In the early morning hours of Friday, December 19, you could spot the visitor from another solar system in the sky ― with a serious telescope, that is. Comet 3I/ATLAS flew by Earth at a distance of 270 million kilometers (168 million miles). That was its closest approach to our planet.
From here, the comet will continue its journey and pass by Jupiter in early 2026 before crossing the orbits of Saturn, Uranus and Neptune by 2028 and then leaving our solar system.
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 was about 670 million kilometers (416 million miles) away from the sun when it was first spotted. It poses no danger to planet Earth.
The comet is thought to be about 20 kilometers (12.4 miles) wide and traveling at about 60 kilometers per second (an impressive 134,000 miles per hour).
This data 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 confirmed it as an interstellar comet.
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 Richard Moissl, who heads the European Space Agency's Planetary Defense office, in an email to DW.
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 Earth. It's scheduled to launch in 2029.
The short (and obvious) answer is that comets, like 3I/ATLAS, 1I/'Oumuamua and 2I/Borisov, come from other planetary systems.
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Much like comets and asteroids 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 continue monitoring 3I/ATLAS to assess its composition and behavior. You could spot it from Earth — but you need "a big telescope to see it," wrote Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics in an email to DW.
Edited by: Fred Schwaller, Carla Bleiker
Editor's note: This article was originally published on July 4, 2025, and updated on December 19, 2025, with information on the comet passing by Earth.
