On Friday, Dec. 19, the interstellar object known as comet 3I/ATLAS reached its nearest point to Earth. At approximately 1 a.m. EST (0600 GMT), the comet passed within about 168 million miles, or 270 million kilometers, of our planet.
After this flyby, which allowed astronomers a rare chance to closely examine a visitor from beyond the solar system, 3I/ATLAS has begun its journey back toward the outer reaches of the solar system. From there, it will eventually exit entirely and continue traveling through the Milky Way.
Comet 3I/ATLAS is only the third confirmed interstellar object ever observed passing through our solar system. The first two were 1I/‘Oumuamua in 2017 and comet 2I/Borisov in 2019. This makes 3I/ATLAS especially valuable, as it provides scientists with a rare opportunity to study material formed around stars other than the sun.
The comet was first detected on July 1, 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS). Early observations of its trajectory quickly revealed that it did not originate within our solar system, but instead came from elsewhere in the Milky Way.
Further analysis of its path suggests that 3I/ATLAS likely formed in a region of the galaxy that predates the solar system itself. While the solar system is about 4.6 billion years old, this comet appears to come from a much older stellar population.
Researchers believe the water-rich comet originated in the Milky Way’s “thick disk” of stars. This region formed earlier than the thin disk, which includes the sun, indicating that 3I/ATLAS could be as old as 7 billion years.
“All non-interstellar comets, such as Halley’s Comet, formed alongside our solar system and are therefore up to about 4.5 billion years old,” said University of Oxford astronomer Matthew Hopkins in a statement released in July. “Interstellar visitors, however, can be significantly older, and based on our statistical analysis, 3I/ATLAS is very likely the oldest comet ever observed.”
Throughout its passage through the solar system, 3I/ATLAS has continued to puzzle astronomers. As it approached perihelion, its closest point to the sun on Oct. 29, the comet brightened far more than scientists had predicted.
Normally, comets grow brighter as they near the sun because solar heat causes frozen material in their nuclei to turn directly into gas. This process enlarges the surrounding cloud, known as the coma, and enhances the comet’s glowing tail.
However, observations from the STEREO-A and STEREO-B spacecraft, the Solar and Heliospheric Observatory (SOHO), and the GOES-19 weather satellite showed that 3I/ATLAS brightened at an unusually rapid rate. The exact cause of this behavior remains unknown.
“The reason for 3I’s rapid brightening, which greatly exceeds that of most Oort cloud comets at similar distances from the sun, is still unclear,” wrote Qicheng Zhang of the Lowell Observatory and Karl Battams of the U.S. Naval Research Laboratory in a paper published on the arXiv research repository.
Although 3I/ATLAS is now nearing the end of its stay in the solar system, the wealth of data collected during its visit will continue to be analyzed for years. These findings are expected to deepen scientists’ understanding of the broader Milky Way and the materials that exist beyond our own planetary neighborhood.
For those interested in following its journey, comet 3I/ATLAS can still be tracked using NASA’s interactive Eyes on the Solar System application, which shows its current position and future path through space.