Double cosmic treat: 2 dazzling comets set to swing by Earth this month — don’t miss the show

Sky-watchers get ready to be dazzled this month as two comets make their closest passes to Earth, offering a rare celestial show. 

Composed of ice, frozen gases, and rock, comets glow spectacularly when the sun’s heat vaporizes their icy surfaces, releasing gas and dust that form the iconic glowing tails.

The first comet, C/2025 A6 Lemmon, was spotted on January 3, while the second, C/2025 R2 SWAN, was detected much more recently on September 10 during its close approach to the sun.

This information was shared by Qicheng Zhang, a postdoctoral fellow focusing on small body astronomy at Lowell Observatory in Flagstaff, Arizona.

For those eager to catch a glimpse, now is a perfect time to observe these visitors using binoculars or telescopes.

Their long, oval-shaped orbits mean they’ll be near Earth only briefly. Comet SWAN won’t return for another 650 to 700 years, while Lemmon won’t be back for an astounding 1,300 years, according to Carrie Holt, postdoctoral fellow and astronomer at Las Cumbres Observatory, a global network of observatories.

“SWAN is best seen in early evening shortly after sunset,” explained Quanzhi Ye, associate research scientist at the University of Maryland’s astronomy department, in an email.

“Lemmon appears now just before sunrise but will soon be visible in the evenings.” Because both comets appear near the sun from Earth’s perspective, there’s only a small daily window to spot them.

On October 6, comet SWAN was captured in an image by the Virtual Telescope Project, showing its characteristic green gas and trailing tail.

These comets act like time capsules from the solar system’s outskirts.

SWAN will pass closest to Earth on October 20, coming within 24 million miles (38.6 million km), while Lemmon will follow on October 21 at 55 million miles (88.5 million km), according to University of Maryland’s research scientist Quanzhi Ye.

Both show similar greenish hues surrounded by long dust tails.

Astronomers explain that the green hues observed in the comas (the fuzzy gaseous envelopes around their nuclei) of comets C/2025 R2 (SWAN) and C/2025 A6 (Lemmon) are caused by the presence of diatomic carbon (C₂) molecules.

These molecules are released from the comets' icy nuclei as they heat up while approaching the Sun.

Astronomers are especially interested in these long-period comets, whose orbits span 200 years or more.

They likely originated near giant planets like Jupiter before gravitational nudges sent them to the distant Oort Cloud — a cold, spherical shell of icy bodies far beyond Neptune. There, they’ve remained frozen for billions of years.

These long-period comets contain organic compounds including carbon-rich ices; as solar radiation vaporizes the ices, C₂ forms in the coma.

“When one gets nudged back inward, we see materials that have hardly changed since the solar system’s birth,” Holt wrote.

“As their ices sublimate, we glimpse the original components that helped form planets and can better understand how planetary systems develop.”

Visibility varies by hemisphere: Lemmon is currently more visible for Northern Hemisphere observers, whereas SWAN favours the Southern Hemisphere but is now becoming accessible to northern sky-watchers as well.

Lemmon is expected to brighten through early November, while SWAN will fade soon.

Starting in November, Lemmon will disappear behind the sun and reappear only for Southern Hemisphere viewers, explained Ye.

Comet SWAN was discovered through images taken by the Solar Wind ANisotropies (SWAN) instrument on the Solar and Heliospheric Observatory spacecraft, a joint NASA-European Space Agency mission to study the sun.

In contrast, Lemmon was found by the Mount Lemmon Observatory in Arizona during the NASA-funded Catalina Sky Survey, which hunts for near-Earth objects like potentially hazardous asteroids.

Carson Fuls, director of the Catalina Sky Survey and operator of the telescope that discovered Lemmon, recalled it wasn’t obvious at first that it was a comet.

“It was too faint and distant initially to show the dramatic tails visible now,” he told CNN.

Fuls believes Lemmon could be the best comet of the year for casual observers because it may become visible with just binoculars or even the naked eye from a dark location — a rare treat in backyard astronomy.