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This week, Earth gets a front-row seat to one of the solar system’s oldest relics. On October 21, Comet C/2025 A6 (Lemmon) will pass closest to Earth—its first and only appearance for more than a thousand years.
“Comets are very common, but Comet Lemmon is definitely the best comet to view from Earth this year,” says Rhonda Stroud, the director of the Center for Meteorite Studies at Arizona State University in Tempe, Arizona.
Where did this comet come from—and how can you spot it in the night sky? Here’s what to know before it passes.
Where did the comet come from?
Space isn’t completely empty. Within our solar neighborhood, it’s sprinkled with ice and dust particles that can be found between planets, stars, and even galaxies. Comets and asteroids are evidence of the ice and dust found in our solar system—the debris left over from the solar system’s formation from a huge, dense cloud of gas and dust about 4.6 billion years ago.
(This interstellar comet could be one of the oldest objects in the entire galaxy.)
After the sun ignited, the leftover gas and dust began clumping together. Far from the heat, in the cold outer reaches, comets formed. Those icy outskirts later became the Kuiper Belt and the even more distant Oort Cloud—vast reservoirs filled with frozen comets.
“Comets are fantastic to study because they are treasure troves of the original building blocks of our solar system,” says Stroud. “The frozen state of comets means that much of the dust and ice is essentially unchanged over billions of years.”
Why are so many comets called Lemmon?
When Comet Lemmon first appeared on January 3, it wasn’t much to look at—just a faint dot against the night sky. Carson Fuls, director of the Catalina Sky Survey and the observer on duty that night, says that’s typical. “Sometimes we won't see them until they ‘turn on,’” says Fuls, or when they get close enough to the sun for their ices to turn to gas and form the characteristic tail of the comet.
(These are the first images from Earth’s giant new telescope.)
Despite the name, this is far from the first Comet Lemmon. Fuls says it is one of around 70 “Comet Lemmons,” and that comets are often named after the observatory that detected them, or the person who discovered it, if they recognize it right away as a comet. For Fuls, this Comet Lemmon wasn’t “turned on” yet, so it more resembled an asteroid than a comet with a tail.
“I usually see a few [comets] each night when running one of our survey telescopes, but they are still a treat. It never gets old seeing so deep into such large sections of the night sky,” says Fuls.
Why does Comet Lemmon glow green?
While dozens of comets pass through the inner solar system each year, Comet Lemmon’s brightness, distinctive green glow, and proximity to Earth make it the most striking comet of 2025. The emerald hue is due to the presence of diatomic carbon (C2), a molecule that gets broken down by solar radiation and emits green light.
The light blue tail is actually made up of two tails: one made of ice and dust from the comet itself, and one made of ions, or energetic particles that are excited when the comet “turns on” as it gets closer to the sun’s radiation.
(These are the best stargazing sites in North America.)
Common comet ingredients include carbon monoxide, carbon dioxide, and water ice, but the ratios between the molecules vary, says Stroud. “Every comet that we have imaged up close with a spacecraft looks different from the others.”
Comets are also unusually dynamic. Their shapes and brightness can change within hours as sunlight and heat reshape their frozen surfaces and blow material off of the main part of the comet.
“You can sometimes see disruption events as chunks of the comet are flung off. These look like ripples in the tail that you can watch slowly evolve,” says Fuls. “It’s rare to see something so dynamic out in space.”
For many researchers studying comets, the next scientific holy grail would be a sample return mission to bring back a frozen chunk of a comet and study its ancient ice and dust, right as it appeared in space.
“The more I learn about [comets], the more I feel connected to the history of the universe,” says Stroud. “Following the comet dust is like following the breadcrumbs to solve the mystery of how our solar system, and ultimately you and I, came to be.”
How to see Comet Lemmon
Comet Lemmon will be closest to Earth on October 21, coinciding with a new moon, a dark-sky advantage that makes faint celestial objects easier to see. Look for a soft green glow near the constellations Scorpius or Libra, low on the western horizon shortly after sunset. The comet should remain visible from mid-October through early November, fading gradually as it moves away from Earth.
(Here are nine other night sky events to see in October.)
“While it is fun to look with just your eyes, I recommend binoculars and taking pictures with a good phone or digital camera. You’ll be able to see more of the coma,” says Stroud, the glowing cloud of gas and dust that surrounds a comet’s icy core.
Adding to the spectacle, October 21 also marks the peak of the Orionid meteor shower, offering a rare double feature in the night sky. For the best viewing experience, astronomers suggest using stargazing apps or NASA’s Sky Events calendar to check exact rise and set times for your location.
