Comet 3I/ATLAS, a rare visitor from another star system, made its closest planetary encounter with Mars on October 3, 2025, passing approximately 18 million miles (29 million kilometers) from the Red Planet. This event provides scientists with a unique opportunity to study the composition and origins of objects from beyond our solar neighborhood. Discovered on July 1, 2025, by the ATLAS telescope in Chile, 3I/ATLAS is the third known interstellar object to traverse our solar system, following 1I/ʻOumuamua and 2I/Borisov. Its hyperbolic trajectory confirms its interstellar origin, ensuring it will continue its journey back into deep space. The comet is estimated to be from 0.32 to 5.6 kilometers wide and travels at approximately 68 kilometers per second relative to the Sun.
Multiple space agencies, including NASA and the European Space Agency (ESA), coordinated observations around Mars. ESA's Mars Express and ExoMars Trace Gas Orbiter and Mars Reconnaissance Orbiter (MRO) captured images as the comet passed within 30 million kilometers of Mars. NASA's Hubble Space Telescope had previously imaged 3I/ATLAS on July 21, 2025, when it was 277 million miles from Earth. The comet reached its perihelion, its closest approach to the Sun, on October 29, 2025, passing just inside Mars' orbit. Its closest approach to Earth will occur on December 19, 2025, at a distance of approximately 270 million kilometers.
While not visible to the naked eye, comet 3I/ATLAS can be observed with telescopes of 150–200 mm aperture. It was expected to be visible in the early morning sky in the constellation Virgo in late November 2025, with an estimated magnitude of 12–13. The study of such interstellar objects is crucial for understanding planetary formation and the diversity of celestial bodies across the galaxy. Early observations suggest that 3I/ATLAS possesses a chemical composition similar to many comets within our solar system, including water and carbon dioxide, hinting at potentially universal building blocks for planets. However, some research indicates unusual properties, such as a high nickel-to-iron ratio in its gaseous crust, and extreme negative polarization of light, which challenges current models of comet formation and activity. Some researchers also discuss its trajectory, suggesting it might not be accidental. This unique composition, along with its trajectory and speed, offers a rare window into the conditions and materials present in other star systems. Coordinated efforts by NASA and ESA, utilizing both Earth-based and space-based assets, underscore the scientific community's commitment to unraveling the mysteries of these cosmic travelers and expanding our understanding of the universe.