Thanks to the Hubble Space Telescope, we now have the sharpest image yet of the interstellar visitor 3I/ATLAS, showing that it is clearly a comet, with a coma filled with dust particles and the first hints of a tail.
Of course, 3I/ATLAS is no ordinary comet. Discovered on July 1, 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), 3I/ATLAS is the fastest comet ever seen. Racing in-system at 130,000 mph (209,000 kph), it is hurtling through space so fast that it will escape the sun‘s gravitational grasp. Its origin is somewhere beyond the solar system, in interstellar space where it has traveled for aeons, being sped up by gravitational slingshots every time it encounters a star. As a result, 3I/ATLAS is just passing through, and will gain another slingshot from our sun to send it on its way back into interstellar space, never to be seen again.
“No one knows where the comet came from,” said David Jewitt of the University of California, Los Angeles in a statement. Jewitt was the science lead on the Hubble observations of 3I/ATLAS. “It’s like glimpsing a rifle bullet for a thousandth of a second. You can’t project that back with any accuracy to figure out where it started on its path.”
Even though 3I/ATLAS’ origins are shrouded in mystery, astronomers now have the rare chance to learn as much as possible about a comet that was born around another star and is quite possibly older than our solar system.
Comets have four primary components. They all have a solid nucleus. And, when comets get close to the sun, they warm up, causing outgassing that produces a coma around the nucleus as well as two tails — a dust tail left along its path, and an ion tail pointing away from the sun.
The Hubble observations show that 3I/ATLAS’ nucleus is shrouded by a coma made from small particles of dust that have been lifted off the interstellar comet’s surface. This coma is hiding the nucleus, but, thanks to Hubble’s careful observations, astronomers have now placed upper and lower limits on the size of that core. It is potentially as large as 3.5 miles (5.6 kilometers) across, while the smallest it can be is 1,000 feet (320 meters). That’s quite a large size range in the context of cometary bodies.
Hubble also saw a dust plume emanating from the sun-facing, warm side of the comet and feeding the coma, plus the barest hints of a dust tail. These are all typical features of a comet that is still 3.8 astronomical units (Earth-sun distances; one AU is about 93 million miles, or 149.6 million km) from the sun. So in that sense, 3I/ATLAS is behaving very much like a comet native to the solar system. So far, only its velocity and hyperbolic trajectory mark it out as different.
Despite being discovered by a network of telescopes designed to spot hazardous asteroids, 3I/ATLAS is no danger to Earth. The closest it will come is 1.8 AU (167 million miles, or 270 million km), and, even when it reaches its closest point to the sun, called perihelion, on Oct. 29, it will barely be closer to the sun than Mars. In fact, after the comet has entered solar conjunction in the sky as seen from Earth and becomes lost in the sun’s glare, it will still be visible from Mars, and our spacecraft at the Red Planet will continue to observe it even after it has moved out of sight from Earth. The comet will reappear in Earth’s sky in December 2025.
Astronomers aim to track 3I/ATLAS’ rise in activity as it gets closer to the sun for as long as possible. As the amount of outgassing ramps up with the sublimation of various ices, spectroscopic observations could perhaps reveal something of the comet’s composition. Astronomers could then compare that composition to the native comets of our solar system. For example, a previously discovered interstellar object, 2I/Borisov, was a comet that had a greater abundance of carbon monoxide than the solar system’s comets.
3I/ATLAS is the third interstellar interloper to be discovered, with the first, 1I/’Oumuamua coming in 2017 and 2I/Borisov following in 2019. However, it is believed that this trio is just the tip of the iceberg, with some estimates claiming that there could be as many as 10,000 such objects of various sizes passing through the solar system at any one time.
However, because we can’t actually see the nucleus of 3I/ATLAS to measure its size exactly, it means that it is not possible to use its discovery to make more accurate predictions about how many interstellar objects there are. For example, we expect there to be more interstellar objects that are closer to 1,000 feet (300 m) in diameter than larger ones of 3 miles (5 km) or so. If 3I/ATLAS were one of the larger ones, then, based on the fact that it is just the third interstellar object that we’ve discovered, we’d need to revise our estimates of how many 3-mile objects there are. That’s because, statistically, we’d be unlikely to have found one as the third interstellar object if objects of that size are very rare.
We can’t even use how likely it was that ATLAS would spot 3I/ATLAS as a guide to how many interstellar objects there are, because it was not the size of the nucleus but rather the dust in the coma scattering light and making 3I/ATLAS appear brighter than it otherwise would have that made it visible to the ATLAS telescope network.
However, not even the presence of a comet’s coma can hide the secrets of such objects forever. With the Vera C. Rubin Observatory in Chile now almost fully operational, the expectation is that it will find at least one interstellar visitor per year on average, and at greater distances from the sun — for many of them, before they’ve had the chance to warm up enough for cometary activity to emerge.
“This latest interstellar tourist is one of a previously undetected population of objects bursting onto the scene that will gradually emerge,” said Jewitt. “This is now possible because we have powerful sky survey capabilities that we didn’t have before. We’ve crossed a threshold.”
These latest findings about 3I/ATLAS have been accepted for publication in The Astrophysical Journal Letters, and a pre-print of the study is available now.
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