Thanks to data collected by the Color and Stereo Surface Imaging System (CaSSIS) on board 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 on board ESA's Trace Gas Orbiter. Image credit: ESA/TGO/CaSSIS.
3I/ATLAS Discovered by the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) research telescope in Rio Hurtado, Chile on July 1, 2025.
Until September, figuring out the location and trajectory of the interstellar comet relied on ground-based telescopes.
From 1 to 7 October TGO turned around his gaze is directed towards 3I/ATLAS from its orbit around Mars.
The comet passed relatively close to Mars, coming within about 29 million km during its closest phase on October 3.
The Mars probe came about 10 times closer to 3I/ATLAS than telescopes on Earth, and observed the comet from a new angle.
Triangulating the TGO/CaSSIS data with data from Earth helped make the comet's predicted path much more accurate.
Although the scientists initially expected a modest improvement, the result was an impressive 10-fold jump in accuracy.
“It was not easy to use data from the Mars orbiter to refine the path of an interstellar comet in space,” the TGO team said.
“The CaSSIS instrument was designed to point to the nearby surface of Mars and view it in high resolution.”
“This time the camera was pointed at the sky above Mars to catch tiny, distant 3I/ATLAS streaking across the starry background.”
Astronomers from the planetary defense team in Near-Earth Object Coordination Centre, ESAused to determine the trajectories of asteroids and comets, had to take into account the specific location of the spacecraft.
“Typically, trajectory observations are made from stationary observatories on Earth and sometimes from spacecraft in Earth orbit, such as the NASA/ESA Hubble Space Telescope or the NASA/ESA/CSA James Webb Space Telescope,” the researchers said.
“Astronomers are good at taking their location into account when determining the future location of objects called ephemeris.”
“This time, the 3I/ATLAS ephemeris, and in particular the accuracy of the forecast, depended on the exact location of TGO: on Mars and in rapid orbit around it.”
“This required the collaboration of multiple ESA teams and partners, from flight dynamics to science and instrument teams.”
Problems and subtleties that would normally be ignored had to be addressed in order to reduce margins as much as possible and achieve the highest possible accuracy.”
