On 1 July 2025, the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile, reported sightings of a comet that's entered our Solar System.
The interstellar comet, officially known as 3I/ATLAS or C/2025 N1, originated from interstellar space, NASA says, and has arrived in our Solar System from the direction of the constellation Sagittarius.
It's currently about about 420 million miles (670 million kilometres) away from Earth.
What we know about 3I/ATLAS
When 3I/ATLAS was first discovered, astronomers began going back through archive observations made before the comet was even discovered, to see if they could spot it in old data.
They trawled the archives of three different ATLAS telescopes around the world, as well as the Zwicky Transient Facility at the Palomar Observatory in California.
The so-called 'pre-discovery' observations go all the way back to 14 June 2025, and soon other telescopes began reporting further observations made since 3I/ATLAS was first reported.
The interstellar visitor is thought to be as big as to 20 kilometres wide and travelling about 60 km/s relative to the Sun.
3I/ATLAS is what's known as an 'active comet', which means it heats up as it gets close to the Sun.
This is potentially exciting news for comet observers, as it could lead to frozen gases sublimating into vapour, carrying dust and ice particles and forming a glowing coma and tail.
Observations of 3I/ATLAS
Since its discovery, some of humanity's most powerful space telescopes have studied 3I/ATLAS, revealing more about its shape, its size and its chemical composition.
In July 2025, astronomers announced that the Gemini North Telescope had taken a look at comet 3I/ATLAS.
The telescope's Multi-Object Spectrograph (GMOS-N) enabled astronomers to get a better view the comet’s coma, which is a cloud of gas and dust surrounding its icy nucleus.
"The sensitivity and scheduling agility of the International Gemini Observatory has provided critical early characterization of this interstellar wanderer," said Martin Still, NSF program director for the International Gemini Observatory.
In early August 2025, astronomers released images of 3I/ATLAS captured by the Hubble Space Telescope.
Hubble's observations amount to the sharpest view yet of the comet, enabling scientists to get a better view of the size of its nucleus.
The Hubble Space Telescope even managed to capture a dust plume ejected from the comet, and a glimpse of a dust tail streaming away from its nucleus.
In late August, astronomers announced that the SPHEREx mission and James Webb Space Telescope had taken a look at 3I/ATLAS.
Observations by these two powerful space observatories revealed the presence of water ice and carbon dioxide.
Just as Solar System comets that orbit our Sun are remnants of the formation of our Solar System, so too are interstellar comets likely to be remnants from the formation of distance star systems.
This means interstellar comets are primordial leftovers; pristine samples that can tell astronomers more about the formation of distance systems across the Galaxy.
Citizen science views of the comet
In July 2025, the smart telescope manufacturer Unistellar announced that citizen scientists using Unistellar scopes had captured images of 3I/ATLAS.
"We’ve caught it early enough to coordinate observation campaigns with our Unistellar citizen science network and other citizen science telescopes to be able to capture images of it," said Franck Marchis, Senior Astronomer at the SETI Institute and Chief Science Officer at Unistellar.
"Combining these observations, which allow us to track its trajectory, with those taken by major ground-based and space-based observatories, could be a unique chance to study its composition and behaviour as it warms up while approaching the Sun."
Will 3I/ATLAS come close to Earth?
NASA says "the comet poses no threat to Earth," and it's expected to remain at a distance of at least 1.6 astronomical units (AU), i.e. 1.6 times the distance between Earth and the Sun.
That's about 150 million miles or 240 million km.
For context, comet 3I/ATLAS is currently 4.5 AU (416 million miles or 670 million km) from the Sun.
From the information gathered about 3I/ATLAS so far, astronomers are able to calculate what its orbit looks like.
It's expected to reach its closest approach to the Sun around 30 October 2025 at a distance of 1.4 AU (130 million miles or 210 million km), putting it just inside the orbit of Mars.
What next for the comet?
NASA says 3I/ATLAS's "size and physical properties are being investigated by astronomers around the world."
And, according to NASA, 3I/ATLAS should be visible through ground-based telescopes until September 2025.
After that time it will pass too close to the Sun to observe, but will reappear on the other side of the Sun by early December 2025, at which point it can be studied again.
