A newly discovered object speeding through our solar system is sparking excitement among astronomers because it’s not from around here. Believed to be a comet, the object is only the third celestial body from beyond our solar system ever to be observed in our corner of the universe.
This interstellar visitor, now officially named 3I/ATLAS, became known when the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope in Chile reported spotting it on Tuesday. Since then, astronomers reviewing archival observations from multiple telescopes have tracked the object’s movements as far back as June 14 and found that the comet arrived from the direction of the Sagittarius constellation.
The comet’s speed and path through the solar system are two strong indicators that it originated beyond our solar system, said Gianluca Masi, astronomer and astrophysicist at the Bellatrix Astronomical Observatory in Italy and founder and scientific director of the Virtual Telescope Project. Masi has been making observations of the comet and will stream a live view of the object on the Virtual Telescope Project’s website.
The comet is moving at nearly 60km per second — or about 214,364 km per hour — too fast to be a “local” object in our solar system, said Teddy Kareta, an assistant professor at Villanova University near Philadelphia.
“Objects bound to the sun — denizens of our solar system — take paths around it that return to the same point,” Kareta wrote in an email. “The Earth’s orbit is mostly circular, Pluto’s orbit is a stretched oval, and many comets are very highly ‘eccentric’ — their orbits are very long and narrow ellipses. This object’s path through the solar system is very nearly a straight line.”
Tracking the object’s orbit also reveals the path it has taken to reach our solar system, said Dr Paul Chodas, director of NASA’s Center of Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory in Pasadena, California.
“When we extrapolate its motion backwards in time we see that it clearly originated from outside our Solar System,” Chodas wrote in an email. “It must have originated from another Solar System and probably has been travelling through interstellar space for millions of years until it happened to encounter our Solar System.”
Since the initial sighting of the comet, located 675 million km from Earth, astronomers have rushed to observe the object with telescopes around the world. One of those astronomers is Kareta, who observed the comet, using the Lowell Observatory’s Lowell Discovery Telescope in Flagstaff, Arizona, as soon as he heard about it on the night of its discovery. He said he believes it will only be a couple of weeks before just about every large telescope on Earth and in space has made time to spot and track the comet.
“People are excited. Almost every planetary astronomer I know immediately ran to a telescope or sent emails requesting telescope (observing) time in the next few days,” said Kareta, formerly a postdoctoral researcher at the Lowell Observatory. “While we might have several months to study this fascinating object, the earlier we can figure out how it works — how it is evolving, what strange or unexpected properties it might have — the quicker we can plan for the rest of its passage through the solar system.”
Anatomy of an interstellar object
Comet 3I/ATLAS follows two other intriguing interstellar objects, called ISOs, that once passed through our solar system: ‘Oumuamua in 2017 and 2I/Borisov in 2019. Both objects, also thought to be interstellar comets, stirred intense interest. The accelerated movements of cigar-shaped ‘Oumuamua even ignited claims that it could be an alien probe.
Little is known so far about comet 3I/ATLAS. Astronomers estimate its diameter to be 20km, with significant uncertainty due to the object’s brightness, Masi said.
However, the comet seems to be the brightest and fastest of the three interstellar objects discovered so far, Kareta noted.
3I/ATLAS is approaching our solar system from the Milky Way’s galactic center, a different direction than the previous objects, Chodas said.
The object has shown signs of cometary activity, including that it appears to be losing mass like a comet. Comets are made of ice, frozen gases and rock, and as they near stars such as the sun, heat causes them to release gas and dust, which creates their signature tails. But it’s not yet clear what kind of material is releasing from 3I/ATLAS or what process is causing it, Kareta said.
“Considering the lingering disagreements about what caused the orbital acceleration of the first ISO ‘Oumuamua, I’d be surprised if diagnosing and understanding this wasn’t a priority for most,” Kareta wrote in an email. “We don’t know where (3I/ATLAS) came from yet, but as our understanding of the object’s orbit (increases) we might be able to make some good guesses in a few months.”
Astronomers said that the comet poses no threat to Earth and will remain at least 240 million km from our planet. The comet is currently about 670 million km away from the sun and will make its closest approach to our star around October 30 at a distance of 210 million km, according to NASA.
The comet will also whip by Mars on October 2 at 30 million km from the red planet. This is a relatively close pass, astronomically speaking. For reference, Earth is about 150 million km from the sun.
The nearest the comet will come to Earth is 270 million km on December 19, Masi said.
Masi said the comet is currently visible in the Sagittarius constellation, which is best viewed from the southern sky in the middle of the night. While the full moon on July 10 will make 3I/ATLAS difficult to observe, observations even with small telescopes should improve in the coming months, he added.
Astronomers expect that the comet will remain visible for ground-based telescope observations through September before disappearing from view. It should reappear on the other side of the sun in early December, enabling follow-up observations. It will be observable well into mid-2026, Chodas said.
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Further study could reveal whether comets look the same in other solar systems, Kareta said.
Studying interstellar objects is also crucial to gaining a broader understanding of planets beyond our solar system and how they form, he added, describing these visitors as “some of the most fascinating things we’ve discovered.”
“They’re comets and asteroids which formed around other stars — the building blocks of planets around those faraway stars — which got ejected into interstellar space which we later find as they zip through our solar system,” Kareta said. “We want to measure everything we can about these objects to compare them to our own local comets and asteroids. They’re big questions, but the fact that we can make any progress on them by studying these fascinating objects should tell you why planetary astronomers are so excited to learn everything we can about them.”
