Rare double comet event to light up the October night sky

It’s time for stargazers to turn their telescopes toward the night sky. October is setting the stage for two cosmic visitors.

Two newly discovered comets, C/2025 A6 Lemmon and C/2025 R2 SWAN, will sweep across the sky this month. 

C/2025 R2 (SWAN) will make its closest approach on October 20, coming within 24 million miles of Earth. Just a day later, on October 21, C/2025 A6 (Lemmon) will sweep past at about 55 million miles.

As they draw closer to Earth, the comets are expected to glow brightly enough to be seen with the naked eye.

Skywatchers in the northern hemisphere are in for a rare double treat. Comet Lemmon has already been lighting up northern skies for weeks.

Comet SWAN, once visible only south of the equator, is beginning to appear in northern skies too.

A once-in-a-millennium opportunity 

An astronomical observatory sits at 9,100 feet atop Mount Lemmon, operated by the University of Arizona. During one of the routine surveys, a team of researchers spotted something new in the sky. 

The celestial object appeared so dim that astronomers initially mistook it for an asteroid. When they examined the images later, they spotted a faint tail and a coma.  

This led them to confirm that what they had spotted was actually a comet, as asteroids do not have comas. 

Comets that are visible from Earth

Comets are small, icy bodies made up of rock, frozen gases, and organic molecules orbiting around the Sun. As they approach the Sun, their surface starts heating up, turning all the solids into gases. 

As a result, a thin, glowing cloud of gas and dust forms around the comet’s nucleus, which is called a coma. The coma can expand thousands of kilometers across and is often what makes a comet visible from Earth.

Without the coma, astronomers might have missed a once-in-a-millennium event. Comet Lemmon hasn’t visited the inner solar system in about 1,350 years.

A green visitor from far away  

Comet C/2025 R2 (SWAN) was discovered using the Solar Wind Anisotropies instrument (SWAN) aboard NASA’s solar observation spacecraft. Amateur astronomer Vladimir Bezugly spotted it on September 11, 2025. 

The discovery came just one day before the comet reached perihelion, its closest point to the Sun. Until then, it had remained hidden by the Sun’s intense glare.

Experts estimate Comet SWAN’s orbital period around the Sun to be approximately 20,000 years. It likely hails from the distant Oort Cloud, a vast reservoir of icy bodies far beyond Pluto.

The Oort Cloud might be home to billions of icy bodies preserved since the solar system’s earliest days. Occasionally, a disturbance sends them sunward, transforming them into comets. 

Both Comet SWAN and Lemmon appear to have a greenish tail and coma. This vivid coloration arises when solar radiation splits apart diatomic carbon molecules around the comet’s coma. 

Spotting the comets in the sky 

Between the two-tailed visitors, Comet Lemmon is the brightest. The comet was relatively dim when astronomers first spotted it.

In August, Lemmon began to gain brightness. On October 21, as it makes its closest approach to Earth, we can get a glimpse of the comet without binoculars or telescopes. 

By mid-October, the Comet Lemmon will be in the evening sky near bright stars like Arcturus in Boötes. The best time to see it is about 90 minutes after sunset, looking toward the northwest.

Comet SWAN is also visible in the evening sky, but in the southwest direction. Since it is dimmer, binoculars or a small telescope would be beneficial. 

As the month of October continues, SWAN will rise higher and become easier to spot from northern latitudes. The best chance to see it occurs around October 20–21, when it’s closest to the Earth.

In hopes of a darker sky

For the best viewing experience, experts suggest finding a dark site away from city lights. Moving away from artificial light will greatly improve your chances of seeing the comet with the naked eye. 

Unfortunately, artificial lights have turned the darkness of the night sky into a luxury. The atlas of artificial night skies shows that more than 80% of the world’s population lives under light-polluted skies. 

To put this in perspective, about one-third of the human population has never experienced the sight of the Milky Way in the night sky.

Don’t worry if city lights rob you of this experience. You can still enjoy the spectacle online on October 20, 2025, starting at 17:30 Coordinated Universal Time (UTC). 

The Virtual Telescope Project 2.0 will livestream both comets through telescopes under the dark skies of Italy.

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