In July 2025, astronomers made a landmark discovery when the sky-survey telescope network Asteroid Terrestrial‑impact Last Alert System (ATLAS) in Chile detected an unusual object dubbed 3I/ATLAS. It soon became clear this was no ordinary comet. With a hyperbolic orbit and speed far beyond what the Sun could impart, it is now recognised as only the third confirmed interstellar object ever observed—a cosmic guest from beyond our Solar System.
A Visitor from Beyond
Objects such as 1I/ʻOumuamua (2017) and 2I/Borisov (2019) prepared us for the possibility of extra-solar explorers, but 3I/ATLAS stands out. Travelling at around 58 km/s (≈ 130 000 mph) relative to the Sun—far faster than typical Solar-System objects—its speed alone is a signature of interstellar origin.
Unlike prior interstellar visitors that approached from the Sun’s motion direction (the “solar apex”), 3I/ATLAS came in from the southern celestial hemisphere—roughly the direction of the Milky Way’s galactic centre—something astronomers did not expect.
Importantly, its path did not include any close encounter with a major planet, ruling out a Solar-System origin via gravitational slingshot; its trajectory truly carries it from interstellar space.
Speed, Size & Composition
Telescopic observations—including data from the Hubble Space Telescope—revealed that 3I/ATLAS is active, showing a coma of dust and gas as it warms by the Sun.
The nucleus’s diameter is still uncertain. Hubble’s sharpest images suggest an upper limit of about 5.6 km (3.5 miles), though it could be considerably smaller (possibly a few hundred metres).
As sunlight strikes the icy surface, jets and tails are forming—typical for comets but here offering a rare opportunity to study a genuinely interstellar one.
Remarkably Old—and Surprising
What truly makes 3I/ATLAS remarkable is its likely age and origin. Studies suggest it comes from the thick-disk stellar population of our galaxy—a very old, metal-poor region far from where the Sun was born. If this holds true, then this ice-rock may be older than the Solar System itself, having drifted between the stars for billions of years before being swept into our neighbourhood.
This contrasts with previous interstellar objects that originated from younger stellar populations, making 3I/ATLAS a unique probe of galactic history and cosmic evolution.
Why It Matters
The arrival of 3I/ATLAS opens new windows for science:
Diversity of interstellar visitors: Until now, we had only two known interstellar objects. A third one allows comparisons and deeper insights.
Galactic archaeology: Because it may hail from a very old stellar region in the galaxy, it carries information about chemical and dynamical conditions different from those of our Sun’s birthplace.
Planet-system formation insights: By studying its composition, orbit and behaviour, scientists hope to learn how objects are ejected from other star systems and how common such wanderers might be.
Testing theory: Its unexpected origin direction and speed challenge preconceptions about how many interstellar objects we might detect and from which directions.
Looking Ahead
3I/ATLAS will make its closest approach to the Sun (perihelion) around October 30, 2025, at a distance of about 1.4 astronomical units—just inside Mars’ orbit. It poses no threat to Earth: its minimum distance from our planet is about 1.8 AU (≈270 million km).
With state-of-the-art telescopes on the ground and in space fully engaged, much more data will come in the next few months. These observations may refine the object’s size, composition, and perhaps reveal surprises.
Whatever we learn, the story of 3I/ATLAS is a reminder that our Solar System is not isolated—but connected to a vast galactic sea of objects, many ancient and mysterious.
Final Thoughts
The discovery of 3I/ATLAS is a thrilling moment in astronomy. A visitor from another star system, likely billions of years old and bearing the signature of a different galactic epoch—it challenges our sense of cosmic neighbourhood. For scientists and the public alike, it’s a rare chance to reach beyond our Solar System without leaving it.
As telescopes train on its journey, we may soon uncover secrets not just about a single comet, but about how countless star systems eject material, how our galaxy evolves, and what lies beyond our own little corner of space.
—
Source
NASA Science: What We Know About 3I/ATLAS.
ABC News: New interstellar ‘comet’ 3I/ATLAS discovery.
Universe Today: Tracking the interstellar objects’ origins.
Space.com / ScienceAlert: Jets and dust activity on 3I/ATLAS.