The discovery of comet 3I/ATLAS, the third known interstellar object, presented a unique opportunity for astronomers. While it poses no danger, its rapid trajectory through our Solar System offered a valuable test case for planetary defence. Initially, predictions relied solely on Earth-based telescopes, but a breakthrough came from an unexpected source: the ExoMars Trace Gas Orbiter (TGO) at Mars.
Between 1-7 October, the TGO spacecraft, orbiting Mars, turned its camera towards the comet. This provided a new viewing angle and a crucial advantage; the Mars probe was ten times closer to 3I/ATLAS than any Earth-based instrument. By triangulating this new data with existing Earth observations, astronomers at ESA’s Near-Earth Object Coordination Centre achieved a ten-fold improvement in the accuracy of the comet’s predicted path, or ephemeris. This was a complex challenge, as the CaSSIS instrument was designed for the martian surface, not for tracking a faint, distant comet against a starry backdrop. Accounting for the precise, fast-moving location of the TGO spacecraft was essential.
This success marks the first time that astrometric measurements from a spacecraft orbiting another planet have been officially accepted into the Minor Planet Center (MPC) database. This rehearsal demonstrates the power of leveraging diverse orbital assets to refine the trajectory of fast-moving objects, a critical skill for any future asteroid threat. The improved prediction allows astronomers to confidently aim their instruments at the vanishing comet, enabling more detailed science on this interstellar visitor.
Looking ahead, observations continue with the Jupiter Icy Moons Explorer (Juice), which will study the comet in its more active post-perihelion state. For future planetary defence, ESA is developing the dedicated Neomir mission to detect objects hidden by the Sun’s glare. Furthermore, the upcoming Comet Interceptor mission aims to visit a pristine comet, potentially even an interstellar one, deepening our connection to the wider galaxy.
