Comet C/2025 A6 (Lemmon) is approaching Earth  — will it become visible to the naked eye?

In the coming weeks, skywatchers will have an opportunity to view a new comet that is currently inbound on its way toward the sun. This new visitor to the inner solar system is expected to brighten, perhaps becoming a fairly easy object to see in small telescopes or good binoculars.

And there’s hope that it may even become bright enough to glimpse with the naked eye under dark non-light polluted skies (for those who have access to such viewing sites).

The new comet is catalogued as C/2025 A6 (Lemmon). It was discovered by the Mount Lemmon Survey, utilizing a 60-inch (1.52-meter) Cassegrain reflecting telescope, equipped with a 10560 x 10560-pixel camera. The telescope is located at the Mount Lemmon Observatory and is operated by the University of Arizona’s Steward Observatory located in the Santa Catalina Mountains to the northeast of the city of Tucson.

The Mount Lemmon Survey (MLS) continuously scans the sky looking for near-Earth objects  — asteroids or comets  — whose orbits bring them within a certain distance of the sun and potentially Earth’s orbit, meaning they can pass close to our planet. The vast majority of MLS’s discoveries have been of asteroids (to date more than 50,000), but every so often the survey will catch sight of a new comet, as is the case with C/2025 A6.

Originally, when first photographed by astronomer David Fuls on Jan. 3, it was believed that the MLS had found yet another asteroid. It appeared as nothing more than a tiny starlike speck of light with a magnitude of +21.5; that’s one million times dimmer than the faintest star on the threshold of naked-eye visibility. Follow-up images showed the object to be, in fact, a comet, and an even fainter pre-discovery image was located dating back to November 2024.

An orbit based on 117 observed positions between Nov. 12, 2024 and Aug. 14 has been calculated by Syuichi Nakano of the Central Bureau for Astronomical Telegrams. Nakano found that the comet will pass perihelion — its closest point to the sun — on Nov. 8 at a distance of 49.25 million miles (79.25 million km). It will come closest to Earth (its perigee) on Oct. 20 when it will be 55.41 million miles (89.16 million km) away.

Welcome back!

Comet Lemmon has apparently been here before. Mr. Nakano’s orbital computations indicate that at the far end of its orbit (aphelion), it is situated as far as 22.6 billion miles (36.3 billion km) from the sun. Its orbital period is estimated at approximately 1,350 years. However, back on April 16, the comet passed within 216.6 million miles (348.5 million km) of Jupiter. That giant planet’s gravitational field served to sap some of comet Lemmon’s orbital energy and as such will shorten its period by some 200 years.

We know that comets are composed primarily of frozen gases that are heated as they approach the sun and made to glow by the sun’s light. We call this cloud of gas the head or coma.

As the gases warm and expand, particles of dust that were embedded in the comet’s nucleus are also released into space. The solar wind blows this material out into an appendage we call the tail. To observers of antiquity, comets resembled a stellar head trailed by long hair, so they called comets, “hairy stars.”

Bright among “common” comets

Comets can be broken down into two basic categories:

Bright comets — the kind that can excite those of us without binoculars or telescopes — appear on average perhaps two or three times every 15 to 20 years. The last such comet to do that was this past January with comet C/2024 G3 (ATLAS), nicknamed the “Great Comet of 2025.”

Then there are the common comets, of which most are only visible either with good binoculars or a telescope. The vast majority of comets fall into this category, but comet Lemmon may end up ranking as rather bright so far as most common comets go, since for a short while it may hover right around the cusp of naked-eye visibility (for those fortunate enough to be blessed with dark, non-light polluted night skies).

How bright?

A number of different predictions have been made regarding the brightness of Comet Lemmon as it passes closest to Earth during the third week of October. To date, the most optimistic brightness forecasts are those issued by Japanese comet expert Seiichi Yoshida and Dutch comet expert Gideon Van Buitenen. Both are projecting that the comet will peak somewhere between magnitude +4 and +5, meaning it might become faintly visible with the unaided eye sometime during early October.

