“The Mars probe got about 10 times closer to 3I/ATLAS than telescopes on Earth, and it observed the comet from a new viewing angle,” the European Space Agency said, underlining another important milestone in deep-space navigation and readiness for planetary defense.
Between October 1 and 7, 2025, ESA’s ExoMars Trace Gas Orbiter, TGO, turned its Colour and Stereo Surface Imaging System, CaSSIS, away from Mars and toward the interstellar comet 3I/ATLAS during its closest recorded planetary encounter-just 18 million miles (29 million kilometers) from the Red Planet on October 3. This viewpoint, along with Earth-based telescope data, allowed for a triangulation that bettered the comet’s predicted trajectory by a factor of ten, far exceeding scientists’ expectations of a modest gain.
Such precision relies on calculating an object’s ephemeris-its future positions in space-by integrating multiple observation points. From Mars orbit, TGO’s CaSSIS faced technical challenges: The system is optimised for imaging Mars’ bright surface at distances of hundreds to thousands of kilometres, not faint targets tens of millions of kilometres away. Engineers had to account for the orbiter’s rapid motion around Mars, its exact position during each exposure, and the comet’s own 155,000 mph (250,000 km/h) velocity. The resulting astrometric measurements were accurate enough to be accepted into the Minor Planet Center’s database-the first time such data came from a spacecraft orbiting another planet.
The gain made in trajectory prediction is more than an academic success. Indeed, planetary defense strategies depend on early and precise orbit determination to maximize warning times and inform potential mitigation measures. As noted by Richard Moissl of ESA, the exercise with 3I/ATLAS enabled coordination between ESA, NASA, and several mission teams without pressure or hazard, building confidence that similar operations could be carried out for genuinely hazardous objects.
The comet itself presents a wealth of scientific returns. Discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System, 3I/ATLAS is only the third confirmed interstellar object, following 1I/‘Oumuamua and 2I/Borisov. Originating from the direction of the Milky Way’s thin disk, it likely spent billions of years in interstellar space before entering the solar system. Early spectroscopic and photometric campaigns showed a 16.16-hour spin period, light curve amplitude of 0.3 magnitudes, and dust mass-loss rates between 0.3 and 4.2 kg/s-evidently of a weakly active comet dominated by large dust grains. Near perihelion on October 29, 2025, it began displaying multiple jets as solar radiation drove volatile release.
Such tracking of a fast-moving, faint target from multiple points in the solar system is rare. Before TGO’s intervention, observations came only from Earth-based telescopes or Earth-orbiting platforms like the James Webb Space Telescope, each constrained by geometry and distance. This Mars-based perspective reduced the positional uncertainty so that astronomers could point instruments with confidence during the short window of visibility the comet will have before it disappears into interstellar space.
The approach that ESA uses is no different from methods applied elsewhere, such as location-based services making use of several cell towers, extended to astronomical scales. With large baselines-on the order of tens of millions of kilometers between Mars and Earth-the angular measurement differences are amplified into substantial improvements in trajectory precision. This concept can be extended to future missions: platforms positioned at different planetary orbits or even at Lagrange points could constitute a distributed network for continuous monitoring of potentially hazardous objects.
The next step in observation will be provided by ESA’s Jupiter Icy Moons Explorer, JUICE, presently on the far side of the Sun from Earth. Between November 2 and 25, 2025, JUICE captured data during the most active phase of 3I/ATLAS, with a bright coma and extended tail. These observations, delayed by low data transmission rates, are expected to arrive in February 2006 and complement the refined trajectory data with detailed physical characterization.
Beyond the immediate science, the 3I/ATLAS campaign foreshadows the operational readiness for missions like NeoMIR-to monitor Sun-approaching regions invisible to ground-based telescopes-and Comet Interceptor-to rapidly deploy towards newly discovered comets, possibly interstellar ones. Mars-based tracking has demonstrated that interplanetary assets can be repurposed for planetary defense, turning them into nodes in a solar system-wide early warning network.
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