Whether you follow space news or have just caught recent media headlines, you’ve likely heard of the interstellar comet 3I/ATLAS. This visitor from the great beyond zoomed into the astronomical spotlight when it was discovered in July 2025, and has gotten the world talking about interstellar objects once again. The idea of an interstellar object is naturally intriguing, so it’s no surprise this discovery has been making news. 

To ensure you’re well-equipped to dazzle your friends with interstellar comet knowledge at your next dinner party, here are some of the most important and interesting things we know about comet 3I/ATLAS. 

It isn’t the first known interstellar object

3I/ATLAS, also called C/2025 N1 (ATLAS), zoomed into the astronomical spotlight in July 2025 when it was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station in Chile. It is the third interstellar object ever discovered, after 1I/ʻOumuamua (discovered in October 2017) and 2I/Borisov (discovered in August 2019). 

All three interstellar objects studied so far are thought to be comet-like, which supports theories about planet formation and the ejection of icy bodies, but they are all slightly different as well. When ʻOumuamua entered the scene, its strange, seemingly oblong shape sparked debate about its nature, even prompting speculation that it could be an alien artifact (although nearly all scientists believe it isn’t). 3I/ATLAS, on the other hand, is clearly a comet, as is Borisov. 3I/ATLAS is likely larger, though, with a nucleus estimated to be about a kilometer in diameter — around the length of ten football fields. 

It looks like it has a backwards tail

One particularly unusual feature of 3I/ATLAS is that it appears as though its tail points toward the Sun. Normally, comet tails point away from the Sun due to solar radiation pressure, and 3I/ATLAS does have a faint tail like this. But it also has a stream of dust grains coming from it that are too large and heavy to be blown back by the solar radiation. Instead, the comet’s own internal activity pushes these dust grains away. Because the Sun’s energy increases such activity, dust is primarily pushed out in the direction of the Sun. 

It’s old

Initial studies suggest that 3I/ATLAS may be between 3 and 14 billion years old, potentially meaning that it’s older than the Solar System (which is 4.6 billion years old). This would also make it the oldest comet yet seen.

We don’t know where it came from

While astronomers can pinpoint the direction from which 3I/ATLAS came into the Solar System, this doesn’t tell them where in the galaxy it originated. It’s not as simple as tracing a straight line — if this object has been travelling through space for millions of years, its trajectory will have been nudged countless times by stars, molecular clouds, even the spiral arms of the galaxy itself. 

What we can deduce is that it likely started off in orbit around a star. Researchers think 3I/ATLAS was likely flung out of its star system by a close encounter with a giant planet or another nearby star. Observations by the space telescopes JWST and SPHEREx have shown that the comet is rich in carbon dioxide, which suggests that it formed far from its parent star, where temperatures are cold enough for carbon dioxide to become a solid. 

We do know where it’s going

3I/ATLAS is heading toward the Sun at the time of writing, and will reach perihelion (its closest point to the Sun) around Oct. 29, 2025, at a distance of about 1.4 astronomical units (210 million kilometers, or 130 million miles), just inside the orbit of Mars. 

After perihelion, it will pass 0.65 astronomical units (97 million kilometers; 60 million miles) from Venus on Nov. 3, 2025. Soon after that, the European Space Agency’s Juice (Jupiter Icy Moons Explorer) spacecraft that is en route to Jupiter will attempt to observe 3I/ATLAS using its cameras, spectrometers, and a particle sensor.

The closest this interstellar object will come to Earth is 1.80 astronomical units (269 million kilometers; 167 million miles) on Dec. 19, 2025, posing no threat of impact. It will carry on through the Solar System, passing 0.36 astronomical units (54 million kilometers; 33 million miles) from Jupiter on March 16, 2026. 

After that, it isn’t expected to make any more planetary visits and will make its way out of the Solar System and continue on its journey through interstellar space. 

It’s fast!

As it approaches the Sun, comet 3I/ATLAS speeds up due to the Sun’s gravitational pull. This speed will reach a maximum at perihelion, at which point the comet is expected to reach a speed of 68 kilometers per second (42 miles per second) with respect to the Sun. 

It won’t be the last

With powerful survey telescopes like the Vera Rubin Observatory, astronomers expect to discover many more interstellar objects soon — perhaps even a “4I” within a few years. Each new visitor gives us an opportunity to study something from outside our local cosmic neighborhood to find out how commonplace certain traits are among objects throughout the Universe. ESA’s Comet Interceptor mission could even rendezvous with a future interstellar comet, potentially unlocking more of the Cosmos’ many secrets.