Two Comets Are Moving Into Your Night Skies in October: How to Watch
The comets A6 (Lemmon) and R2 (SWAN) are visitors from the chilly fringes of our solar system, and could even be visible at the same time.
If you like comets, this month is shaping up to be a good season. An assortment of the objects are passing through our cosmic neighborhood, trailed by wisps of gas and dust. This month, skywatchers in the Northern Hemisphere will have the opportunity to observe not one but two once-in-a-lifetime comets in the fall skies.
The celestial visitors, known to scientists as C/2025 A6 (Lemmon) and C/2025 R2 (SWAN), traveled here from the very edges of the solar system where the sun appears as a pinprick of light in the darkness.
A6 (Lemmon) was spotted in January by the Mount Lemmon Survey, which catalogs near-Earth objects from a mountaintop observatory in Arizona.
R2 (SWAN) turned up in early September and is more of an unexpected visitor. It was discovered by Vladimir Bezugly, an amateur astronomer in Ukraine. He found the object in publicly available images from SWAN, or the Solar Wind Anisotropies instrument of the Solar and Heliospheric Observatory, a spacecraft stationed nearly a million miles from Earth.
The comet “caught everybody by surprise,” said Quanzhi Ye, an astronomer at the University of Maryland. That’s because the comet arrived from the direction of the sun, a trajectory that kept the object hidden in the glow of our star during its approach, out of view from sky-scanning telescopes around the world.
What is a comet?
Comets are ancient chunks of ice and rock, leftovers from the creation of the solar system. During an encounter with the sun, a comet gets warmed up. Some ice sublimates into gas, which streams away into space, dragging along dust from the object and producing a shimmery tail.
When will R2 (SWAN) be visible?
After dazzling stargazers in the Southern Hemisphere, R2 (SWAN) is starting to swing into view in the northern evening sky this week and will remain visible until the end of October.
To spot the comet, astronomers recommend going to a dark location with an unobstructed view to the southwest and with as little light pollution as possible. The comet’s brightness is expected to remain out of range of the naked eye, so you’ll need binoculars or a small telescope. About 45 minutes to one hour after sunset, scan low along the horizon. The comet will appear as a “fuzzy ball,” said Yoonyoung Kim, a comet researcher at the University of California, Los Angeles.
Mr. Ye suggests a trick for locating it with your smartphone: Set the camera exposure to a few seconds and take some pictures of the sky.
Astronomers expect observing conditions for R2 (SWAN) to steadily improve until the comet makes its closest approach to Earth on Oct. 20. The object will move higher in the sky each night, toward the darkness and away from the lingering glow of the setting sun.
Comets are as unpredictable as cats, as the astronomy adage goes: They too have tails, and tend to do whatever they want. That means R2 (SWAN) may further surprise us: It brightened considerably when first discovered, and could undergo a similar outburst, perhaps even enough to be visible without binoculars, Mr. Ye said.
There is also the possibility that the comet could disintegrate entirely.
“We are all wondering what it’s going to do next,” he added.
How do I see A6 (Lemmon)?
A6 (Lemmon) is currently gracing the northern morning sky. To observe the comet, you’ll need binoculars or a small telescope — and the fortitude to wake up before dawn.
Look for a fuzzy object in the northeast, several hours ahead of sunrise, just below the ladle-shaped Big Dipper. From Earth’s perspective, A6 (Lemmon) will perform a switcheroo halfway through the month, appearing in the evening sky in the west. If predictions hold, A6 (Lemmon) will brighten in late October and early November, possibly enough to be spotted with the naked eye in very dark conditions.
Will it be possible to see both comets at once?
Stargazers could potentially spot both comets sometime around Halloween, said David Dickinson, an amateur astronomer and author of “The Backyard Astronomer’s Field Guide.”
A6 (Lemmon) will closely hug the horizon, though, and could be swallowed up by twilight, Mr. Dickinson said. R2 (SWAN) will hover in the sky for hours after sunset, so that comet may be your best bet for an evening dose of cosmic wonder.
Where did these comets come from?
Far-flung comets like these two originate in the Oort cloud, a bubble-shaped realm of frozen objects that surround the solar system at its very edges. According to Carrie Holt, an astronomer at Las Cumbres Observatory in California, such comets are likely “perturbed inward to the inner solar system a very, very, very long time ago,” perhaps jostled by a passing star or the galactic tides of the Milky Way.
According
to astronomers’ latest calculations, A6 (Lemmon) circles the sun about every 1,351 years, while R2 (SWAN) completes its loop in 642 years.
Other comets reside closer in, within the Kuiper belt region beyond Neptune, and usually take less than 200 years to orbit the sun. Still other comets arrive from interstellar space, such as
3I/ATLAS, which is zooming through the inner solar system now but will be on the wrong side of the sun for viewing this month.
For comet researchers, visits from the Oort cloud are drive-by lessons in cosmic history. Because of their lack of exposure to the sun, these comets contain materials that remain as pristine as they were more than four billion years ago. That offers scientists a glimpse of our celestial beginnings.
If they don’t disintegrate, both comets will return to the chilly hinterlands of the solar system, permanently altered by their sojourn past the sun.
Out there, “it’s actually pretty boring because it’s cold, and there’s not much going on,” Mr. Ye said. This flyby of Earth may be the most exciting thing that has happened to these comets in decades, and it is unfolding right overhead, Mr. Ye said, “so why not just go see?”