New Delhi : On July 1st, 2025, astronomers at the Gemini South Observatory got their first look at 3I/ATLAS, the third interstellar object (ISO) ever observed passing through our Solar System. Since then, astronomers worldwide have been working to constrain its origin and predict where it might be headed next. This requires accurate calculations about the comet’s trajectory, which has improved considerably thanks to an innovative approach using data provided by the ESA’s ExoMars Trace Gas Orbiter (TGO), currently in orbit around Mars.
In addition to tracing the comet’s path, this research provides a valuable test case for planetary defense. By knowing an object’s trajectory, astronomers can determine whether a comet or asteroid poses a threat to Earth. Before October, when 3I/ATLAS was at its closest approach to Mars (within ~29 million km; 18 million mi), the location and trajectory of the ISO could only be determined from ground-based telescopes. But when 3I/ATLAS flew past Mars, the ESA’s ExoMars TGO and Mars Express orbiters were both able to view the comet from an entirely different angle.
As we reported during the release of the orbiters’ images, capturing 3I/ATLAS was a major challenge. The main imaging cameras for both missions, including the TGO’s Colour and Stereo Surface Imaging System (CaSSIS), were designed to study brightly lit features on the Martian surface from orbit. In this case, they were trying to capture images of a small object roughly 30 million km (18.64 million mi) away. In addition, there was the problem of accounting for TGO’s movement as it captured images of 3I/ATLAS.
This task fell to the planetary defense team at the ESA’s Near-Earth Object Coordination Center (NEOCC), who are used to determining the trajectories of asteroids and comets in our Solar System. Typically, trajectory observations are performed using ground-based telescopes, with occasional input from space-based telescopes such as Hubble and the JWST. This time, predicting the ephemeris of 3I/ATLAS depended on accounting for the exact location of ExoMars TGO as it orbited at speeds of up to 14,000 km/h (8,700 mph).
This required a combined effort involving several ESA teams and partners, who triangulated the TGO data and combined it with Earth-based telescope data to improve their predictions of 3I/ATLAS’ path by a factor of ten. This represents a first for astronomers: the first time data from a spacecraft orbiting another planet were used to triangulate the position and trajectory of a celestial object. Having made its closest pass to the Sun on Oct. 30th, the comet is now traveling at speeds of up to 250,000 km/h (~155,350 mph) and will pass by Earth on Dec. 19th.
The comet will pass at a safe distance of 270 million kilometers (170 million miles), almost twice the distance between the Sun and the Earth, on its way out of the Solar System. The improved trajectory will mean that telescopes and spacecraft can train their instruments with greater accuracy, enabling detailed observations that will reveal more about the third interstellar object ever detected.
The refining of 3I/ATLAS’ trajectory based on the TGO’s data was also a valuable exercise for planetary defense. While this latest interstellar visitor poses no threat to Earth, its passage around the Sun allows scientists to test their ability to monitor, track, and predict the paths of objects in our Solar System. In short, this “rehearsal” with 3I/ATLAS demonstrates the value of triangulating Earth-based data with spacecraft observations, which may be closer to the objects and better positioned to provide data and measurements.
