Last Updated on October 15, 2025 by Practical Astrophotography Staff
As autumn evenings deepen and the night sky clears of summer's haze, October 2025 offers a rare astronomical treat: not one, but two bright comets gracing our view. Comet C/2025 A6 (Lemmon) and Comet C/2025 R2 (SWAN) are hurtling through the inner Solar System, their icy nuclei vaporizing into glowing comas and ethereal tails under the Sun's heat. These "dirty snowballs"—remnants from the Solar System's formation 4.6 billion years ago—represent once-in-a-millennium opportunities. Comet Lemmon won't return for about 1,150 years, while SWAN's orbit spans over 22,000 years. With the Orionid meteor shower peaking around the same time and a new moon on October 21 ensuring dark skies, this could be the skywatcher's highlight of the year. Here's everything you need to know about these cosmic wanderers and how to spot them before they fade into the dawn.
Comet Lemmon: The Steady Brightener from Mount Lemmon
Discovered on January 3, 2025, by the Mount Lemmon Survey—a NASA-funded program hunting near-Earth objects from Arizona's Santa Catalina Mountains—Comet C/2025 A6 (Lemmon) started as a faint speck at magnitude 21.5, visible only to large telescopes 4.5 AU from the Sun. Follow-up observations quickly revealed its cometary nature: a compact coma about 2.2 arcseconds across and a short tail. Precovery images from Pan-STARRS telescopes pushed its detection back to November 12, 2024.This non-periodic comet follows a highly elliptical orbit, having last visited the inner Solar System around 675 AD during Europe's early Middle Ages—perhaps glimpsed by a forgotten monk. Inbound, its orbital period was roughly 1,350 years; after slingshotting around the Sun, it'll shorten to about 1,150 years. Perihelion, its closest solar approach, arrives on November 8, 2025, at 0.53 AU (about 79 million km), roughly Mercury's average distance from the Sun. Earth gets a slightly closer shave on October 21, when Lemmon passes 0.60 AU (90 million km) away—over 55 million miles, safe but spectacular.
What began as a predicted peak of magnitude 10 has exploded into something far brighter. As of October 13, it's at magnitude 5.4, with an ion tail stretching 3 degrees (six full Moons long) and a dust tail emerging. Optimistic forecasts peg its maximum at magnitude 3.5 to 4.4—bright enough for naked-eye visibility under dark skies, potentially outshining most stars in its path through Leo Minor and Ursa Major. Early images show a lime-green glow from diatomic carbon in its coma, evoking a cosmic absinthe hue. Though not a "great comet" like NEOWISE, it's brighter than 90% of visitors and could develop a dramatic tail if outbursts continue.
Comet SWAN: The Unexpected Green Streak
If Lemmon is the reliable veteran, SWAN is the wild card. Officially Comet C/2025 R2 (SWAN), it was serendipitously spotted on September 11, 2025, by Ukrainian amateur astronomer Vladimir Bezugly sifting through images from the Solar Wind Anisotropies (SWAN) instrument aboard NASA's Solar and Heliospheric Observatory (SOHO). This ultraviolet camera, designed to study solar wind, has a knack for comet-hunting— this marks the 20th discovery from its feeds. Initially dubbed SWAN25B, it earned its IAU name on September 15.
At discovery, SWAN was already a showstopper: magnitude 7.4 with a 2-degree tail (four full Moons), nestled in Virgo near Spica. It had just passed perihelion on September 12 at 0.5 AU from the Sun, emerging from solar glare with a greenish coma and an impressive ion tail up to 2.8 degrees long—about five full Moons. This long-period interloper boasts an orbital eccentricity of 0.996, circling the Sun every 22,554 years (though some estimates suggest 1,400 years; the dynamically old path hints it's not fresh from the Oort Cloud). Earthward bound, it zips closest on October 20 at just 0.26 AU (39 million km; 24 million miles)—closer than Venus at inferior conjunction.
As of mid-October, SWAN shines at magnitude 6.0, observable in 50mm binoculars with 50-degree solar elongation. Its green hue stems from the same carbon compounds as Lemmon's, but its post-perihelion fade and potential outbursts make it unpredictable: it could brighten to naked-eye limits (magnitude 4.5 or better) or fizzle. A faint meteor shower might sparkle around October 5-6 as Earth crosses its path, though the stream's dilution over millennia likely means just a few shooting stars near the Sun. Ground images from September 12 show a condensed core and striking tail, confirming it's one of SWAN's brightest catches.
