This week, Earth gets a front-row seat to one of the oldest relics in the Solar System. October 21 Comet C/2025 A6 (Lemmon) will make its closest approach to Earth in its first and only appearance in more than a thousand years.
“Comets are very common, but Comet Lemmon is really the best comet to see from Earth this year,” says Rhonda Stroud, 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 you need to know before it’s gone.
Where did the comet come from?
Space is not completely empty. In our solar neighborhood, it is 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 formation of the solar system from a huge, dense cloud of gas and dust about 4.6 billion years ago.
(This interstellar comet may be one of the oldest objects in the entire galaxy.)
As the sun ignited, the remaining gas and dust began to accumulate. Far from the heat, the comets formed at the cold edges. These icy fringes 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. “Frozen comets mean that most of the dust and ice is essentially unchanged over billions of years.
Why are so many comets called Lemmons?
When Comet Lemmon first appeared on January 3rd, it wasn’t much to look at, just a faint dot in the night sky. Carson Fuls, director of the Catalina Sky Survey and an observer on duty that night, says this is common. “Sometimes we won’t see them until they ‘turn on,'” says Fuls, or when they get close enough to the sun for their ice to turn into gas and form the comet’s characteristic tail.
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Despite its name, this is far from Lemmon’s first comet. Fuls says it’s one of about 70 “Lemmon comets” and that comets are often named after the observatory that discovered them, or the person who discovered them if they’re immediately recognizable as a comet. To Fuls, this Comet Lemmon was not yet “on” so it looked more like an asteroid than a comet with a tail.
“I usually see a few [comets] running one of our survey telescopes every night, but they are still a treat. It never gets old seeing so deep into such large swaths of the night sky,” Fuls says.
Why does Comet Lemmon glow green?
Although 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 brightest in 2025. comet The emerald hue comes from diatomic carbon (C2), a molecule that breaks down in sunlight to emit green light.
The light blue tail actually consists of two tails: one of ice and dust from the comet itself, and the other of ions, or energetic particles, that are excited when the comet “turns on” as it approaches solar radiation.
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Common ingredients in comets are carbon monoxide, carbon dioxide and water ice, but the molecular ratios vary, Stroud says. “Each comet we’ve photographed up close with the spacecraft looks different from the others.
Comets are also unusually dynamic. Their shapes and brightness can change within hours as sunlight and heat alter their frozen surfaces and blow material away from the main body of the comet.
“Sometimes you can see disruption events where pieces of the comet are thrown off. It looks like ripples in the tail that you can watch slowly evolve,” says Fuls. “It’s rare to see something so dynamic in space.
For many comet researchers, the next scientific holy grail would be a flagship return mission to bring back a frozen piece of a comet to study its ancient ice and dust as it appeared in space.
“The more I learn about [comets]the more connected I feel to the history of the universe,” Stroud says. “Following comet dust is like following breadcrumbs to solve the mystery of how our solar system and ultimately you and I came about.
How to see Comet Lemmon
Comet Lemon will be closest to Earth on October 21 when the new moon coincides, the advantage of a dark sky that makes faint celestial objects easier to see. Look for a soft green glow near the constellations of Scorpius or Libra, low on the western horizon shortly after sunset. The comet should remain visible from mid-October to early November, gradually fading as it moves away from Earth.
(Here are nine more Night sky events to see in October.)
“While it’s fun to see with your eyes alone, I recommend using binoculars and taking pictures with a good phone or digital camera. You’ll see more of the coma,” says Stroud, the glowing cloud of gas and dust surrounding the comet’s icy core.
The impression is enhanced by the fact that on October 21 also marks the peak of the Orionid meteor shower, providing a rare double feature in the night sky. For the best viewing experience, astronomers suggest using stargazing apps or NASA’s Celestial Events Calendar to check the exact rise and time in your area.