A celestial spectacle is unfolding in the night sky this month as multiple comets, distinguished by their vibrant green comas, make their closest approaches to Earth. These icy visitors from the outer solar system, primarily Comet C/2025 A6 (Lemmon) and Comet C/2025 R2 (SWAN), are offering a rare viewing opportunity for astronomers and stargazers alike, with at least one potentially becoming visible to the naked eye under dark sky conditions. The convergence of these comets provides a valuable window into the chemical makeup of these ancient bodies and the dynamics of our solar system.

The comets have been rapidly brightening as they near the sun, a process that causes their frozen materials to vaporize and glow. Comet Lemmon, in particular, is being hailed as the brightest comet of the year, reaching its peak visibility around October 21 when it passed just 55 million miles from Earth. Its companion, Comet SWAN, made an even closer pass at 24 million miles on October 20. For observers in the Northern Hemisphere, these events present a limited-time opportunity to witness these transient objects, which journey from the distant reaches of space, put on a brief, luminous show, and then recede into the darkness, with Comet Lemmon not expected to return for more than a thousand years.

The Featured Celestial Visitors

This month’s astronomical display is headlined by two prominent green comets: C/2025 A6 (Lemmon) and C/2025 R2 (SWAN). A third comet, C/2025 K1 (ATLAS), is also present, though it is currently fainter. Comet Lemmon has garnered the most attention due to its significant and rapid increase in brightness. Discovered on January 3, 2025, by the Mount Lemmon Survey near Tucson, Arizona, it was not initially expected to become a major celestial event. However, it began to intensify dramatically in August, exceeding initial brightness predictions.

Comet SWAN was confirmed in September 2025 from images captured by the Solar and Heliospheric Observatory (SOHO) spacecraft’s SWAN camera. It had already passed its closest point to the sun a day before its discovery, having gone undetected in the sun’s glare. While it is not expected to brighten further, its close proximity to Earth on October 20 made it a notable target for observers. Together, these comets represent the first binocular-visible comets since the appearance of Comet Tsuchinshan-ATLAS in October 2024, providing a welcome sight for comet watchers.

The Science of the Green Glow

The characteristic emerald hue observed in these comets is not merely a beautiful anomaly but a direct result of their chemical composition and interaction with sunlight. The green color originates from the presence of diatomic carbon (C₂), a simple molecule composed of two carbon atoms bonded together, within the comet’s coma—the nebulous envelope of gas and dust surrounding its nucleus. As a comet approaches the sun, the increasing solar radiation heats its icy body, causing frozen gases to sublimate, or turn directly from a solid into a gas.

This process releases various molecules, including diatomic carbon. When these C₂ molecules are struck by the sun’s ultraviolet radiation, they absorb the energy and are then broken apart in a process called photodissociation. As they break down, they emit light, specifically in the green part of the visible spectrum. This gives the coma its distinct green glow. The amount of diatomic carbon varies between comets, which is why the intensity of the color can differ. Interestingly, this green color is almost always confined to the head of the comet; the tail, which is composed of dust and ionized gas pushed away by solar wind and radiation pressure, does not typically exhibit the same hue.

A Guide to Observation

Optimal Viewing Times and Locations

Catching a glimpse of these celestial travelers requires some planning. Both Comet Lemmon and Comet SWAN are best observed from the Northern Hemisphere. The key to a successful sighting is to find a location with minimal light pollution, as far away from city lights as possible. Observations should begin approximately 90 minutes after sunset, once the sky has sufficiently darkened. During mid-October, Comet Lemmon appears low on the northwestern horizon, while Comet SWAN can be found in the southwestern sky, below the bright star Altair in the Summer Triangle asterism.

Tools for Stargazing

While Comet Lemmon may approach naked-eye visibility around Halloween under perfect dark-sky conditions, using binoculars or a small telescope is highly recommended for a much better viewing experience. These instruments will reveal the comet’s glowing coma and possibly a faint tail, details that are difficult to discern with the unaided eye. For precise positioning, stargazing apps such as Sky Guide, Stellarium, and SkySafari are invaluable tools for locating the comets in the night sky. Aspiring astrophotographers can also capture the comets, though it requires specialized camera equipment and techniques.

Trajectory and Future Visibility

The paths of these comets through the inner solar system are fleeting. Comet Lemmon made its closest approach to Earth on October 21, passing at a safe distance of 55 million miles. It is scheduled to reach perihelion, its closest point to the sun, on November 6. Following this solar encounter, it will begin its long journey back out into the depths of space. Its highly elliptical orbit means it will not return to the inner solar system for at least 1,300 years, making this the only opportunity for anyone alive today to see it.

Comet SWAN reached its closest point to our planet on October 20, coming within 24 million miles. Having already passed its perihelion before its discovery, it is expected to grow dimmer as it moves away from both the sun and Earth. Viewers hoping to see these comets should take advantage of the remaining clear nights in October and early November before they fade from view. Weather, of course, plays a critical role, as cloud cover can easily obstruct the view of these relatively faint objects.

Concurrent Celestial Events

The appearance of the green comets coincides with other astronomical events, offering a richer experience for skywatchers. The Orionid meteor shower is also active, peaking on the night of October 21. While not an exceptionally strong shower, producing about 20 meteors per hour under ideal conditions, it adds another layer of interest to a night of observation. Additionally, the planet Saturn is visible as a bright point of light in the southeastern sky during the evening. With a good pair of binoculars, it may even be possible to resolve its famous rings, providing a planetary target to observe alongside the transient beauty of the comets.