A rare cosmic traveller is on its way towards Earth this month, and skywatchers are eagerly waiting. Comet Lemmon, with its unusual green glow, is making a close approach that could make it visible without telescopes. Scientists say this celestial visitor will offer a spectacular show before heading back into deep space.
What makes Comet Lemmon special?
Comet C/2025 A6, known as Lemmon, is a nonperiodic comet with an orbital period of around 1,350 years. Most of its time is spent in the distant Oort Cloud, a region filled with icy objects on the solar system’s edge. It was first spotted on 3 January by astronomers at the Mt. Lemmon SkyCenter in Arizona as it raced toward the Sun at nearly 130,000 miles per hour.
The comet will make its closest approach to Earth on 21 October, coming within 56 million miles of the planet. Although this is more than 230 times the Moon’s distance, it is considered a relatively close pass in astronomical terms. Lemmon will then reach its nearest point to the Sun, known as perihelion, on 8 November before beginning its long journey back to the outer solar system.
Why does it glow green?
One of the most remarkable aspects of Lemmon is its emerald coma, which is the gas and dust cloud around it. The green glow is thought to be produced by diatomic carbon, an unusual type of carbon where two atoms are stuck together. When sunlight hits this molecule, it releases a bright green light, something that has been observed in other comets in recent times, like C/2022 E3 and 12P/Pons-Brooks.
The comet also has a vibrant tail composed of gas, ice and dust, driven back by solar radiation. Towards the end of September, solar winds made the tail ripple wildly, contributing to its viewing beauty. Astronomers believe Lemmon’s close pass around the Sun could alter its orbit, shortening its journey by up to 200 years, according to Space.com. This change is attributed to the gravitational forces from a recent Jupiter encounter.
How and when can you observe it?
Comet Lemmon is already visible to the naked eye using simple stargazing equipment and will grow easier to see as it brightens in the weeks ahead. Some predict it will come within a magnitude of 4, thus being barely visible to the naked eye. To see it best, astronomers advise searching for it in the constellation Lynx, which lies between Jupiter and the Big Dipper.
The optimal time to catch a glimpse of it will be prior to dawn or shortly after dusk, particularly in mid-October when the comet should be brightest. Stargazers in the Northern Hemisphere have the best hopes of seeing it, and if lucky, might get to see other transient comets such as SWAN R2 and 3I/ATLAS simultaneously.
This astronomical display might not occur again in over a thousand years, so Comet Lemmon's flyby represents an opportunity to see a radiant guest from the distant reaches of our solar system.
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