An interstellar comet named 3I/ATLAS is now dazzling astronomers as it speeds toward the Sun, growing a tail and coma in visible view. Discovered on July 1, 2025, this object is only the third confirmed visitor from outside our solar system following ‘Oumuamua (2017) and 2I/Borisov (2019).
Speed, Size, and Origin
Speed & Trajectory: 3I/ATLAS is racing through space at about 130,000 miles per hour (≈ 209,000 km/h). That’s its velocity as it plunges inward toward our Sun.
Size: Early images and measurements suggest a nucleus diameter ranging from roughly 0.3 to 5.6 km (about 0.2 to 3.5 miles) depending on assumptions. Some more generous estimates have placed it even larger, but the most consistent size seems to be a few kilometers.
Origin & Age: It was likely ejected from another star system in the Milky Way. If it came from the thick disk of our galaxy, it could be 7 billion years old or more—making it older than our solar system in many respects.
Composition: What Makes It Special
What sets 3I/ATLAS apart is its unusual chemistry:
It has an extremely high carbon dioxide (CO₂) to water (H₂O) ratio about 8:1. That means its coma (the cloud of gas and dust around it) is dominated by CO₂ emission, much more than most comets we’ve seen.
Water, carbon monoxide (CO), and even carbonyl sulfide (OCS) have also been detected, but in smaller amounts.
Observations from the SPHEREx mission show strong absorption features from water ice, plus a large, extended CO₂ coma.
This chemical signature suggests that 3I/ATLAS may have formed in a cold region far from its parent star, perhaps near the “CO₂ ice line,” where carbon dioxide can freeze. It also might have been exposed to higher radiation or other processes that allowed CO₂ ice to remain abundant.
Tail & Coma Growth
When activity began: The comet was already active when it was well beyond Jupiter’s orbit. It’s unusual for comets to start showing bright activity (coma, gas/dust emission) that far out. This early activity is likely tied to the sublimation of volatile ices like CO₂.
Recent images: On August 27, 2025, astronomers used the Gemini South telescope in Chile to take high-resolution images that clearly show the growing coma and an anti-solar tail particles and dust being pushed away from the Sun.
Visibility limits: The comet will reach its closest point to the Sun (perihelion) on October 29, 2025, at about 1.36 astronomical units (between Earth and Mars). But during that time, from Earth’s vantage point, it will be hiding behind the Sun making observations difficult. It will become more observable again afterward.
What We Learn & Why It Matters
Clues to other planetary systems: Comets like 3I/ATLAS carry ice and dust from distant star systems. Their composition gives insight into how planets and icy bodies might form elsewhere. The CO₂ richness suggests conditions different from those in our solar neighborhood.
Understanding early activity: Because it becomes active far out from the Sun, 3I/ATLAS helps astronomers understand sublimation behavior of different ices (not just water) under cold, distant conditions.
Rare opportunity: Interstellar comets are extremely rare. Every new observation (from telescopes like James Webb, Hubble, and ground-based observatories) is precious. 3I/ATLAS is a chance to see materials that formed in other star systems and compare them to ours.
What to Expect Next
As it moves past perihelion and comes closer to Earth, 3I/ATLAS’s tail and coma are expected to grow much larger and more luminous, provided geometry and sunlight allow.
After being hidden behind the Sun during closest approach, it may reappear for ground-based observers in December 2025, when it will be about 170 million miles away from Earth.
Interstellar Comet 3I/ATLAS is a rare, breathtaking visitor from far beyond. Its fast journey, strange mix of ices, and early tail development make it stand out among comets. This object doesn’t only fascinate sky-watchers it holds clues about the building blocks of planets in other star systems. As we watch it brighten and evolve, 3I/ATLAS reminds us of how much more of the universe there is left to explore.