Why Interstellar Comet 3I/ATLAS Is 'Missing' Its Tail — Here's What We Know

Astronomers have captured new images of the interstellar comet 3I/ATLAS — a bright green object currently speeding away from the Sun — and say that despite appearances, the comet has not lost its tail. The latest observations were made on November 5 by researcher Qicheng Zhang of the Lowell Observatory in Arizona, using the facility's Discovery Telescope. The comet, which recently passed behind the Sun, is once again visible as it travels deeper into space. Comets form a glowing atmosphere, or coma, when they near the Sun. The heat causes their frozen cores to release gas and dust, which then reflects sunlight. Through a green filter, 3I/ATLAS appears especially bright — a feature Zhang says is linked to its chemical composition. "The comet contains

many large hydrocarbons — molecules made of hydrogen and carbon — which break apart when exposed to ultraviolet light from the Sun," Zhang told Live Science. "It's sort of the same reason that if we stay out in the sun too long without sunscreen, we get sunburnt," he said. "The UV rays are destroying our DNA — which, like these hydrocarbons, are large, carbon-based molecules."Also Read: Rare Interstellar Comet 3I/ATLAS Approaches Earth - Check Date, Distance And How To Spot It This breakdown process produces diatomic carbon (C₂) — two carbon atoms bonded together — which emits a distinctive green glow.

The Mystery of the Missing Tail

Images of 3I/ATLAS taken after it re-emerged show no clear dust tail. Instead, the comet appears brighter on one side. Zhang explains that this is an optical illusion: the tail is present, but positioned almost directly behind the comet from Earth's viewpoint, curving slightly to one side. Since its discovery in July, 3I/ATLAS has intrigued astronomers as only the third known interstellar object to pass through our Solar System. It is thought to have originated from a distant, unidentified stellar system in the Milky Way — and could be billions of years older than the Sun. The comet made its closest approach to the Sun (perihelion) on 29 October, before reappearing in early November. This period is when comets are typically most active, offering researchers valuable data on their chemistry and structure. Preliminary findings suggest that 3I/ATLAS may have developed a thick, irradiated crust after long exposure to cosmic radiation, meaning it could now be releasing altered material rather than pristine samples from its home system. On October 31, Zhang used the Lowell Discovery Telescope to capture the first optical images of 3I/ATLAS after its perihelion passage, observing it at dawn as it rose above the north-eastern horizon. In earlier work published on October 28, Zhang and a colleague reported that the comet had brightened rapidly before perihelion and appeared slightly blue compared with the Sun. Filtered images confirmed this effect, showing the comet's light concentrated in shorter blue-green wavelengths. With its position now improving in the night sky, 3I/ATLAS can be observed by more telescopes around the world — and even by experienced amateur astronomers using instruments as small as 15 cm in diameter. "It's an exciting opportunity," Zhang said. "We can watch an interstellar visitor evolve in real time as it leaves our Solar System behind."