How to see two bright, once-in-a-lifetime comets with your naked eye
Two rare bright comets are visiting our neighborhood this month, making for once-in-a-lifetime viewing.
The comets, C/2025 A6 (Lemmon) and C/2025 R2 (SWAN), are already showing off their green tails to cameras and telescopes. But the best chances to see them with the naked eye from the northern hemisphere should be in the second half of the month.
“What stands out about these two is their potential brightness,” said Erika Gibb, an astrobiologist at the University of Missouri-St. Louis. “For sky-gazers, it is relatively uncommon for comets to be observable, and they really are beautiful objects.”
It helps that both comets are expected to be closest to Earth around the time of the Oct. 21 new moon, which should help them stand out in the night sky.
Comet Lemmon should be most visible between Oct. 12 and Nov. 2, with peak brightness around Halloween, according to Star Walk. The best opportunities to see Comet SWAN may be between Oct. 18 and Oct. 21.
Their appearances coincide with the Orionids meteor shower, which peaks Oct. 20-21, making for an extra spectacular display.
Comets are objects made of ice, dust and rock left over from the formation of the solar system. As the sun’s gravity pulls them close, solar heat defrosts them and produces an enveloping cloud and tail.
When seen at high magnification, two tails are visible: a broad white tail of dust and a narrower, colored tail of microparticles ionized by ultraviolet radiation. The color of that second tail depends on the composition of the comet — carbon vapor produces a green glow.
Both of the current visitors come from the cold outer region of the solar system known as the Oort Cloud and take a long time to complete their orbit around the sun. Comet Lemmon won’t be visible again from Earth for about another 1,300 years, while Comet SWAN won’t be back for about 700 years.
Another comet, C/2025 K1 Atlas, is also passing by this month but will probably be too faint to see without a telescope. NASA instruments are also watching Comet 3I/ATLAS, only the third confirmed object to visit from outside our solar system, although it won’t provide great viewing for the public.
It’s a special treat for comet fans and scientists that two comets are visible in the dark skies, Gibb said. Many comets are around during the daytime, but can’t be seen because of the overwhelming light from the sun.
For best viewing, get away from city lights. Here are some more tips for catching a glimpse of Comet Lemmon and Comet SWAN.
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How to see Comet Lemmon
Comet Lemmon was discovered in January via a telescope in Mount Lemmon, Arizona. But it “only really started to become interesting when it reappeared from behind the sun brighter than expected,” said Nick James of the British Astronomical Association.
Since last month, people have been able to spot the comet — and its active tail — using binoculars and telescopes. In the United States, amateur astronomers and photographers have reported seeing it from viewing points in Virginia and Texas. But viewing should get even better in the coming weeks.
Initially, the best viewing window will be before sunrise, but as the month goes on, that will shift to after sunset.
By Oct. 16, the comet will be in the vicinity of Cor Caroli, the brightest star in the Canes Venatici constellation (often seen as the shape of hunting dogs), according to Star Walk. By the new moon, Oct. 21, it will be in the kite-shaped Boötes constellation and north of Arcturus, the brightest star in the northern sky. By Halloween, it will be visible above the western horizon after dusk in the Ophiuchus constellation (sometimes referred to as the serpent bearer).
Websites or apps like Stellarium can help find the comet from your location.
Although comet brightness can be hard to predict, scientists expect Lemmon to be the brightest of this year.
“From recent observations, I’m pretty confident that this will be a good comet,” James said.
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How to see Comet SWAN
Comet SWAN was discovered last month by a Ukrainian amateur astronomer looking at solar data captured by the Solar Wind Anisotropies (SWAN) instrument.
At first it favored sky-gazers in the southern hemisphere, but it was seen from Mexico in the second week of September and will be best viewed from the northern hemisphere this month.
The window for Comet SWAN is at dusk. You’ll want an unobstructed view of the southwestern horizon.
If you manage to spot the comet this week, it will appear within the Ophiuchus constellation, passing near the star Sabik on Oct. 14. Around the new moon, and potentially prime viewing, it can be found near Sagittarius. And by Oct. 24, you should be able to see it to the north of the Capricorn constellation.
There’s a chance of further brightening during these next weeks, as well as a chance that people may end up needing binoculars.
By early November, according to StarWalk, Comet SWAN should fade from view.