**Comet 3I/ATLAS Glows Green and Hides Its Tail, But There’s Nothing Wrong**
A new image of comet 3I/ATLAS reveals that the interstellar visitor is glowing green and seemingly hiding its tail — but that doesn’t mean anything is amiss.
Qicheng Zhang, a researcher at the Lowell Observatory in Arizona, used the observatory’s powerful Discovery Telescope to capture fresh observations of the comet as it sped away from the Sun on Wednesday (Nov. 5). The comet recently became visible again after swinging around the far side of our star.
Comets develop an atmosphere, or coma, as they fly close to the Sun. This cloud of gas and dust grows larger and brighter as solar heat causes ice and other materials on the comet’s surface to sublimate into gases, which astronomers can then observe. In the case of 3I/ATLAS, the coma is brightest when viewed through a green filter — like most comets that pass near the Sun.
Zhang used a specialized filter to detect diatomic carbon (C2) particles, which glow green. He noted that the comet contains a significant amount of large molecules composed of carbon and hydrogen (hydrocarbons). When the comet approaches the Sun, ultraviolet (UV) light breaks these molecules apart.
“It’s sort of for the same reason that if we stay out in the sun too long without sunscreen, we get sunburnt,” Zhang explained to Live Science. “The UV rays are destroying our DNA [in our skin cells], which is kind of a similar type of molecule in the sense that it’s big and contains carbon.”
When UV light breaks down these hydrocarbons, some of the fragments are diatomic carbon molecules — two carbon atoms bonded together — which glow green and are easy for astronomers to detect.
### The Hidden Tail
The comet appears to lack a dust tail in the image, but it is still there. Zhang pointed out that if you look closely, the comet’s left side is slightly brighter than the right. This subtle asymmetry occurs because we’re essentially seeing the tail head-on; it lies directly behind the comet, curving slightly to the left.
In other words, the comet’s apparent absence of a tail isn’t anything unusual or cause for concern.
### A Rare and Intriguing Interstellar Visitor
Since its discovery in July, comet 3I/ATLAS has become a celestial celebrity. Much of the excitement stems from speculation that it might be an alien spacecraft. However, most astronomers agree it’s a comet traveling through the Milky Way from an unknown star system.
Calling 3I/ATLAS just a regular comet doesn’t fully capture its significance. It’s only the third interstellar visitor ever recorded and could be the oldest comet observed, with some studies suggesting it’s around 3 billion years older than our solar system.
The comet recently reappeared in Earth’s skies after briefly disappearing behind the Sun and reached its closest point to our star, known as perihelion, on Oct. 29. This post-perihelion phase offers a crucial window for astronomers to study the comet’s gases and composition, as comets tend to be most active near perihelion.
Preliminary research indicates that prolonged exposure to space radiation may have formed a thick, irradiated crust on 3I/ATLAS, obscuring its original characteristics from its home star system. If this is true, scientists might find it challenging to trace the comet’s origins since it would be venting irradiated material rather than pristine ancient material.
### Observing 3I/ATLAS Through Different Filters
Zhang previously used the Lowell Discovery Telescope to capture the first optical, post-perihelion images of 3I/ATLAS from Earth on Halloween (Oct. 31). Like his latest observations, these were made during morning twilight as the comet rose above the northeastern horizon, moving northward from our perspective.
He took multiple images using different filters. The diatomic carbon image he shared on his Cometary blog on Nov. 5 roughly shows what the comet might look like to the naked eye.
On Oct. 28, Zhang and a colleague posted a study to the preprint server arXiv suggesting that 3I/ATLAS brightened rapidly before perihelion and appeared distinctly bluer than the Sun. The green color in the new image doesn’t indicate a post-perihelion color change; the comet likely had this characteristic earlier.
In astronomy, “bluer” or “redder” usually refers to shorter or longer wavelengths of light. Zhang said the comet is much brighter when viewed through bluer filters compared to redder ones. Though these “bluer” filters are actually a mix of green and blue, they are not very sensitive to pure blue light.
“It’s brightest in the bluest filter that we have,” Zhang noted.
### Viewing the Comet Now and in the Coming Months
The Lowell Discovery Telescope is among the largest telescopes capable of pointing low enough on the horizon to observe 3I/ATLAS just after perihelion. However, the comet is now high enough above the horizon that many large telescopes can conduct observations. Even small personal telescopes with lenses as modest as 6 inches (15 centimeters) can spot it.
Astronomers can expect a flurry of interesting findings on comet 3I/ATLAS in the coming months as it continues its journey through our solar system and beyond.
https://www.livescience.com/space/comets/irradiated-comet-3i-atlas-glows-green-and-hides-its-tail-in-new-image