New telescope images of the interstellar comet 3I/ATLAS confirm that the mysterious object has gotten brighter and greener since its close approach to the sun in late October. This increase in activity could portend new, bright outbursts of cometary material as 3I/ATLAS careens toward its closest encounter with Earth next week.
Taken on Nov. 26 with the Gemini North telescope atop Hawaii’s dormant Mauna Kea volcano, the new images capture the comet in one of its most active phases yet. Recently heated by intense solar radiation, ice on the comet is sublimating and spewing into space along with tons of dust, forming a bright, cloudy atmosphere (a coma) around the comet’s main body and a long, glowing tail behind it.
Why is 3I/ATLAS going green?
Don’t rush to blame little green men for the comet’s new greenish glow. Among the gases spilling out of 3I/ATLAS is diatomic carbon (C2) — a molecule of two carbon atoms that emits a greenish light, according to a statement from the National Science Foundation’s NOIRLab, which operates the Gemini North telescope along with its twin Gemini South telescope in Chile.
Many solar system comets give off a similar green hue when activated by the sun, including last year’s “Mother of Dragons” comet 12P/Pons-Brooks and the recently discovered Comet C/2025 F2 (SWAN).
However, it’s noteworthy that 3I/ATLAS appeared distinctly redder when it was first observed by Gemini South in late August, months before its close flyby of the sun, according to NOIRLab. This is evidence that 3I/ATLAS is releasing new molecules into space as it heats up, offering fresh hints into its mysterious makeup.
Is another outburst coming?
As the comet approaches its closest point to Earth on Dec. 19 (coming within a comfortable 170 million miles, or 270 million kilometers, of our planet), we may be in for even more surprises.
“What remains unknown is how the comet will behave as it leaves the Sun’s vicinity and cools down,” NOIRLab representatives wrote in the statement. “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.”
3I/ATLAS is the third interstellar object ever discovered, after 1I/’Oumuamua and 2I/Borisov. The comet was detected in late June as it was speeding through our solar system at an estimated 130,000 mph (210,000 km/h), zooming by on a hyperbolic (U-shaped) orbit that will never bring it through our neighborhood again.
3I/ATLAS is probably the largest and, very likely, the oldest interstellar object seen so far. While it shows many intriguing features — including large, sun-facing jets and signs of being irradiated by its billions of years spent in interstellar space — the vast majority of astronomers and space agencies agree that it is a typical comet — and not an artificial piece of alien technology, as some viral claims have suggested.
Dozens of observatories and spacecraft around the solar system have been closely monitoring 3I/ATLAS to better understand its size, trajectory, composition and origins. Studying it in depth could reveal new details about the mysterious frontier of our galaxy and how some of the earliest star systems in the Milky Way formed.