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The interstellar object known as 31/ATLAS, more formally designated 3I/ATLAS, will make its closest-ever approach to Earth on Friday, Dec. 19, giving astronomers and the public a once-in-a-universe opportunity to observe a visitor from another star system. The Virtual Telescope Project is planning a live global broadcast of the event. 

Scientists say this fleeting encounter offers an unprecedented chance to study how planetary systems beyond our own may form and evolve.

Discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile, 3I/ATLAS is only the third confirmed interstellar object ever detected passing through our solar system. It follows 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019.

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A Once-in-a-Cosmic-Lifetime Encounter

According to Space.com, Dec. 19 marks the moment when “this erratic wanderer will be the closest to Earth that it will ever be over the lifetime of the entire universe.” At that point, 3I/ATLAS will be about 168 million miles (270 million kilometers) from Earth.

While that may sound distant, astronomers emphasize that this is as close as humanity will ever get to this object.

“After billions of years of galactic solitude and just moments in the warmth of our sun, interstellar comet 3I/ATLAS makes its closest approach to Earth on Friday,” said Dr. Gary Blackwood, PhD, a retired NASA Jet Propulsion Laboratory astronomer. “On a hyperbolic orbit originating from the center of the Milky Way, this rare visitor will depart our solar system never to be seen again.”

Earlier in its journey, 3I/ATLAS reached perihelion—its closest point to the sun—on Oct. 29, passing within about 126 million miles (203 million kilometers) of our star.

31/ATLAS Alien Spacecraft?

Speculation surrounding 3I/ATLAS intensified after Harvard astrophysicist Dr. Avi Loeb suggested the object could be artificial in origin. However, follow-up observations strongly support a natural explanation.

Data from NASA’s Psyche mission and the European Space Agency’s Mars Trace Gas Orbiter show that the object is accelerating due to outgassing—jets of vaporized gas escaping from its surface as it warms, a hallmark of cometary behavior.

“We measured the non-gravitational acceleration using long-baseline astrometry,” said Marshall Eubanks, lead author of a paper published in Research Notes of the AAS, in comments to Spaceweather.com. “The results are pretty typical of ordinary comets, and certainly not record-breaking.”