Interstellar comet 3I/ATLAS has once again surprised astronomers, this time showing a faint blue tint after a sudden and unexplained brightening while it was positioned behind the Sun. The unexpected shift in colour and brightness has renewed scientific curiosity around the cosmic visitor.
Why is 3I/ATLAS turning blue?
A recent yet-to-be-published research study notes that the comet developed a noticeable blue shade in its coma shortly after it became visible again in late October. Scientists believe this could be related to icy gases — such as carbon monoxide or ammonia — leaking from the comet’s surface, causing a cooler, bluish appearance. The study further notes that 3I/ATLAS brightened dramatically while it was out of Earth’s direct view, though researchers are still unsure how much the Sun’s proximity alone could have influenced this surge.
This is the third recorded colour change for 3I/ATLAS this year. In July, the comet appeared red, likely due to sunlight reflecting off dust particles. Then, in September, it shifted to green — a colour potentially linked to molecules like cyanide or dicarbon. In each instance, the change faded after a short period, leaving scientists without a definitive explanation for the comet’s unusual pattern of colour cycling.
An image captured by astrophotographers Michael Jäger and Gerald Rhemann on September 7 showed the comet emitting a striking green glow.