On July 1, 2025, astronomers at the Gemini South Observatory observed 3I/ATLAS, the third interstellar object detected in our solar system. Since then, scientists worldwide have been working on pinpointing its origin and predicting its trajectory. This endeavor has been significantly enhanced by data from a spacecraft orbiting Mars. Until October, when 3I/ATLAS made its closest approach to Mars, its location and path were determined primarily through ground-based telescopes. However, as the comet passed by Mars, ExoMars TGO and Mars Express observed it from a new angle. The primary cameras of both missions, designed for observing well-lit objects on Mars’s surface from orbit, attempted to image a faint object approximately 30 million kilometers away.
Determining the trajectory of 3I/ATLAS became a task for the Planetary Defense Team at the ESA’s Near-Earth Object Coordination Centre (NEOCC), typically involved in calculating the trajectories of asteroids and comets within our solar system. This time, predicting 3I/ATLAS’s coordinates hinged on the precise positioning of ExoMars TGO, which orbits at speeds reaching 8,700 mph. This challenge required concerted efforts from multiple teams to triangulate TGO data with ground-based telescope data, enhancing the accuracy of 3I/ATLAS’s trajectory prediction tenfold. This marked the first instance of using spacecraft data from another planet to triangulate a celestial object’s position and trajectory.
The comet reached its closest point to the Sun on October 30 and is now speeding through space at up to 155,000 mph. On December 19, it will pass Earth at a safe distance of 167 million miles, almost double the distance between Earth and the Sun.
Refining 3I/ATLAS’s trajectory with TGO data has also provided invaluable experience in planetary defense. Although this comet poses no threat to Earth, it allows scientists to test capabilities for monitoring, tracking, and predicting solar system object trajectories. ESA continues to monitor 3I/ATLAS using the Jupiter Icy Moons Explorer (JUICE), targeting Jupiter’s largest moons (Ganymede and Europa). Data collected by JUICE is expected to be released next year.
Since emerging from behind the Sun, 3I/ATLAS has entered a more active state, becoming brighter, exhibiting an unexpected blue hue, and showing increased gas emissions. These findings could reveal insights into its composition, possibly suggesting a higher presence of volatile materials, unleashing gases as it reacts to solar heat. As scientific interest grows, continued observations could unlock broader understandings of similar interstellar travelers.
