Lowell Discovery Telescope – The interstellar traveller known as comet 3I/ATLAS appears green — and curiously, seems to be hiding its tail. But astronomers say there’s no cause for concern. On Wednesday, November 5, Qicheng Zhang, a researcher at the Lowell Observatory in Arizona, captured new images of the comet using the facility’s Discovery Telescope.
After circling behind the sun, 3I/ATLAS is once again visible as it speeds away into the depths of space. Comets develop a diffuse atmosphere, or coma, as they approach the sun.
Solar radiation heats their icy cores, causing the frozen material to sublimate into gas and dust, which then glows as it expands outward. When observed through a green filter, comet 3I/ATLAS appears particularly bright — much like most comets nearing the sun.
Story continues below this ad The glow of diatomic carbon Zhang used a filter to detect diatomic carbon (C₂), the molecule responsible for the comet’s green hue. “The comet contains many large hydrocarbons — molecules made of hydrogen and carbon — which break apart when exposed to ultraviolet (UV) 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 explained. “The UV rays are destroying our DNA — which, like these hydrocarbons, are large, carbon-based molecules.
” When this process happens on a comet, astronomers can easily spot diatomic carbon, composed of two bonded carbon atoms that emit a distinct green glow. Explaining the ‘missing’ tail Recent observations show 3I/ATLAS without a visible dust tail, instead displaying an asymmetric brightness that appears stronger on its left side. This, Zhang says, is an optical illusion — the tail is there but lies directly behind the comet, slightly curved to the left, creating a head-on perspective.
Story continues below this ad Despite speculation, this does not diminish scientific enthusiasm. Since its discovery in July, 3I/ATLAS has fascinated researchers as a confirmed interstellar object, likely originating from a distant, unidentified stellar system in the Milky Way.
Some have even speculated — fancifully — that it could be an extraterrestrial probe. What is certain, however, is that 3I/ATLAS is only the third recorded interstellar visitor and may be up to three billion years older than our solar system.
Post-perihelion findings The comet passed perihelion on October 29, its closest approach to the sun, before reappearing in early November. This period marks peak activity for most comets, allowing astronomers to study their chemistry in detail. Preliminary data suggests that 3I/ATLAS may have developed a thick, irradiated outer crust from prolonged cosmic exposure — meaning it could now be releasing altered material rather than pristine samples from its original star system.
On October 31, Zhang used the Lowell Discovery Telescope to conduct the first optical observations of 3I/ATLAS post-perihelion, capturing it at dawn as it drifted northward from the northeastern horizon. Story continues below this ad Brightness and colour shifts Earlier on October 28, Zhang and a colleague published findings on arXiv describing a rapid brightening of the comet before perihelion and a noticeably blue hue relative to the sun. In an accompanying blog post, Zhang shared several filtered images, including one highlighting diatomic carbon — showing how the comet might appear to the naked eye.
Their analysis confirmed that the comet’s blue appearance corresponds to shorter wavelengths of light. As Zhang noted, 3I/ATLAS appears significantly brighter through blue-green filters, which capture these shorter wavelengths most effectively.
While only a few large telescopes, such as the Lowell Discovery Telescope, could observe the comet immediately after perihelion, its rising position in the sky now allows many observatories — and even skilled amateur astronomers using 6-inch (15 cm) telescopes — to track its journey.