When we look back on 2025 from an astronomical perspective, comet 3I/ATLAS is sure to be considered one of the highlights of the year.
This interstellar visitor from beyond our Solar System has captured our attention – and, in some cases, our wild imaginations – since it was first discovered back in June 2025.
It's an interstellar comet, meaning that, unlike the vast majority of comets we know of, which originated from within our Solar System, 3I/ATLAS originated from elsewhere in the galaxy.
As a result, learning more about 3I/ATLAS can teach astronomers about what other star systems are like, and how interstellar spacerocks compare to those found within our own cosmic neighbourhood.
James Webb Space Telescope observations, for example have found water ice, water vapour, carbon monoxide and carbonyl sulfide at 3I/ATLAS.
In fact, so far, observations suggest the dust and ice found at 3I/ATLAS is similar to those found at comets that formed within our Solar System.
3I/ATLAS discovery
Let's take ourselves back to the start of summer 2025, when this incredible interstellar comet was discovered.
It was found by the ATLAS (Asteroid Terrestrial-impact Last Alert System), which is a project developed by the University of Hawaii and funded by NASA.
ATLAS is an early warning system, designed to detect potentially dangerous asteroids and give humanity a heads-up about any that look to be coming our way.
The whole setup is automated and consists of four telescopes in total: two in Hawaii, one in Chile and one in South Africa.
These telescopes are fitted with cameras that automatically scan and photograph the whole sky several times a night.
The system takes four images of each part of the sky, at intervals separated by 12 minutes.
It then looks for objects moving quickly against the background sky by comparing the images and spotting differences between them.
Stars, for example, will be in the same position from one image to another.
But something much closer and moving fast, like a comet or asteroid, will have changed position significantly when multiple images of the same patch of sky are compared.
The computer system detects objects that have moved or which have altered in brightness, and picks out potential asteroids and comets from other transient objects like variable stars or supernovae, for example.
This technique is a well-established astronomical method for discovering new objects, known as 'blinking'.
Pluto, for example, was discovered using the 'blinking' method, by comparing images of the same patch of sky and spotting an object moving quickly, relative to the background stars.
Of course, back in 1930 when Pluto was discovered, the blinking was done manually by astronomer Clyde Tombaugh, but nowadays computer systems do the blinking work automatically.
Comet 3I/ATLAS was discovered using this technique, as the images above show.
The animation you can see further up this page is made of a series of frames captured by the ATLAS system, clearly showing an object moving across the sky.
It was, in fact, the ATLAS survey telescope in Chile that first reported that the comet comes from interstellar space.
And since its discovery, some of Earth's most powerful telescopes have been pointed at 3I/ATLAS, including the Very Large Telescope, the Hubble Space Telescope and the James Webb Space Telescope.
As the comet approaches the Sun, Earth and Mars this autumn, there's even talk of other spacecraft like Mars rovers and orbiters, or the NASA Juno mission at Jupiter, catching a glimpse.
3I/ATLAS won't go into orbit around the Sun: it's merely passing through our Solar System on its journey across the Galaxy.
But one thing's for sure, astronomers are doing everything they can to learn as much as they can about this strange, mysterious visitor, while it happens to be in the neighbourhood.
Future sky surveys like those conducted by the Vera Rubin Telescope, which has just begun science observations, may open up many more of these interstellar objects for observation.
We could be on the brink of a new era in our understanding of primordial spacerocks from the depths of the cosmos.
