A spectacular cosmic event unfolds this October as the bright green comet C/2025 A6 (Lemmon) nears Earth, offering skywatchers a once-in-a-decade celestial display visible even to the naked eye.
A New Visitor from the Outer Solar System
The comet was discovered on January 3, 2025, by astronomers at the Mount Lemmon Observatory in Arizona using a 60-inch telescope. Since then, it has been steadily brightening as it approaches the Sun.
Its closest approach to Earth will occur on October 21, at a distance of about 89 million kilometers, while it will reach perihelion — the nearest point to the Sun — on November 8.
How the Comet’s Brightness Is Changing
Since its discovery, Comet Lemmon’s magnitude has increased dramatically — from +21.5 in January to about +7 by late September. If this trend continues, it could soon be visible without telescopes.
“If the brightness continues to rise, the comet may be visible to the naked eye under dark skies,” noted specialists at Mount Lemmon Observatory.
For reference, the human eye can detect celestial objects with a brightness of up to +6.5 magnitude. However, astronomers caution that cometary behavior is notoriously unpredictable — sudden brightening can be brief, followed by fading.
Where to Find the Comet
In early October, Comet Lemmon can be observed in the pre-dawn sky between the constellations Ursa Major and Leo. Even a pair of binoculars can reveal it as a faint, fuzzy patch of light.
The best viewing conditions are expected in mid-October when the Moon’s glare will not interfere with visibility.
A Cosmic Photoshoot
As it draws closer to the Sun, Lemmon reveals its true colors — a turquoise coma and twin tails made of dust and ionized gas.
Astronomer Aleix Roig captured the comet on September 22 over Catalonia, showing an emerald halo and a thin blue tail. Dan Bartlett of California produced a long-exposure series revealing how the tail changes shape under solar wind pressure. Chris Schur of Arizona recorded a striking image showing two distinct tails — a golden dust plume and a long blue gas stream diverging like a fan.
“The comet feels alive — its tail shimmers and shifts with the slightest changes in the solar wind,” wrote Schur in his comments.
Key Characteristics of Comet Lemmon
| Parameter | Value |
|---|---|
| Full name | C/2025 A6 (Lemmon) |
| Discovered | January 3, 2025 |
| Observatory | Mount Lemmon, Arizona (USA) |
| Closest distance to Earth | 89 million km |
| Perihelion date | November 8, 2025 |
| Orbital period | 1,351 years |
| Predicted maximum brightness | +7 to +5 magnitude |
| Coma color | Green — due to cyanogen and diatomic carbon |
A 1,351-Year Journey
After its perihelion passage, Lemmon will leave the inner Solar System, embarking on a long journey that will bring it back only after more than a millennium. Its orbit extends nearly 243 times farther than the Earth–Sun distance.
This makes it a long-period comet — a visitor from the distant reaches of the Solar System, possibly originating in the Oort Cloud.
Interesting Facts
- The green color is caused by the fluorescence of C₂ and CN molecules under ultraviolet sunlight.
- Despite its bright appearance, the comet’s nucleus is quite small — estimated at only 1–2 kilometers in diameter.
- Mount Lemmon Observatory has discovered numerous comets over the years, but C/2025 A6 may become one of the most spectacular in recent history.
What Astronomers Say
“Comets like this remind us of the dynamic nature of our Solar System. They carry ancient material from which planets once formed, offering us a glimpse into cosmic history,” said astrophysicist Dan Bartlett.
Comet Lemmon is not just an astronomical event — it is a rare opportunity to witness a messenger from the depths of space. In the coming weeks, it will reach its brightest phase, and if predictions hold, the October sky will deliver one of the most breathtaking spectacles of the decade.
So look up — you might just see a glowing emerald streak cutting through the darkness between the stars.
