When 3I/ATLAS was first seen, it caught attention with its striking red hue. Now, it has shifted to a vibrant green. Why the change? Scientists are still piecing it together, but it involves some interesting cosmic chemistry.
Discovered in early July 2025, 3I/ATLAS is an exciting interstellar comet traveling through our Solar System. NASA has confirmed it’s not an alien spaceship, though! This comet is a solid mass of ice and gas, surrounded by a cloud called a coma. It’s only the third object from another star that we’ve directly observed, following ‘Oumuamua and Comet 2I/Borisov.
In July, early observations noted its red coma, but that changed quickly. On September 7, photographers Michael Jäger and Gerald Rhemann captured images of 3I/ATLAS during a lunar eclipse in Namibia. To their amazement, it appeared green, with hints of blue. Jäger said, “Our images show a gas-rich coma for the first time, clearly visible in both blue and green filters,” as reported by SpaceWeather.com.
It’s not totally strange for comets to glow green. This often happens due to molecules of diatomic carbon (C₂) in the coma when they absorb sunlight. Yet, 3I/ATLAS is known to be low in carbon chains. This raises questions about what’s causing its emerald glow.
Avi Loeb, a professor at Harvard, suggests that the color change could be linked to increased cyanide production in the coma. This was noted by the European Southern Observatory’s Very Large Telescope.
Beyond its color, 3I/ATLAS is impressive in size. Loeb and his colleagues estimate it weighs about 33 billion tons and features a nucleus diameter of 5 kilometers (3.1 miles). That’s quite hefty for a comet!
To gather more insights, further observations will be crucial. However, there’s a catch: from late October 2025, 3I/ATLAS will disappear behind the Sun from our view until early December 2025. While we’ll get another chance to observe it later, expect less news about this fascinating interstellar wanderer in the meantime.
For more about celestial phenomena, check out NASA’s website on comets and their ongoing research.
