Thanks to observations from the European Space Agency’s Trace Gas Orbiter (TGO) currently orbiting Mars, scientists have dramatically improved the accuracy of predicted trajectory for the interstellar comet 3I/ATLAS. The new data, gathered by the Colour and Stereo Surface Imaging System (CaSSIS) instrument, has refined the comet’s path by a factor of ten—a significant leap in precision.
Discovery and Early Tracking
The comet was first detected on July 1, 2025, by the NASA-funded ATLAS survey telescope in Chile. For months afterward, Earth-based telescopes were the sole source of tracking data. This meant that calculations about its orbit were less precise than they could be.
The TGO Advantage
Between October 1 and 7, the TGO adjusted its focus to observe 3I/ATLAS as it passed relatively close to Mars—within approximately 29 million kilometers at its closest approach on October 3. Unlike Earth-bound observers, the Mars orbiter was roughly ten times closer to the comet, providing a unique vantage point.
The combination of TGO’s data with existing ground-based observations allowed for a powerful triangulation effect, dramatically enhancing the accuracy of the comet’s predicted trajectory.
Unique Challenges in Space-Based Tracking
The TGO team faced several unique hurdles. The CaSSIS instrument is designed to image the Martian surface at high resolution. Adapting it to track a distant, fast-moving comet required significant adjustments.
“It was a challenge to use the Mars orbiter’s data to refine an interstellar comet’s path through space,” said team members in a statement.
Furthermore, planetary defense astronomers had to account for the spacecraft’s specific location—orbiting Mars at high speed—when calculating the comet’s ephemeris. This is unlike standard calculations, which rely on fixed observatories on Earth or spacecraft in near-Earth orbit like Hubble or Webb.
“This time, the ephemeris of 3I/ATLAS, and in particular the prediction’s precision, depended on accounting for the exact location of TGO: at Mars and in a fast orbit around it.”
The success required close coordination between ESA teams, including flight dynamics, science, and instrument specialists. Subtle factors typically ignored in trajectory calculations became critical to maximizing accuracy.
The improved tracking is a testament to how multiple perspectives—Earth-based and space-based—can work together to refine our understanding of interstellar objects. The ability to precisely predict the path of such comets is crucial for planetary defense, even though 3I/ATLAS poses no immediate threat. This exercise demonstrates a new method for tracking objects in deep space.
