Surprising new photos of comet 3I/ATLAS taken during last week's total lunar eclipse hint that the "interstellar visitor" may be turning bright green as it approaches the halfway point on its journey through the solar system. This unexpected transformation, if confirmed, is likely the result of the comet's increasing proximity to the sun, experts say.
3I/ATLAS is a roughly 7-mile-wide (11 kilometers) comet that was first spotted in early July, zooming toward us at more than 130,000 mph (210,000 km/h) from beyond the orbit of Jupiter. Astronomers quickly realized that the superfast object did not originate within our cosmic neighborhood, and was instead passing through on a one-way trip. It was likely ejected from a distant star system within the Milky Way and is probably much older than the solar system.
The comet is now closing in on a flyby of Mars next month, before reaching its minimum distance from the sun on Oct. 29. As the interstellar interloper gets closer to the sun, it has started to soak up more solar radiation, causing more ice, gas and dust to be expelled from its core, which has allowed it to start growing a traditional cometary tail.
But on Sept. 7, astrophotographers Michael Jäger and Gerald Rhemann snapped new shots of the comet in the dark skies over Namibia. These images were taken during the "blood moon" total lunar eclipse, when the full moon passed through the darkest part of Earth's shadow, meaning that the skies were darker than normal for that time of month. The resulting photos show 3I/ATLAS with a surprising emerald hue.
The new images suggest that the comet's increasing proximity to the sun has caused it to "turn green" as new rarer chemicals are expelled from its core, Spaceweather.com reported. However, it is still too soon to tell for sure, as no other photographers or observatories have so far witnessed this change.
This is not the first time that astronomers have spotted a green glow coming from a comet. In recent years, there have been several other emerald ice balls, including the aptly nicknamed "green comet" C/2022 E3, which passed by us in early 2023. The explosive "devil comet" 12P/Pons-Brooks also turned green as it neared the sun in 2024, while astronomers spotted another green comet, named SWAN25F, earlier this year.
The most common cause of this rare coloring is the presence of "dicarbon" in the clouds of ice and gas that surround comets, known as their comas. This molecule, also called diatomic carbon, is a form of carbon where two atoms are bonded together. Normally, pure carbon exists as solitary, unbonded atoms or within larger structures, such as diamonds.
So far, spectroscopic observations of 3I/ATLAS have not detected dicarbon in the comet's coma. However, it is possible that the molecule was locked away beneath layers of ice that have since been melted away by solar radiation, Spaceweather.com representatives wrote. "Or the green glow might come from some other mix of gases or dust, mimicking a classic cometary color with unfamiliar chemistry," they added.
Avi Loeb, an astronomer at Harvard University who is renowned for studying interstellar objects for possible signs of alien technology, has another explanation. On his personal blog, he wrote that the color could be caused by cyanide, which was spotted in the comet's coma in late August by the Very Large Telescope in Chile's Atacama Desert.
As a result, more photos and observations will be needed in the coming weeks to confirm the new coloration and uncover what may be causing it.
Unfortunately, the comet will soon disappear from view as it passes the sun on the opposite side from Earth. It will reappear in a few months, shortly before reaching its closest point to Earth in December, when it will be around 700 times further away from us than the moon.
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