sun. Astronomers say the change in color and increased activity could signal ongoing or future outbursts as the comet approaches its nearest point to Earth.

What the Images Reveal

Gemini North photographed 3I/ATLAS on Nov. 26 from Mauna Kea using four filters (blue, red, orange and green). The images capture the comet during one of its most active phases: solar heating is driving sublimation of ices and releasing large quantities of gas and dust that form a bright coma around the nucleus and a glowing tail.

After viewing the comet through red, orange, blue and green filters, NSF researchers found the comet is glowing much greener than it was several months ago. The blurred lines in the background are stars, which appear to be in motion while the telescope stays fixed on the comet. | Credit: Gemini North / NSF NOIRLab

The coma now shows a subtle greenish hue. That color is attributed to diatomic carbon (C2), a common cometary molecule that fluoresces green when illuminated by sunlight. NOIRLab, which operates Gemini North and Gemini South, noted the green glow was not apparent in earlier observations.

Color Change and What It Means

Earlier Gemini South images from late August showed 3I/ATLAS appearing distinctly redder, months before it passed closest to the Sun. The shift from red to green indicates the comet is releasing different molecules as it warms, providing valuable clues about its composition and internal structure.

A second Gemini North image, taken on Nov. 26, has been adjusted to correct the motions of background stars. According to NSF researchers, "an almost invisible, unknown main belt asteroid is photobombing the image to the lower right of the comet." | Credit: Gemini North / NSF NOIRLab

"What remains unknown is how the comet will behave as it leaves the Sun's vicinity and cools down," NOIRLab representatives wrote. "Many comets have a delayed reaction in experiencing the Sun's heat due to the lag in time that it takes for heat to make its way through the interior of the comet. A delay can activate the evaporation of new chemicals or trigger a comet outburst."

Trajectory, Size and Scientific Interest

Discovered in late June, 3I/ATLAS is the third known interstellar object to visit our solar system after 1I/'Oumuamua and 2I/Borisov. It is traveling on a hyperbolic trajectory at roughly 130,000 mph (210,000 km/h) and will not return to our neighborhood.

The comet is likely one of the largest and possibly the oldest interstellar visitors observed so far. Observers have reported large, Sun-facing jets and signs of long-term irradiation from time spent in interstellar space. Despite occasional speculation online, the scientific consensus is that 3I/ATLAS is a natural comet, not an artificial or extraterrestrial craft.

Close Approach and Ongoing Monitoring

3I/ATLAS will make its closest approach to Earth on Dec. 19, passing at a safe distance of about 170 million miles (270 million kilometers). Dozens of ground-based observatories and spacecraft are monitoring the object to refine measurements of its size, trajectory and composition. Continued observations may reveal further color changes, new chemical signatures or sudden outbursts as interior ices respond to solar heating.

Why It Matters: Studying 3I/ATLAS gives astronomers a rare opportunity to sample the chemistry and physical properties of material formed around other stars, helping to improve our understanding of how planetary systems — including our own — formed and evolved.