Comet 3I/ATLAS

Comet 3I/ATLAS, interstellar object and possibly the oldest known comet. It was discovered by the robotic survey project Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025.

Comet 3I/ATLAS is the third known interstellar object, that is, an object that has entered the solar system from interstellar space. Such objects have been scattered through gravitational interaction from where they originated in a planetary system around other stars. These objects give astronomers a rare chance to study conditions in other planetary systems up close.

Comet 3I/ATLAS has a velocity relative to the Sun of 58 km (36 miles) per second, the highest of the three known interstellar objects. The specific star where Comet 3I/ATLAS originated is unknown, but its high velocity suggests that it comes from the Milky Way Galaxy’s thick disk, a population of old stars, thus making it possibly the oldest known comet, with an age as high as 11 billion years. If Comet 3I/ATLAS is indeed that old, it would be likely that interstellar objects could have been easily formed early in the Milky Way Galaxy’s history.

Hubble Space Telescope observations of Comet 3I/ATLAS place its diameter between 320 meters and 5.6 km (1,050 feet and 3.5 miles). The comet had its closest approach to the Sun on October 29, 2025, at a distance of 1.36 astronomical units, or between the orbits of Earth and Mars. Its closest approach to Earth was at a distance of 270 million km (168 million miles) on December 19. There are no plans to send spacecraft to rendezvous with Comet 3I/ATLAS.