Interstellar comet 3I/ATLAS is turning bright green: Scientists explain why

When astronomers talk about interstellar objects, they’re usually describing something small and fleeting. But every so often, a true spectacle arrives, and right now, it’s in the form of a 7-mile-wide (that’s about the size of Manhattan end to end) comet nicknamed 3I/ATLAS.

What is comet 3I/ATLAS?

First detected in July, the comet has been hurtling inward at over 130,000 mph (170 times faster than a speeding bullet) on what will be a one-time visit before it vanishes back into deep space.

It’s worth noting that the comet didn’t come from anywhere near our planetary neighborhood. Instead, it was likely ejected from a far-off star system millions – perhaps billions – of years ago. After skimming past Mars next month, it will swing by the sun in late October before disappearing from Earth’s skies until December, when it re-emerges on the opposite side of its orbit.

Why is comet 3I/ATLAS green?

Astrophotographers – you know, those people who just love snapping celestial objects and events – in Namibia, using the unusually dark skies of last week’s lunar eclipse, captured images of the comet that startled scientists. The icy traveler now appears to be glowing emerald. If confirmed, this would put it in the company of a handful of other recent “green comets” that lit up telescopes in 2023 and 2024.

Normally, comets get that green coloration from dicarbon, a rare molecule formed when sunlight breaks down carbon-based gases. But early telescope readings haven’t yet found it in 3I/ATLAS. Some researchers think hidden layers of ice may just now be releasing the chemical, while others suggest the hue might come from cyanide or another exotic gas.

For now, the comet keeps racing onward, growing a tail and teasing astronomers with its mystery. The universe doesn’t give out many neon-green postcards, so this one has scientists watching very closely. And many of us amateur sky-gazers too.

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