TGO provides more precise information on the path of 3I/ATLAS through the solar system
Using data collected by the Color and Stereo Surface Imaging System (CaSSIS) aboard ESA’s Trace Gas Orbiter (TGO) spacecraft orbiting Mars, scientists have improved the predicted location of interstellar comet 3I/ATLAS by a factor of 10.
This image of interstellar comet 3I/ATLAS was captured on October 3, 2025 by the CaSSIS instrument aboard ESA’s Trace Gas Orbiter. Image credit: ESA/TGO/CaSSIS.
3I/ATLAS was discovered by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope in Rio Hurtado, Chile, on July 1, 2025.
Until September, determining the location and trajectory of the interstellar comet relied on ground-based telescopes.
Between October 1 and 7, TGO looked toward 3I/ATLAS from its orbit around Mars.
The comet passed relatively close to Mars, coming within about 29 million kilometers during its closest phase on October 3.
The Mars probe got about 10 times closer to 3I/ATLAS than terrestrial telescopes and observed the comet from a new viewing angle.
Triangulating the TGO/CaSSIS data with Earth data helped make the comet’s predicted trajectory much more accurate.
While scientists initially expected a slight improvement, the result was an impressive gain in accuracy of a factor of 10.
“It was a challenge to use data from the Mars orbiter to refine the path of an interstellar comet in space,” TGO team members said in a statement.
“The CaSSIS instrument was designed to point toward the nearby Martian surface and observe it in high resolution.”
“This time, the camera was aimed at the sky above Mars to capture the tiny and distant 3I/ATLAS flying over a starry background.”
Astronomers from the planetary defense team at ESA’s Near-Earth Object Coordination Center, accustomed to determining the trajectories of asteroids and comets, had to take into account the spacecraft’s particular location.
“Usually, trajectory observations are made from fixed observatories on Earth, and sometimes from a spacecraft in near-Earth orbit, such as the NASA/ESA Hubble Space Telescope or the NASA/ESA/CSA James Webb Space Telescope,” the researchers explained.
“Astronomers are accustomed to taking their location into account when determining the future locations of objects, called ephemeris.”
“This time, the 3I/ATLAS ephemeris, and in particular the accuracy of the prediction, depended on the exact location of TGO: on Mars and in a fast orbit around it.”
“This required the joint work of several ESA teams and partners, from flight dynamics to scientific and instrumental teams.”
Challenges and subtleties that are usually negligible had to be overcome to reduce the margins as much as possible, in order to achieve the highest possible accuracy.
