Farewell, comet 3I/ATLAS! Interstellar visitor heads for the outer solar system after its closest approach to Earth

On Friday (Dec. 19), the interstellar invader, comet 3I/ATLAS, made its closest approach to Earth, coming to within 168 million miles (270 million kilometers) of our planet at 1 a.m. EST (0600 GMT).

Following this close approach and the opportunity it offers to investigate this interloper from beyond the solar system, 3I/ATLAS will begin to move back out to the outer regions of the solar system, before leaving altogether to continue its voyage through the Milky Way.

3I/ATLAS is just the third body discovered passing through the solar system from interstellar space, its predecessors being 1I/'Oumuamua in 2017 and comet 2I/Borisov in 2019. As such, 3I/ATLAS has been offering scientists the unique opportunity to study the raw materials that came together to form comets, asteroids, and planets orbiting stars other than the sun.

First spotted by NASA's Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025, with its trajectory indicating its origin lies elsewhere in the Milky Way.

In fact, its path through space suggests that this interstellar comet comes from a region of our galaxy that is much older than the 4.6 billion-year-old solar system.

The water-rich comet seems to originate from the Milky Way's "thick disk" of stars, rather than the thin stellar disk of which the sun is a member. The thick disk formed earlier than the thin disk, meaning 3I/ATLAS could be up to 7 billion years old.

"All non-interstellar comets, such as Halley's comet, formed at the same time as our solar system, so they are up to 4.5 billion years old," University of Oxford astronomer Matthew Hopkins said in a statement back in July. "But interstellar visitors have the potential to be far older, and of those known about so far, our statistical method suggests that 3I/ATLAS is very likely to be the oldest comet we have ever seen."

During its time within the solar system, 3I/ATLAS has continued to surprise astronomers. As it began to make its closest approach to the sun, or its perihelion, on Oct. 29, the comet brightened more than scientists had expected.

Comets tend to brighten as they approach our star due to solar radiation heating their icy cores and causing solid ice to transform straight into vapor, which erupts from the comet, growing its halo or "coma" and its characteristic glowing tail.

Quite why 3I/ATLAS brightened more than expected as observed by STEREO-A and STEREO-B, the twin spacecraft that make up Solar Terrestrial Relations Observatory (STEREO), the sun observing Solar and Heliospheric Observatory (SOHO), and the weather satellite GOES-19, is still unknown.

"The reason for 3I’s rapid brightening, which far exceeds the brightening rate of most Oort cloud comets at similar r [radial distance], remains unclear," the scientists behind the research, Qicheng Zhang of Lowell Observatory in Flagstaff, Arizona, and Karl Battams, an astrophysicist at the Naval Research Laboratory (NRL) in Washington DC, wrote in a paper discussing the observation published on the research repository site arXiv.

Though 3I/ATLAS is now in the final stage of its occupation of the solar system, the data collected from it will likely inform scientists for some time to come, continuing to paint a more intricate picture of the Milky Way beyond the solar system.

Using NASA’s Eyes on the Solar System interactive app, you can track comet 3I/ATLAS through the solar system and see where it’s headed next.