Astronomers have confirmed a rare and extraordinary discovery: the third known interstellar object to enter our solar system.
Named 3I/ATLAS, where 3I stands for “third interstellar”, and designated C/2025 N1 (ATLAS), the object was first spotted on July 1, 2025, by the Deep Random Survey remote telescope in Chile, part of the ATLAS (Asteroid Terrestrial-impact Last Alert System) project.
It’s a significant find. But what exactly is it?
Initially referred to by the temporary designation A11pl3Z, 3I/ATLAS drew immediate attention from astronomers because of its peculiar motion. Rapid follow-up observations and reanalysis of previous data led to the preliminary conclusion that the object was not bound by the sun’s gravity. That makes it an interstellar object — only the third ever seen after 1I’Oumuamua in 2017 and 2I/Borisov in 2019. “If confirmed, it will be the third known interstellar object from outside our solar system that we have discovered, providing more evidence that such interstellar wanderers are relatively common in our galaxy,” Mark Norris, Senior Lecturer in Astronomy at the University of Central Lancashire, told Space.com at the time of 3I/ATLAS’s discovery.
Even more exciting? 3I/ATLAS is the largest and brightest interstellar object yet, which means it could help scientists unlock clues about the formation of other star systems.
How do we know it’s interstellar? Could it strike Earth? Can we send a spacecraft to intercept it? Here are all of your questions answered and everything else you need to know about this rare discovery, including why it may be the first of many more interstellar objects to be detected.
How do we know 3I/ATLAS is from another star system?
What makes astronomers certain about the interstellar nature of 3I/ATLAS is its trajectory. The object follows a highly hyperbolic orbit, which means it’s not gravitationally bound to the sun. Its orbital path also has an eccentricity of 6.2. For context, any object with an eccentricity above 1 is on a path that does not loop back around the sun, implying it comes from — and will return to — interstellar space. In comparison, the first known interstellar visitor, 1I/’Oumuamua, had an eccentricity of about 1.2, and 2I/Borisov came in at 3.6. 3I/ATLAS massively outpaces both.
“Some long-period comets could have a brush with Jupiter that modifies its orbit to 1.05, i.e., hyperbolic on the way out, but just barely,” Olivier Hainaut, an astronomer at the European Southern Observatory, told Space.com. “This one is firmly hyperbolic on the way in, so interstellar.”
How is 3I/ATLAS different from 1I/’Oumuamua and 2I/Borisov?
Aside from being significantly more hyperbolic, the most striking difference is size.
“3I/ATLAS is much larger than the other two — it’s about 15 kilometers (km) [9 miles] in diameter, with huge uncertainty, compared to 100m for 1I/’Oumuamua and less than 1km for 2I/Borisov,” said Hainaut. 3I/ATLAS may even be as wide as 12 miles (20 km). However, that conclusion could change with more observations.
What is 3I/ATLAS?
What 3I/ATLAS and 2I/Borisov have in common is that they are both comets. Shortly after its discovery, signs of a comet-like coma and tail became evident, giving it an additional designation of C/2025 N1 (ATLAS), the naming convention for comets.
Since 1I/ʻOumuamua was observed only as it was leaving the solar system, it was difficult for astronomers to get enough data on it to confirm its exact nature — hence the crazy theories about it being an alien spaceship — though it’s almost certainly an asteroid or a comet.
Could 3I/ATLAS strike Earth?
Right now, 3I/ATLAS is within Jupiter’s orbit, about 323 million miles (520 million km) from Earth and 420 million miles (670 million km) from the sun.
3I/ATLAS will reach approximately 167 million miles (270 million km) from Earth on Dec. 19, and at no point will it pose a threat. It will get to within 18 million miles (30 million km) of Mars on Oct.2 and to within 130 million miles (210 million km) of the sun — its closest point (perihelion) — on Oct. 29. At perihelion, it will be traveling at around 42 miles (68 km) per second/second or about 152,000 miles (245,000 km) per hour.
Is 3I/ATLAS visible in the night sky?
