October unfurls an extraordinary celestial tapestry, presenting a rare opportunity to observe not one, but two brilliant greenish comets, C/2025 A6 (Lemmon) and C/2025 R2 (SWAN), as they sweep through our inner solar system. Adding to this spectacle, an enigmatic interstellar comet, 3I/ATLAS, also makes its presence known, offering a chance to glimpse cosmic material from beyond our stellar neighborhood. This definitive guide prepares skywatchers in the Northern Hemisphere to locate and appreciate these icy wanderers, providing unparalleled context and viewing strategies for a truly memorable astronomical event.
October 2025 marks a truly extraordinary period for skywatchers, as a rare alignment brings not just one, but two vibrant greenish comets into view from the Northern Hemisphere. This dual appearance in the same month is highly unusual, offering a unique opportunity for both amateur and seasoned astronomers. To further enhance this cosmic convergence, an interstellar comet, the third of its kind ever confirmed, is also making a fleeting visit, providing a once-in-a-lifetime chance to observe material from beyond our solar system.
These icy wanderers, named C/2025 A6 (Lemmon) and C/2025 R2 (SWAN), are celestial time capsules, remnants from the dawn of our solar system. Their current orbits bring them through our cosmic neighborhood, making them accessible to observation with modest equipment. Understanding their origins, their unique characteristics, and how to spot them will enrich the experience of this exceptional month.
The Green Glow: Comets Lemmon and SWAN Unveiled
Both Comet Lemmon and Comet SWAN are characterized by a distinct greenish hue and a dramatic long tail, a feature captured by astrophotographer Julien De Winter in late September. This emerald glow originates from the presence of diatomic carbon (C₂) in the gaseous cloud, or coma, surrounding the comet’s nucleus. As these icy bodies approach the Sun, its heat causes the frozen gases and dust to sublimate, creating the visible atmosphere and tail.
The discovery stories of these comets are as fascinating as their appearance. Comet Lemmon was first identified in January by the Mount Lemmon Survey, an initiative that utilizes telescopes in Arizona’s Santa Catalina Mountains to track near-Earth objects. Comet SWAN, however, was spotted much more recently in early September by an amateur astronomer sifting through images from the SWAN instrument aboard NASA’s Solar & Heliospheric Observatory (SOHO), a spacecraft dedicated to studying the Sun since its launch in 1995. This highlights the vital role of both professional observatories and citizen science in astronomical discoveries, as noted by the NASA SOHO mission homepage.
These are considered long-period comets, meaning their orbits around the Sun last 200 years or more. They originate from the frigid edge of our solar system in the Oort Cloud, a spherical shell of icy bodies. These comets likely formed near gas giants like Jupiter and Saturn billions of years ago before being gravitationally nudged to the solar system’s outskirts, where they have been preserved in a deep freeze. When such a comet is drawn back towards the Sun, it offers scientists a rare glimpse into the primordial materials and original building blocks from which our solar system formed, providing clues about its early history.
Your Viewing Guide: When and Where to Look
For eager skywatchers in the Northern Hemisphere, October presents an excellent, albeit limited, window to spot these cosmic travelers. Both comets are best viewed with binoculars or a small telescope, though Comet Lemmon holds the potential for naked-eye visibility under ideal conditions. To maximize your chances, seek out locations with minimal light pollution, clear weather, and dark skies.
Spotting Comet Lemmon
- From now until mid-month, Comet Lemmon appears near the Big Dipper before sunrise. Look northeast in the sky in the hours before dawn, slightly below the distinct bowl and handle of the star grouping, as advised by EarthSky.
- In about a week, it will transition to the evening sky, rising in the west. From then until the end of October, it may become faintly visible to the unaided eye as it rapidly brightens.
- Its closest approach to Earth, when it will be at its brightest, is anticipated on or around October 21, at a distance of approximately 55 million miles (88.5 million kilometers). This coincides with a new moon, ensuring darker skies.
Spotting Comet SWAN
- Comet SWAN will be visible in the evening sky through the end of the month.
- The optimal viewing time is roughly 90 minutes after sunset, facing southwest.
- While generally expected to be dimmer than Lemmon, it will appear higher on the horizon after sunset in the coming days and may brighten before its closest approach to Earth on or around October 20, coming within 24 million miles (38.6 million kilometers) of our planet. Binoculars or a small telescope will be essential for spotting this comet.
Under truly superb conditions, with transparent skies and complete darkness, it might even be possible to witness both comets on the same night around the end of the month. Adding to the celestial celebration, the Orionid meteor shower is also set to peak on the night of October 20-21. This synchronization with the new moon provides the darkest possible conditions, offering the incredible possibility of seeing streaking comets against the backdrop of an active meteor shower.
An Interstellar Enigma: Comet 3I/ATLAS
Beyond the homegrown spectacle of Lemmon and SWAN, another truly remarkable visitor is making its way through our solar system this month: Comet 3I/ATLAS. This is only the third such interstellar visitor ever confirmed, meaning it originated from an entirely different star system beyond our own. The rarity of such an object makes its observation a profound scientific event.
This enigmatic comet was recently photographed by a spacecraft orbiting Mars, a testament to humanity’s ongoing efforts to explore and understand the cosmos. Comet 3I/ATLAS is projected to make its closest approach to the Sun on or around October 30. The study of interstellar comets like 3I/ATLAS provides invaluable insights into the composition of other planetary systems and the conditions of star formation across the galaxy.
A Once-in-a-Lifetime Opportunity
The convergence of these three celestial objects offers an unparalleled opportunity for anyone with an interest in the night sky. Comet Lemmon is not expected to reappear for another 1,300 years, and Comet SWAN for another 650 to 700 years. This truly makes their current appearance a once-in-a-lifetime event for most observers.
For those eager to pinpoint their exact locations, a multitude of free planetarium software and mobile applications, such as Stellarium or KStars, can show the comets’ real-time positions relative to stars. Even a phone camera, with settings adjusted for longer exposure, can be used to attempt capturing these faint objects. Take this rare chance to connect with the cosmos and witness some of the most ancient and distant objects our universe has to offer.
