In the spirit of the season, the interstellar comet 3I/ATLAS is changing color as it leaves the sun behind — shifting from a reddish tint to a faint green glow. That visible change signals the comet is reacting to solar heat and releasing new gases in space, giving scientists fresh clues about what it is made of and how it behaves.

Harvard physicist Avi Loeb is puzzled by the interstellar comet 3I/ATLAS after it turned a shocking green colour near the sun. Is this a natural phenomenon, or a sign of alien technology?

Discovered on July 1 by the NASA-funded ATLAS telescope in Chile, 3I/ATLAS is only the third confirmed visitor from another solar system, following 1I/'Oumuamua in 2017 and comet 2I/Borisov in 2019.