It seems Halloween had a visitor from beyond our solar system. Astronomers at Arizona’s Lowell Observatory managed to snap what’s believed to be the first optical image of interstellar comet 3I/ATLAS after its fiery swing around the Sun — a cosmic trick-or-treater caught on camera by the 4.3-metre Discovery Telescope.Qicheng Zhang, a postdoctoral fellow at the observatory, subsequently showed that the comet is also visible through small amateur instruments across much of the Northern Hemisphere, posting an example on his Cometary blog on Sunday (November 2).“All you need is a clear sky and a very low eastern horizon,” Zhang told Live Science on Friday. “It won’t look very impressive, it’s just a smudge, but it will be an increasingly visible smudge over the next few days.” Discovered in July, 3I/ATLAS is only the third interstellar object ever recorded. It is travelling through the solar system at more than 130,000 mph (210,000 km/h) along an unusually flat and straight trajectory. The comet briefly disappeared from view as it slingshotted around the sun, reaching perihelion on Thursday (October 29) at 1.4 astronomical units—130 million miles (210 million kilometres)—from our star. Researchers and amateur astronomers continued to track its path using data from space telescopes while it was obscured. On October 28, Zhang and a colleague posted a study to the preprint server arXiv suggesting that 3I/ATLAS underwent rapid brightening ahead of perihelion and was distinctly bluer than the sun, indicating that gas emissions accounted for a substantial fraction of its visible brightness. Zhang added that the comet might still be brightening but that further data are required to confirm this. According to Zhang, the Lowell Discovery Telescope is one of the few large instruments capable of pointing close enough to the horizon to observe 3I/ATLAS so soon after perihelion. The comet is moving northward away from the northeastern horizon, creating a narrow window for morning-twilight observations when the sun is sufficiently below the horizon. To prepare for his scheduled time on the Discovery Telescope, Zhang tested conditions with a small six-inch-lens telescope. He successfully imaged the comet when it lay about 16 degrees from the sun (five degrees above the horizon). He noted that radio observations were made throughout perihelion and that another party may have obtained an optical post-perihelion image, although he was unaware of any prior to his own. Scientists now expect an important period of observations as sublimation of surface ice should reveal more about the comet’s composition as it recedes from the sun. Despite some media speculation that 3I/ATLAS might be an alien spacecraft, the majority of astronomers regard it as a routine comet originating in another star system within the Milky Way. Preliminary research suggests it could be around 3 billion years older than the solar system and that prolonged space-radiation exposure has formed an irradiated crust, making its origins harder to trace. “The comet is rapidly rising from the sun,” Zhang said. “I think in one week it’s going to be something like 25 or 30 degrees away from the sun, by which point there will be a large number of other big telescopes around the world that will also be able to start to follow it up.”
