When 3I/ATLAS was first spotted a few months ago, it seemed to emit a reddish color. But earlier this month, some astronomers noted it was glowing green. So why is our latest interstellar visitor appearing to switch colors like a traffic light? The answer isn’t certain, but it has something to do with cosmic chemistry. The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

First observed in early July 2025, 3I/ATLAS is a giant interstellar object that’s currently blasting through our Solar System. And sorry, folks, but NASA says it’s not an advanced alien spacecraft.

It’s an active comet consisting of a solid icy nucleus surrounded by a haze (known as a coma) of gas and ice. It’s particularly exciting because it’s only the third time we ha

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