Comet 3I/ATLAS: A Visitor from Beyond the Solar System
The NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile, first reported observations to the Minor Planet Center of comet 3I/ATLAS on July 1, 2025. Since the first report, observations made before the discovery were gathered from the archives of three different ATLAS telescopes around the world and Caltech’s Zwicky Transient Facility at the Palomar Observatory in San Diego County, California. These “pre-discovery” observations extend back to June 14.
Comet 3I/ATLAS is the third known object from outside our solar system to be discovered. Astronomers have categorized this object as interstellar because of the hyperbolic shape of its orbital path. (It does not follow a closed orbital path about the Sun.) When the orbit of 3I/ATLAS is traced into the past, the comet clearly originates from outside our solar system.
Comet 3I/ATLAS poses no threat to Earth and will remain far away. The closest it will approach our planet is about 1.8 astronomical units (about 170 million miles, or 270 million kilometers). 3I/ATLAS will reach its closest point to the Sun around Oct. 30, 2025, at a distance of about 1.4 au — just inside the orbit of Mars.
Comets are generally named for their discoverer(s), in this case the ATLAS survey team. The letter “I” is for “interstellar,” indicating that this object came from outside our solar system. It’s the third known interstellar object, hence the “3” in the name.
3I/ATLAS formed in another star system and was somehow ejected into interstellar space, which is the space between the stars. For millions or even billions of years, it has drifted until it recently arrived at our solar system. It has been approaching from the general direction of the constellation Sagittarius, which is where the central region of our galaxy, the Milky Way, is located. When discovered, 3I/ATLAS was about 410 million miles away from the Sun, within the orbit of Jupiter.
3I/ATLAS appeared to be travelling at 245,000 kilometres per hour, making it the fastest object ever detected in our Solar System.
It was also huge. Early estimates suggest the object could be up to 20 km in size. Finally, scientists believe it may even be older than our Sun.