This photo suggests that interstellar comet 3I/ATLAS may have started to glow green. However, scientists have not yet confirmed the color change. (Photo: Michael Jaeger/Gerald Rehmann)
Stunning new images of Comet 3I/ATLAS taken during last week's total lunar eclipse suggest that the “interstellar visitor” may be turning a bright green as it approaches the middle of its journey through the solar system. The unexpected transformation, if confirmed, is likely the result of the comet's increasingly close approach to the sun, experts say.
3I/ATLAS is a comet about 11 kilometers in diameter, first spotted in early July. It is hurtling toward us at over 210,000 km/h (130,000 mph) from beyond Jupiter's orbit. Astronomers quickly realized that this super-fast object did not originate in our cosmic neighborhood, but was moving in one direction. It was likely ejected from a distant star system within the Milky Way, and is likely much older than the Solar System.
The comet is now making its closest approach to Mars next month, before reaching its closest approach to the Sun on October 29. As the interstellar alien gets closer to the Sun, it begins to absorb more solar radiation, causing more ice, gas and dust to be ejected from its nucleus, allowing it to begin growing a traditional comet tail.
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NASA's first observations have shown that Comet 3I/ATLAS is unusually “active.”
But on September 7, astrophotographers Michael Jäger and Gerald Rehmann captured new images of the comet in dark skies over Namibia. The images were taken during a total lunar eclipse called a “blood moon,” when the full moon passed through the darkest part of the Earth’s shadow, meaning the sky was darker than usual for that time of month. In the resulting images, Comet 3I/ATLAS took on a striking emerald hue.
The new images suggest that the comet's increasing proximity to the Sun has caused it to “green up” as new, rarer chemicals are released from its nucleus, Spaceweather.com reports. However, it is too early to draw firm conclusions, as no other photographer or observatory has yet captured the change.
In recent weeks, 3I/ATLAS has also begun to grow its traditional comet tail.
This isn’t the first time astronomers have spotted a green glow coming from a comet. Several other emerald icy balls have been spotted in recent years, including the aptly nicknamed “green comet” C/2022 E3, which flew past us in early 2023. The explosive “devil comet” 12P/Pons-Brooks also turned green as it approached the Sun in 2024, and earlier this year, astronomers spotted another green comet, SWAN25F.
The most common cause of this rare coloration is the presence of dicarbon in the clouds of ice and gas that surround comets, known as their comas. Also called diatomic carbon, this molecule is a form of carbon in which two atoms are bonded together. Pure carbon usually exists as individual, unbonded atoms or as part of larger structures such as diamonds.
3I/ATLAS spectroscopic observations have not yet detected dicarbon in the comet's coma. However, the molecule may have been trapped under layers of ice that were subsequently melted by solar radiation, Spaceweather.com writes. “Or the green glow may be caused by some other mixture of gases or dust that mimics the classic cometary color with an unfamiliar chemical composition,” they added.
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Avi Loeb, a Harvard University astronomer known for his studies of interstellar objects for possible signs of alien technology, offers another explanation. He wrote on his personal blog that the color could be caused by cyanide, which was detected in the comet’s coma in late August by the Very Large Telescope in Chile’s Atacama Desert.
Therefore, additional photographs and observations will be needed in the coming weeks to confirm the new coloration and determine its possible cause.
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 it reaches its closest approach to Earth in December. At that point, it will be about 700 times farther away from us than the Moon.
Harry BakerNavigate Social LinksSenior Staff Writer
Harry is a senior writer for Live Science based in the UK. Before becoming a journalist, he studied marine biology at the University of Exeter. He covers a wide range of topics, including space exploration, planetology, space weather, climate change, animal behaviour and palaeontology. His recent work on solar maximum won the 2024 Aerospace Media Awards in the Best Space Story category and was shortlisted for the 2023 NCTJ Awards for Excellence in the Breaking News category. He also writes Live Science’s weekly series Earth from Space.
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I watched scientists observe the interstellar comet 3I/ATLAS in real time. This is what they saw.
Watch Newly Discovered 'Interstellar Visitor' 3I/ATLAS Fly Towards Us in First Live Stream
NASA's first observations have shown that Comet 3I/ATLAS is unusually “active.”
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