Earth has just said goodbye to one of the rarest cosmic visitors ever observed. On December 19, interstellar comet 3I/ATLAS made its closest approach to our planet before beginning its long journey back out of the solar system, offering scientists a brief but invaluable opportunity to study material formed far beyond our own stellar neighborhood, as reported by Space.com.
At its nearest point, the comet passed within about 168 million miles, or 270 million kilometers, of Earth. While that distance posed no danger, it was close enough for telescopes and space-based observatories to gather detailed data. Following this flyby, 3I/ATLAS is now heading outward, destined to leave the solar system entirely and continue its lonely voyage through the Milky Way.
What makes 3I/ATLAS so special is its origin. It is only the third confirmed interstellar object ever detected passing through our solar system. The first was the mysterious 1I/?Oumuamua in 2017, followed by comet 2I/Borisov in 2019. Unlike ordinary comets that formed alongside the planets around our sun, interstellar visitors are born around other stars and later ejected into interstellar space.
First spotted on July 1, 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System, or ATLAS, the comet’s unusual trajectory quickly revealed that it did not originate within the solar system. Further analysis suggests something even more remarkable. Scientists believe 3I/ATLAS likely comes from the Milky Way’s thick disk, an older population of stars that formed well before the sun.
That origin implies the comet could be extraordinarily ancient. While comets native to our solar system are at most about 4.5 billion years old, researchers estimate that 3I/ATLAS could be as old as 7 billion years. As University of Oxford astronomer Matthew Hopkins explained earlier this year, that would make it the oldest comet humanity has ever observed.
During its passage, the comet also surprised astronomers with its behavior. As it approached the sun in late October, it brightened far more rapidly than expected. Comets usually grow brighter as solar heat vaporizes their icy surfaces, forming glowing comas and tails. However, observations from spacecraft including STEREO, SOHO, and GOES-19 showed that 3I/ATLAS outpaced typical comets at similar distances from the sun. The reason for this unusually rapid brightening remains unclear.
Although 3I/ATLAS is now leaving, its scientific impact is far from over. The data collected during its brief visit will be studied for years, helping researchers better understand how planetary systems form around other stars and how material moves between them. In that sense, while the comet is departing, the story it tells about the wider galaxy is only just beginning.
