Comet 3I/Atlas

Discovery

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.

Overview

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 (130 million miles, or 210 million kilometers) — just inside the orbit of Mars.

The interstellar comet’s size and physical properties are being investigated by astronomers around the world. 3I/ATLAS should remain visible to ground-based telescopes through September 2025, after which it will pass too close to the Sun to observe. It will reappear on the other side of the Sun by early December 2025, allowing for renewed observations.

How This Comet Got Its Name

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.

FAQs

Where did this interstellar object come from?

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 (670 million kilometers) away from the Sun, within the orbit of Jupiter.

How was it determined that 3I/ATLAS didn’t originate in our solar system?

Observations of the comet’s trajectory show that it is moving too fast to be bound by the Sun’s gravity and that it's on what is known as a hyperbolic trajectory. In other words, it does not follow a closed orbital path around the Sun. It is simply passing through our solar system and will continue its journey into interstellar space, never to be seen again.

How many interstellar objects have been discovered?

This is the third known interstellar object to have been observed. Discovered in 2017, ‘Oumuamua was the first known interstellar object; the second was 2I/Borisov, which was discovered in 2019.

How big is 3I/ATLAS, and is it an asteroid or comet?

Astronomers don’t yet know how big 3I/ATLAS is, but from observations, they can see that it’s active, which means it has an icy nucleus and coma (a bright cloud of gas and dust surrounding a comet as it approaches the Sun). This is why astronomers categorize it as a comet and not an asteroid.

How fast is it moving?

Very fast. When it was discovered, the interstellar comet was traveling about 137,000 miles per hour (221,000 kilometers per hour, or 61 kilometers per second), and its speed will increase as it approaches the Sun.

Is there a danger of this interstellar object hitting Earth?

No. Although the trajectory of the object brings it into the inner solar system, it won’t come close to Earth. As the comet 3I/ATLAS journeys through the solar system, it won’t come closer than 1.6 au (about 150 million miles, or 240 million kilometers) to our planet.