Interstellar comet 3I ATLAS glows green during lunar eclipse | Space photo of the day for Sept. 15, 2025

On Sept. 7 2025, the skies darkened as Earth’s shadow consumed the moon. Skywatchers in many parts of the world saw the moon turn blood red due to a total lunar eclipse.

For amateur astronomers Michael Jäger and Gerald Rhemann in Namibia, the eclipse was not only a sight to behold, but it also gave them an unprecedented opportunity: the chance to capture the interstellar comet 3I/ATLAS under some of the darkest skies on Earth.

Using the eclipse’s natural dimming of the moon, the duo was able to take some deep images of the comet, revealing something surprising: the comet glowed green.

What is it?

3I/ATLAS is only the third confirmed interstellar comet observed passing through our solar system and where it originated from is still a mystery. The comet is classified as interstellar because its trajectory is hyperbolic, meaning it’s not gravitationally bound to the sun and most likely originated in another star system (at least that’s the most likely theory).

It follows in the footsteps of two other famous interstellar objects: ‘Oumuamua, a cigar-shaped body that still sparks debates about its nature, and comet 2I/Borisov, the first confirmed interstellar comet, which displayed more of a typical dusty coma and tail.

Now, 3I/ATLAS provides astronomers with another rare chance to study the chemistry of material formed around a completely different star.

Where is it?

This photograph was taken in the Kalahari Desert in Namibia, Africa.

Why is it amazing?

This is the first image showing a green hue to 3I/ATLAS, hinting at its unique chemistry. With the darkness provided by the lunar eclipse, the first explanation for this green color is that the comet may have diatomic carbon (C₂). This molecule, made of two carbon atoms bonded together, is known to fluoresce a green glow when energized by sunlight.

However, in August, astronomers at Kitt Peak Observatory in Arizona found that 3I/ATLAS is actually poor in carbon-chain molecules, including diatomic carbon. This begs the question however, if diatomic carbon was absent only a month earlier, how is the comet suddenly green now?

One possibility is that heating by the sun caused fresh layers of ice to melt and release hidden diatomic carbon molecules that were previously trapped beneath the surface. Another possibility is that the green glow is coming from other molecules which also glow under sunlight. Because 3I/ATLAS is interstellar, it may contain chemistry not commonly seen in our solar system comets, leading to a false “classic” green glow from an unfamiliar source.

Whatever the cause, 3I/ATLAS remains a mystery in many aspects as astronomers continue to study it as it moves through our solar system.

Want to learn more?

You can read more about imaging 3I/ATLAS and its journey toward the sun.