On July 1, 2025, astronomers at the Gemini South Observatory got their first look at 3I/ATLAS, the third interstellar object (ISO) ever observed traveling through our solar system. Since then, astronomers around the world have been working to narrow down its origin and predict where it might go next. This requires accurate calculations of the comet’s trajectory, which has been significantly improved thanks to an innovative approach using data from ESA’s ExoMars Trace Gas Orbiter (TGO), currently in orbit around Mars.
In addition to tracking the comet’s orbit, 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 came closest to Mars (within about 29 million kilometers), ISO’s location and trajectory could only be determined using ground-based telescopes. But as 3I/ATLAS flew past Mars, ESA’s ExoMars TGO and Mars Express orbiters were able to view the comet from a completely different angle.
As we reported when we published the orbiter images, capturing 3I/ATLAS was a major challenge. The main imaging cameras for both missions, including TGO’s Color and Stereo Surface Imaging System (CaSSIS), were designed to study brightly lit structures on the Martian surface from orbit. In this case, they tried to capture images of a small object about 30 million kilometers away. Additionally, there was the problem of accounting for TGO’s motion when acquiring images from 3I/ATLAS.
*The interstellar object 3I/ATLAS was captured by NASA’s Lucy mission on September 16, 2025. Image credit: NASA/Goddard/SwRI/JHUAPL*
This task fell to the planetary defense team at ESA’s Near-Earth Object Coordination Center (NEOCC), which determines the trajectories of asteroids and comets in our solar system. Typically, trajectory observations are carried out using ground-based telescopes, and occasionally with the help of space-based telescopes such as Hubble and the JWST. This time, 3I/ATLAS’s ephemeris prediction depended on taking into account the exact location of ExoMars TGO, as it orbited at speeds of up to 14,000 km/h (8,700 mph).
This required a collaborative effort by several ESA teams and partners, who triangulated the TGO data and combined it with ground-based telescope data to improve their predictions of 3I/ATLAS’s orbit by a factor of ten. This marks a first for astronomers: For the first time, data from a space probe orbiting another planet has been used to triangulate the position and trajectory of a celestial object. After coming closest to the Sun on October 30, the comet is now traveling at speeds of up to 250,000 km/h (~155,350 mph) and will fly past Earth on December 19.
The comet will pass at a safe distance of 270 million kilometers (170 million miles) on its way out of the solar system, almost twice the distance between the Sun and Earth. The improved trajectory means telescopes and spacecraft can train their instruments more precisely, enabling detailed observations that reveal more about the third interstellar object ever discovered.
Refining the trajectory of 3I/ATLAS based on the TGO 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 orbits of objects in our solar system. In short, this “trial” with 3I/ATLAS demonstrates the value of triangulating Earth-based data with observations from spacecraft, which may be closer to the objects and better positioned to provide data and measurements.
*Infographic showing the path of Comet 3I/ATLAS, the third known interstellar object to enter our solar system. Photo credit: ESA*
ESA continues to observe 3I/ATLAS courtesy of the Jupiter Icy Moons Explorer (JUICE), which is currently en route to the Jupiter system to study some of its largest moons (Ganymede and Europa). Although the data won’t be released until sometime next year, the data collected by JUICE should be very interesting. Now that 3I/ATLAS has emerged from behind the Sun, it is in a more active state and appears much brighter, with an unexpected blue color and more intense outgassing.
Meanwhile, ESA continues to develop the Comet Interceptor spacecraft, a mission expected to launch in 2029. This mission will rendezvous with a comet, either an ISO or one originating from the outer solar system, and study it closely. They are also preparing the Near-Earth Object Mission in the Infrared (NEOMIR), an orbiting asteroid spotter that will monitor Earth’s “blind spot” by searching for comets approaching from the Sun.
These and other missions will provide important data about the icy objects that fill our solar system and the universe at large. What they reveal could be revolutionary, perhaps shedding light on how water, chemical elements and the basic building blocks are distributed in the universe. These studies could answer the most fundamental questions – e.g. B. How did life arise here on Earth and where else in the universe could we find it?
Further reading: ESA
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