On Sept 7th, darkness fell across the Kalahari Desert as Earth's shadow enveloped the full Moon. It was a total lunar eclipse--and the perfect time to photograph an interstellar comet. "We took advantage of the total lunar eclipse to take a deep image of Comet 3I/ATLAS under the dark skies of Namibia," says amateur astronomers Michael Jäger and Gerald Rhemann. Here is what they saw.

The comet was green. "Our images show a gas-rich coma of 2 arcminutes for the first time," says Jäger "It was clearly visible in both the blue and green filters."

The sight is familiar to astronomers: Comets in our own Solar System often glow green. Their emerald hue comes from diatomic carbon (C₂), which fluoresces at 518 nanometers when energized by sunlight.

Is that why 3I/ATLAS is green? The jury's still out. In August, spectroscopy by astronomers at Kitt Peak showed that 3I/ATLAS was strongly depleted in C₂. Indeed, it was one of the most carbon-chain poor comets ever observed. Perhaps C₂ production turned on in September. Or the green glow might come from some other mix of gases or dust, mimicking a classic cometary color with unfamiliar chemistry.

Stay tuned for updates. Its passage is far from over. Each week could bring new chemistry from another star system.

more images: from Efrain Morales of Aguadilla, Puerto Rico