Interstellar Comet 3I/ATLAS Turns Green Ahead Of Closest Approach To Earth

Recent images captured by the Gemini North telescope reveal that the interstellar comet 3I/ATLAS, which is currently passing through our solar system, has taken on a striking green hue and is growing brighter as it approaches Earth, Live Science reported. The comet will be at its closest point to Earth on December 19 as it will come within 170 million miles (270 million kilometres) of our planet.

The interstellar comet was discovered on July 1, 2025, by the ATLAS telescope in Chile. It is the third interstellar object ever detected, following 1I/'Oumuamua and 2I/Borisov. 3I/ATLAS is believed to be the largest and oldest interstellar object observed so far, offering scientists a unique opportunity to study material formed around another star.

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The latest image of the comet was captured by scientists using the Gemini Multi-Object Spectrograph (GMOS) on the 8.1-meter telescope Gemini North telescope atop Hawaii's dormant Mauna Kea volcano on November 26, 2025, four weeks after its perihelion.

After its closest approach to the Sun, the comet is now emitting tonnes of dust and forming a bright coma, along with a tail. To capture the same, scientists observed it using four filters - blue, red, orange and green. They found that gas in the coma now emits a faint greenish hue, which also happened a few weeks ago.

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What's behind the colour change?

The comet's green colouration is caused by diatomic carbon (C2) molecules in its coma, which emit light at green wavelengths when exposed to solar radiation.

This phenomenon is not uncommon in comets, but the recent change in 3I/ATLAS's colour from red to green suggests that new molecules are being released as it heats up.

"What remains unknown is how the comet will behave as it leaves the Sun's vicinity and cools down. Many comets have a delayed reaction in experiencing the Sun's heat due to the lag in time that it takes for heat to make its way through the interior of the comet," the scientists wrote in a statement.