Interstellar object Comet 3I/ATLAS: New image and more
Comet 3I/ATLAS shrinks again
On August 7, 2025, NASA shared a new image of Comet 3I/ATLAS and an updated estimate of its size. 3I/ATLAS, the 3rd-known interstellar object, was originally estimated to have a diameter of 20 km (12 miles). But with data from the Vera C. Rubin Observatory, astronomers re-estimated the size to be around 10 km (6 miles). And now the Hubble data puts the size of 3I/ATLAS’s nucleus at a diameter of 5.6 km (3.5 miles). But it could be as small as 320 meters (1,050 feet) across.
The two previous interstellar objects we know of are 1I/ ‘Oumuamua and 2I/Borisov. ‘Oumuamua’s size is thought to be about 200 meters across at its widest (you’ll recall it has an elongated shape). And Borisov is thought to be less than a kilometer across.
The new Hubble data reveal even more about the comet. The NASA/ESA press release said:
Hubble also captured a dust plume ejected from the sun-warmed side of the comet, and the hint of a dust tail streaming away from the nucleus. Hubble’s data yields a dust-loss rate consistent with comets that are first detected around 480 million kilometers (300 million miles) from the sun. This behavior is much like the signature of previously seen sun-bound comets originating within our solar system.
3I/ATLAS is traveling through our solar system at roughly 210,000 kilometers per hour (130,000 mph), the highest velocity ever recorded for a solar system visitor. This breathtaking sprint is evidence that the comet has been drifting through interstellar space for many billions of years. The gravitational slingshot effect from innumerable stars and nebulae the comet passed added momentum, ratcheting up its speed. The longer 3I/ATLAS was out in space, the higher its speed grew.
An interview with Colin Orion Chandler
Watch the interview below with Colin Orion Chandler of the DiRAC Institute of the University of Washington for more on the comet’s size.
You’ve probably heard by now about the new interstellar object – an object from another star system – hurtling toward our sun. Earthly astronomers have named it 3I/ATLAS. It’s only the 3rd-known interstellar object to visit our solar system. The object was originally estimated to have a diameter of 20 km (12 miles). But the size of 3I/ATLAS has now been re-estimated at around 10 km (6 miles). Hear from one of the astronomers who helped refine 3I/ ATLAS’s size estimate – Colin Orion Chandler of the DiRAC Institute of the University of Washington – speaking with EarthSky’s Deborah Byrd. Watch in the player above, or on YouTube.
It’s probably an old, old comet
Does that change what most astronomers think about this object? No. Most astronomers still think it’s an extremely old comet. To hear directly from one of the astronomers who studied it, and identified it as a comet, watch the video below.
The newly discovered interstellar object 3I/ATLAS – found on July 1 – is likely the oldest comet we’ve ever seen. It could be more than 7 billion years old, predating our solar system by more than 3 billion years! That’s according to University of Oxford astronomer Matthew Hopkins. Hear him explain in the player above, or on YouTube.
Discovery of 3I/ ATLAS
The Asteroid Terrestrial-impact Last Alert System (ATLAS) detected our new visitor on July 1, 2025. And the Minor Planet Center confirmed its interstellar nature on July 2, 2025, naming it 3I/ATLAS (or C/2025 N1). The “3I” means it’s the 3rd interstellar visitor that we’ve found. Its trajectory and speed revealed it as an object not from our solar system, but from another star system.
The Hubble Space Telescope imaged the object on July 21, 2025. See the post from Bluesky below.
Hubble Space Telescope images of interstellar comet 3I/ATLAS are out! These were taken 5 hours ago. Plenty of cosmic rays peppering the images, but the comet’s coma looks very nice and puffy. Best of luck to the researchers trying to write up papers for this… archive.stsci.edu/proposal_sea… ?
— astrafoxen (@astrafoxen.bsky.social) July 21, 2025 at 4:28 PM
From a different part of our galaxy
The object is traveling on a steep path through the Milky Way galaxy. Astronomers have analyzed its trajectory, which shows it is not swinging around the sun and heading back out into the outskirts of our solar system. Instead it’s just flying by. And their analysis suggests Comet 3I/ATLAS originated within, or at least on the border of, the Milky Way’s thick disk. This is an area of ancient stars orbiting above and below the thin galactic plane where our sun resides.
What we know about Comet 3I/ATLAS
Our new visitor will get its closest to the sun – at about 2 astronomical units (AU), or twice as far as Earth is from the sun – in October. As it reaches perihelion – its closest point to the sun – it will be traveling at almost 15,500 miles (25,000 kilometers) per hour.
The speedy nature of Comet 3I/ATLAS is more proof of its interstellar nature. It has to be moving at a blistering pace in order to escape the sun’s gravitational pull.
Marshall Eubanks, a physicist and VLBI radio astronomer and co-founder of Space Initiatives, said the comet will come within about 0.4 AU of Mars in October. That would make it just barely observable by the Mars Reconnaissance Orbiter.
It’s important for science
Having a visitor from another solar system is a rare opportunity for scientists, as NOIRLab said in a press release:
These visitors from faraway regions of the cosmos are valuable objects to study since they offer a tangible connection to other star systems. They carry information about the chemical elements that were present when and where they formed, which gives scientists insight into how planetary systems form at distant stars throughout our galaxy’s history, including stars that have since died out.
Observing the new interstellar object
From now through the beginning of September, Comet 3I/ATLAS will be in the evening sky. For the rest of September and October it will be too close to the sun to see. But by November and December the comet will be bright after just passing the sun and also out of the sun’s glare. At this point it will be a morning object, not far from Venus.
Eddie Irizarry shared maps of the location of 3I/ATLAS. Eddie said in an email to EarthSky:
Although Comet 3I/Atlas is currently dim, advanced amateur observers might be able to photograph the new visitor by taking long exposure images through a telescope.
By August, the new comet should reach magnitude 16 and gradually improve, allowing more astrophotographers to capture this rare object.
The dim space rock is currently at about magnitude 16.7.
Following are star charts for those who want to search for the comet using a telescope or do astrophotography by capturing the comet close to a star or deep-sky object.
Evening star charts here
Morning star charts here
After Comet 3I/ATLAS makes its close approach to the sun, you can find it in the morning sky.
Bottom line: The new interstellar visitor – 3I/ATLAS – was first thought to be 20 km across. Based on Hubble Space Telescope observations, its nucleus is now estimated to be 5.6 km across. But it could be as small as 320 meters.
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