A Double Comet Show: Timing and Paths
Remarkably, both comets reach Earth flyby within a day—SWAN on the 20th, Lemmon on the 21st—positioning them for simultaneous viewing amid the Orionids' peak (up to 20 meteors/hour from Halley's comet debris). Northern Hemisphere observers get prime seats for Lemmon early in the month, while southern viewers favor SWAN initially. By late October, both shift: Lemmon to evening skies in Ophiuchus, SWAN fading southwest in Sagittarius.
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Comet
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Peak Brightness (Est.)
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Best Viewing Dates
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Constellation Path (Mid-Oct)
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Earth Distance (Closest)
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C/2025 A6 (Lemmon)
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Mag 3.5-4.4 (late Oct)
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Oct 15-Nov 2 (N. Hem.); mid-Nov (S. Hem.)
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Ursa Major → Canes Venatici → Ophiuchus
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0.60 AU (Oct 21)
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C/2025 R2 (SWAN)
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Mag 6.0 (possibly brighter)
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Oct 13-31 (S. Hem. first, then N.)
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Virgo → Libra → Sagittarius
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0.26 AU (Oct 20)
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How to See Them: Your October Observing Guide
Comets aren't pinpoint stars—they're fuzzy with extended tails, so wide-field optics shine. Patience pays: let your eyes dark-adapt 20-30 minutes. Apps like Star Walk 2, Stellarium, or Sky Tonight pinpoint exact positions; search "comet finder charts" for tailored maps.
General Tips
- Location: Head to Bortle Class 4 skies or darker—rural spots far from city glow. Clear horizons matter: west for SWAN, north/northwest for Lemmon.
- Timing: Evening for SWAN (post-sunset), predawn for Lemmon early-month. October 20-21: New moon means velvet-black skies.
- Gear: Naked eye if lucky (mag <5); 7x50 binoculars for tails; small telescopes (e.g., 70-100mm refractors) for details. Smart telescopes like ZWO Seestar S50 automate tracking.
- Photography: Use a DSLR/mirrorless on tripod with 10-30s exposures at ISO 800-1600, wide-angle lens (24-50mm). Stack images in software like DeepSkyStacker for coma/tail pops.
- Now (Oct 15): Magnitude 5.4, low in Ursa Major. Predawn: Face northeast; it's 20° below Big Dipper's handle (two fists at arm's length). Binoculars reveal the tail.
- Oct 16: Skims 1° from Cor Caroli in Canes Venatici—use as a guidepost.
- Oct 20-21: Brightest at closest approach; elongation 42° post-sunset. Northwest after dusk, near Big Dipper. Rate: 4°/day southeast.
- Halloween (Oct 31): Evening west in Ophiuchus near Marfik; fading but tailed.
- Pro Tip: Northern latitudes (>48°N) see it circumpolar mid-month. Crosses equator Nov 2.
- Now (Oct 15): Magnitude 6.0 in Libra, southwest post-sunset. Low horizon; southern viewers get higher altitude.
- Oct 20: Peak visibility at perigee; near Zubenelgenubi (Alpha Librae). Elongation 50°—binoculars sweep Virgo-Libra border.
- Late Oct: Dips into Sagittarius; try smartphone long-exposure (3-5s) for green tail.
- Pro Tip: Watch for outbursts; if it surges, naked-eye potential under rural skies. Possible weak meteors Oct 5 near horizon.
Why This Matters: Comets as Time Capsules
These comets aren't just pretty lights—they're pristine fossils, delivering water and organics that may have seeded life on Earth. As they outgas, they sculpt tails: ion (blue, solar-wind driven) and dust (white, curving). Their unpredictability thrills: Lemmon's sixfold brightening defies models, while SWAN's post-perihelion path risks disintegration. In a year of interstellar visitors like 3I/ATLAS, they remind us the sky is alive.
Grab your binoculars this October—these icy envoys won't wait. Under crisp fall stars, with meteors streaking and tails unfurling, you'll witness history. Clear skies!
