Interstellar COMET 3I/ATLAS was observed to become brighter and emit a greenish color as it moved closer to Earth. This finding is based on the latest images from the Gemini North telescope in Hawaii which shows an increase in comet activity since its close pass to the Sun at the end of last October.
Images recorded on November 26 from the top of Mount Mauna Kea show comet 3I/ATLAS in one of its most active phases so far. Heating due to solar radiation causes the ice in the comet to sublimate and burst into space along with large amounts of dust, forming a bright coma around the comet’s nucleus and a long, luminous tail.
Observations were made using four color filters—blue, red, orange, and green. The results show that the gas inside the comet’s coma is now emitting a faint greenish light, which was previously undetectable just a few months ago. This green color comes from diatomic carbon (C2), a gas commonly released by comets when activated by the heat of the Sun.
This color change is considered significant because 3I/ATLAS appeared reddish when it was first observed at the end of August, long before it approached the Sun. This condition indicates the release of new molecules as the comet’s temperature increases, as well as providing new clues about its composition.
NOIRLab stated that the behavior of this comet is still difficult to predict when it starts to move away from the Sun and cools. “What is still unknown is how this comet will behave when it leaves the Sun’s environment and begins to cool,” wrote NOIRLab representatives in a written statement, quoted from the report. Live ScienceDecember 13, 2025.
“Many comets experience a delayed reaction to the Sun’s heat because there is a time lag for the heat to propagate to the interior of the comet. This delay can enable the evaporation of new chemicals or trigger comet flares,” he added.
3I/ATLAS is scheduled to reach its closest point to Earth on December 19, at a distance of about 170 million miles or 270 million kilometers. The researchers estimate that there is still the possibility of further activity occurring before or after that moment.
This comet is the third interstellar object ever discovered, after 1I/’Oumuamua and 2I/Borisov. 3I/ATLAS was detected in late June while passing through the solar system at a speed of about 130 thousand miles per hour or 210 thousand kilometers per hour in a hyperbolic orbit that would not bring it back to the vicinity of the Sun.
Astronomers estimate 3I/ATLAS to be the largest and possibly the oldest interstellar object ever observed. Even though it displays a number of unique characteristics, the scientific community agrees that this comet is a natural object, not an artificial object or alien technology as is claimed on social media.
Dozens of observatories and space probes continue to monitor the movement of 3I/ATLAS to study its size, trajectory, composition and origins. It is hoped that this study will open up a new understanding of interstellar space and the process of forming early star systems in the Milky Way Galaxy.