An intriguing interstellar object that ventured into our solar system has proven to be even stranger than scientists initially thought.
Back in July, the NASA-funded ATLAS survey telescope located in Chile detected 3I/ATLAS, marking the third confirmed object that originated from interstellar space, meaning it came from beyond our solar system. The first two were ʻOumuamua in 2017 and 2I/Borisov in 2019. This latest visitor, however, is the largest among them, with an estimated width of about 7 miles (11.2 kilometers).
Researchers identified 3I/ATLAS as a comet. Similar to other comets, it features a coma—a cloud of gas and dust—and a tail, both resulting from the Sun’s heat acting on its icy surface.
Initially, the comet appeared red in all observations, but recent images reveal a green glow. This color transition could be attributed to a significant rise in cyanide gas (CN) production as the comet approached the Sun, as noted by Avi Loeb, a Harvard professor and the founding director of its Institute for Theory and Computation.
Loeb expanded on this in a Medium article, where he described how the ATLAS team evaluated changes in the comet’s light as it moved away from the Sun. When 3I/ATLAS was beyond 3.3 astronomical units (AU), there was a sharp increase in the brightness of the scattered light. However, this rate of increase slowed as it approached closer.
The observed shift indicates a transition in the comet’s behavior from reflecting sunlight off reddish surface dust to emitting small, bright icy particles that altered the light’s path through the surrounding plume.
Subsequent observations using the Hubble Space Telescope on July 21, 2025, along with data from the James Webb Space Telescope and SPHEREx observatory in August 2025, revealed that the gas plume encircling 3I/ATLAS primarily consists of carbon dioxide (CO₂), accompanied by traces of carbon monoxide (CO) and water (H₂O).
These discoveries, along with the comet’s trajectory closely aligning with the solar system’s ecliptic plane, render 3I/ATLAS a fascinating and distinct object compared to known icy bodies.
Additionally, data from the Very Large Telescope on August 25, 2025, indicated a significant spike in cyanide production, along with an increase in nickel (but not iron). This change in the comet’s composition is likely responsible for its greenish hue.
Various organizations, including NASA, have confirmed that 3I/ATLAS is indeed an interstellar comet. It is expected to make its closest approach to the Sun around Halloween 2025 before it continues its journey back into the depths of space, never to return.
Therefore, if you’re interested in observing this interstellar visitor, you should do so before Halloween, as this cosmic traveler won’t be making a return trip!