Other forecasts, however, are far more conservative. Daniel W.E. Green at the Central Bureau for Astronomical Telegrams, on circular #5594, issued on Aug. 20, “cautiously” suggests a peak magnitude of +7.3 on Oct. 27. That would make it too faint to be seen with bare eyes alone, but certainly within reach of good binoculars or small telescopes.

Where to find it and viewing prospects

Right now, comet Lemmon is a predawn object, located in the faint zodiacal constellation of Cancer. It will cross over into the similarly vague and dim constellation of Lynx on Sept. 9 at a declination near +34°. It will still be quite faint, probably no brighter than magnitude 9 or 10. But from then onward, its brightness and movement against the background stars will progressively increase eastward as it approaches Earth.

For most, the first really good opportunity to try and make a definitive sighting will come on the morning of Oct. 6, when it will move into the southern boundaries of the Great Bear, Ursa Major. On this morning, Comet Lemmon will be passing less than 0.3 degrees to the upper left of the third-magnitude star Tania Australis, a member of the three pairs of stars that mark “The Three Leaps of the Gazelle.” Tania is part of the pair marking the second or middle Gazelle leap. It rises in the north-northeast shortly before 1 a.m. local daylight time and by the break of dawn will be one-third up in the east-northeast. The comet will be in the same field of view and could be as bright as magnitude +6 or +7, making it an easy target with binoculars.

Beginning Oct. 12, the comet will begin to be available to evening skywatchers, low in the northwest about 90 minutes after sunset.

On Oct. 16, the comet will be positioned about 1 degree to the upper left of the third magnitude star Cor Caroli in the constellation of Canes Venatici, the Hunting Dogs. At that time, comet Lemmon will be racing 4 degrees per day, so even a few minutes of watching with a telescope should reveal its shift relative to field stars.

On Oct. 22, about 7:30 p.m. local daylight time, look very low above the west-northwest horizon for the brilliant orange star, Arcturus in Boötes the Herdsman. On that same evening, comet Lemmon will be situated 10 degrees above this star (your clenched fist held at arm’s length also measures 10 degrees in width). The comet will also be positioned about 2 degrees to the left of the second magnitude star Izar, also in Boötes. A quick check of Izar with binoculars should also reveal the comet.

Keep your expectations low

But as compelling as this all may sound; we now must temper any excitement by providing a very important disclaimer.

By early-to-mid October, many people with binoculars and small telescopes will no doubt attempt to follow the path of Comet Lemmon across the night sky. But seeing it will strongly depend on your observing site. From locations that are plagued by light pollution, sighting this comet may prove to be rather difficult. Remember, you’re not looking for a sharp star-like object, but rather something which is spreading its light out over a comparatively large area.

In fact, under a completely dark sky, free of light pollution, perhaps the best instruments for locating the comet will be your own two eyes, especially if you use averted vision.

Recent photographs have shown the comet displaying a distinct greenish color, likely due to a molecule made from two carbon atoms bonded together, called dicarbon. This unusual chemical process is confined chiefly around the comet’s head, not its tail. Comets generally throw off two types of tails; tails composed primarily of gas, and tails composed primarily of dust. Dust tails are far brighter and more spectacular to the eye than gas tails, because dust is a very effective reflector of sunlight.

Comet Lemmon’s tail, however, appears to be primarily composed of gas. Such tails appear much fainter and glow with a bluish hue. The gas is activated by the ultraviolet rays of the sun, making the tail glow in much the same way that black light causes phosphorescent paint to light up.

So, most who ultimately locate Comet Lemmon in their binoculars or telescopes will typically describe it as a nearly circular cloud, appearing noticeably brighter and more condensed near the center. Some might also detect a faint tail appearing as a bit of an elongation of the comet’s coma, but hardly the kind of tail or appendage exhibited by other larger and brighter comets.

A final point to consider: comets are notoriously unpredictable; we can only guess how they ultimately will appear in our sky. It’s not completely out of the question that comet Lemmon could surprise us and become unexpectedly bright.

Then again, it might fail to brighten much at all, perhaps living up to the homonym of its name (a lemon). We’ll post any updates if needed here on Space.com. So, stay tuned!

Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.