Only with the right equipment — and patience.
Right now, 3I/ATLAS is in the constellation Sagittarius in the arc of the Milky Way, low on the southern horizon as seen from mid-northern latitudes in July. Traveling south, it’s around magnitude 18.5, making it about 2.5 million times fainter than Polaris, according to Gianluca Masi at the Virtual Telescope Project, who imaged 3I/ATLAS on July 3. A 150-200mm/6-8-inch aperture telescope with a CCD camera is required to image 3I/ATLAS, while an optical telescope would need an aperture of around 400 mm/16-inch.
“It will not be visible to the naked eye, and I think it will be a challenge for an amateur, but some have impressive equipment these days,” Professor Martin Barstow at the School of Physics & Astronomy at the University of Leicester, told Space.com.
However, that could change because as it gets closer, it’s expected to brighten. “By the time it makes its closest approach, it will be a relatively easy target for amateur astronomers to observe,” said Norris. By then, it could reach magnitude 11. For most, 3I/ATLAS will be a fascinating science story but not a skywatching opportunity.
When will professional telescopes observe 3I/ATLAS?
Most large observatories are in the Southern Hemisphere, where 3I/ATLAS will be best placed, so expect numerous images to be shared over the coming days and weeks.
As it gets close to its bright perihelion, it will be lost in the sun’s glare as seen from Earth, so professional astronomers will study it — just as soon as the bright moon has departed the sky, likely in the weeks following the last quarter moon on July 18.
More observations are necessary because what we know about 3I/ATLAS is based purely on preliminary data. “It was discovered a few days ago and has been observed only with small telescopes,” said Hainaut. “We are scrambling to get the big guys on it as soon as possible.”
Why is 3I/ATLAS so interesting to astronomers?
Although much remains unknown, it is already clear that this object is orders of magnitude larger than ʻOumuamua and Borisov, making it a better target for study.
It could be a valuable opportunity for planetary scientists, as interstellar objects offer a tangible connection to other star systems and carry chemical signatures that can provide insights into how planetary systems form, or even offer evidence of life elsewhere in the galaxy.
“They undoubtedly carry chemical signatures from outside the solar system, so gaining observations tells us a lot about the possibility of material traveling between planetary systems,” Barstow said. “If we could get a sample from one, one day, it would be an incredible breakthrough.”
Can we send a spacecraft to intercept or fly by 3I/ATLAS?
Probably not, it’s just too fast.
“We would need a spacecraft ready to do this in space, fully checked out and with a rendezvous capability,” Barstow said.
The need to have a spacecraft in orbit ready to react to an incoming interstellar object, such as 3I/ATLAS, has been considered before. The European Space Agency is currently readying its Comet Interceptor project for launch in 2029 to deal with intriguing comets that suddenly appear. “However, even this mission might not be able to cope with the high speed of an interstellar traveler,” Barstow said.
Although a sample of 3I/ATLAS is not going to be possible, it would provide a huge shortcut for planetary scientists. “Even with our fastest rockets, it would take tens of thousands of years for us to reach nearby stars,” said Norris. “Thanks to these visitors from outside our solar system, we may not have to travel that far to sample star systems beyond our own [but] we’ll need the technology to catch up and reach them before they pass through our solar system.”
Why are astronomers suddenly finding interstellar objects?
It’s no coincidence.
“Clearly, our telescopes don’t affect the outer solar system, so the fact we get more simply reflects that we are getting better at finding them,” Hainaut said.
And we’re just getting started. The new Vera C. Rubin Observatory, which just released its first images, could discover many more interstellar objects like 3I/ATLAS during its decade-long Large Synoptic Survey Telescope (LSST) project. There could be plenty to find; a 2020 paper estimated that around seven interstellar objects could pass within one Earth-sun distance of the sun each year. We just haven’t been able to see them until now.
“It will be a dramatic improvement,” Hainaut said of the LSST. “Get ready for 4I, 5I … 42I!”
3I/ATLAS may be the brightest and biggest interstellar visitor yet, but it almost certainly won’t be the last.
