3I/ATLAS
Interstellar comet · The third visitor
3I/ATLAS is the third known object from another star system, and the fastest. It is an active comet, growing a tail as it passed the Sun. Some astronomers think it could be billions of years older than our Solar System.
- 1 July 2025Discovered
- ~58 km/sArrived at
- Late October 2025Closest to Sun
Take a closer look
A cosmic speed record
3I/ATLAS entered our Solar System at about 58 kilometres every second – fast enough to cross from London to New York in under two minutes. It came from the direction of Sagittarius, towards the crowded centre of our Milky Way galaxy.
Alien ice
The James Webb Space Telescope found that the gas streaming from 3I/ATLAS is full of carbon dioxide – much more, compared with water, than in most comets from our own Solar System. Clues like this tell scientists about the cold place where it formed, around a star we will never see.
Hiding behind the Sun
When 3I/ATLAS made its closest pass by the Sun, it was on the far side of the Sun from Earth, so telescopes here could not watch. It never came near us: it always stayed more than 250 million kilometres from Earth, much farther away than the Sun.
How we found out
- 2019
Amateur astronomer Gennadiy Borisov discovers 2I/Borisov, the first interstellar comet with a clear tail.
- 2025
On 1 July, the ATLAS survey telescope in Chile discovers a third interstellar visitor.
- 2025
In October it passes Mars, and spacecraft orbiting Mars photograph it. Later that month it makes its closest approach to the Sun.
- 2025–26
Telescopes around the world and in space study it as it heads back out into interstellar space.
How many interstellar visitors are there?
Probably lots – most are just too small and faint to spot. The new Vera C. Rubin Observatory is expected to find many more.
See it yourself
Large telescopes only, and it is fading as it leaves.
From the encyclopedia
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