Beyond the fact that this is the third known interstellar object to have entered our solar system, “we don’t know very much,” said Larry Denneau, co-principle investigator at ATLAS, a telescope in Chile that spotted 3I/ATLAS on July 1, 2025.
It’s not exactly reassuring when scientists say “we don’t know,” but at least it’s honest.
Astronomers do know that 3I/ATLAS is a comet that’s about 670 million kilometers (416 million miles) away from the sun. Based on current projections, it poses .
“Scientists are still determining the velocity and trajectory to a degree that will allow accurate predictions for the future,” wrote Richard Moissl, who heads the ‘s Planetary Defense office, in an email to DW.
The closest it will get to our planet is about 240 million km away, when it will fly by in October. That is more than 1.5 times the distance between us and , and about 624 times the distance between the Earth and our . It is also thought to be about 20 km (12.4 miles) wide and traveling at about 60 km per second (an impressive 134,000 miles per hour).
But this is all relatively basic data — the very data that allowed astronomers at the Asteroid Terrestrial-impact Last Alert System in Chile to spot it. When they saw the object on an unusual trajectory, they immediately began to track and measure it.
Then, other astronomers based at telescopes in Hawaii and Australia, began monitoring the object’s flight progress and confirm it as an interstellar comet.
“We are seeing an onset of [normal] cometary activity,” wrote Moissl.
Open questions about comet 3I/ATLAS?
Comet 3I/ATLAS flew through the heliosphere to enter our solar system. The heliosphere is a barrier that protects us from interstellar winds and radiation.
The heliosphere is, however, an imperfect barrier — some interstellar radiation gets through, and it clearly doesn’t stop icy intergalactic wanderers, like 3I/ATLAS.
Interstellar objects in our solar system are thought to be quite rare, though. The first known interstellar object was 1I/’Oumuamua, detected in 2017, and 2I/Borisov, detected in 2019.
“This is only the third interstellar [object] ever to be detected, hence a precise forecast of the expected frequency is not possible at this point,” wrote Moissl.
But telescopes have got more technologically advanced and scientists do now scan the night sky continuously. So, we may begin to see more of them.
“The Legacy Survey in Space and Time at the Vera Rubins telescope in Chile goes online this year. It is more efficient than existing surveys and expected to detect several new Interstellar Objects over the next 10 years,” said Moissl’s colleague at ESA, Michael Kueppers.
Kueppers is a Comet Interceptor Project Scientist. Comet Interceptor is a spacecraft that will rest in a “parking orbit” and intercept distant comets and asteroids if they come too close to planet Earth. It’s scheduled to launch in 2029.
Where did comet 3I/ATLAS come from?
The short (and obvious) answer is that comets, like 3I/ATLAS, 1I/’Oumuamua and 2I/Borisov, come from other planetary systems.
Much like within our solar system, interstellar objects are considered to be untouched specimens from elsewhere in our galaxy, the Milky Way — if not fragments from the very beginnings of the universe.
Moissl said this new object “came roughly from the direction of the Galactic Center region,” which, as the name suggests, is towards the center of the Milky Way. But astronomers do not know its precise origin or “home star”.
Based on its brightness, 3I/ATLAS appears to be bigger than the other two stray comets — 1I/’Oumuamua and 2I/Borisov — which are thought to have entered our solar system from a different region of the Milky Way.
Astronomers will want to continue monitoring 3I/ATLAS to assess its composition and behavior. ESA said that as an active comet, it may heat up as it gets closer to Earth, and “sublimate” — that’s when frozen gases on a comet turn into vapor, creating a glowing coma and trail of dust and ice particles.
You should be able to see it from Earth with a telescope by September. When it’s closest to Earth, it will be hidden by the sun, but then reappear by early December.
Edited by: Fred Schwaller
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