New observations confirm that the interstellar comet 3I/ATLAS is growing more active as it approaches Earth, displaying a striking greenish glow that wasn’t present just months ago. This phenomenon, driven by heating from the sun, signals potential future outbursts of cometary material. The comet will make its closest pass to Earth on December 19th, offering a rare opportunity to study an object originating outside our solar system.
Increased Activity and the Green Hue
Images captured by the Gemini North telescope in Hawaii reveal the comet’s coma—the bright, cloudy atmosphere surrounding its nucleus—is now distinctly green. This color change isn’t due to extraterrestrial life, but rather the release of diatomic carbon (C2) gas. When heated by the sun, ice sublimates (turns directly into gas) carrying dust and molecules like C2 into space. The C2 emits a green light, a common sight in active comets such as 12P/Pons-Brooks and Comet C/2025 F2 (SWAN).
What’s unusual about 3I/ATLAS is that it initially appeared redder in late August, before its closer approach to the sun. This suggests that the comet is continuously releasing new chemicals as it heats up, offering a dynamic glimpse into its composition.
The Science Behind Interstellar Comets
Interstellar comets, unlike those born within our solar system, arrive from other star systems and are only briefly visible before continuing their journey outwards. 3I/ATLAS is the third confirmed interstellar object to pass through our neighborhood, after 1I/’Oumuamua and 2I/Borisov. It was discovered in June, moving at over 130,000 mph on a hyperbolic orbit, meaning it won’t return.
Astronomers believe 3I/ATLAS is likely the largest and oldest interstellar object observed so far. Despite some fringe theories suggesting artificial origins, the scientific consensus is that it is a typical comet that has spent billions of years in interstellar space.
What to Expect Next
As 3I/ATLAS nears Earth (approximately 170 million miles away on December 19th), its behavior remains unpredictable. Heat may take time to permeate the comet’s interior, potentially triggering delayed outbursts or the evaporation of new compounds. The comet’s reaction to cooling down after its close encounter with the sun is also unknown.
“What remains unknown is how the comet will behave as it leaves the Sun’s vicinity and cools down,” NOIRLab representatives note. This uncertainty makes 3I/ATLAS a valuable object for study, providing insight into the composition and behavior of interstellar visitors.
