The astronomers in the planetary defence team at ESA’s Near-Earth Object Coordination Centre, used to determining the trajectories of asteroids and comets, had to account for the spacecraft’s special location.

Usually, trajectory observations are made from fixed observatories on Earth, and occasionally from a spacecraft in near-Earth orbit, like the NASA/ESA Hubble Space Telescope or NASA/ESA/CSA James Webb Space Telescope. The astronomers are well-practiced in considering their location as they determine the future locations of objects, called ephemeris.

This time, the ephemeris of 3I/ATLAS, and in particular the prediction’s precision, depended on accounting for the exact location of ExoMars TGO: at Mars and in a fast orbit around it. It required working together in a combined effort by several ESA teams and partners, from flight dynamics to science and instrument teams. Challenges and subtleties that are usually negligible, had to be tackled to reduce the margins as much as possible, in order to achieve the highest accuracy possible.

The resulting data on comet 3I/ATLAS is the first time that astrometric measurements from a spacecraft orbiting another planet have been officially submitted and accepted into the Minor Planet Center (MPC) database. The database acts as a central clearing house for asteroid and comet observations, streamlining data collected by different telescopes, radar stations and spacecraft.