Trace Gas Orbiter Captures New Images of Interstellar Object 3I/ATLAS
During its closest approach to Mars on October 3, 2025, 3I/ATLAS was 30 million kilometers from ESA’s ExoMars Trace Gas Orbiter (TGO).
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.
TGO captured the new images from 3I/ATLAS with its Color and Stereo Surface Imaging System (CaSSIS).
“This was a very challenging observation for the instrument,” said Dr Nick Thomas, principal investigator of the CaSSIS instrument at ESA.
“3I/ATLAS is the slightly blurred white dot moving downward near the center of the images.”
“This point is the center of the comet, including its icy, rocky core and surrounding coma.”
“CaSSIS could not distinguish the coma core, because 3I/ATLAS was too far away.”
“The angular resolution of the CaSSIS camera is 11.36 microradians (equivalent to 2.34 arcseconds) per pixel,” said Professor Avi Loeb of Harvard University.
“At the shortest distance of 3I/ATLAS, around 30 million km, this resolution translates to 340 km.”
“This pixel size is 1 to 2 orders of magnitude larger than the expected core diameter of 3I/ATLAS, which falls between a minimum value of 5 km and the maximum value of 46 km.”
“Some of this expansion is visible in the CaSSIS images,” he said.
“The passage of 3I/ATLAS above the Martian sky is observed by Mars orbiters in a direction almost perpendicular to the 3I/ATLAS-Sun axis, allowing a side view of the scale of the glow extension around 3I/ATLAS.”
“The width of the bright glow around 3I/ATLAS in the CaSSIS image is about twice the diameter of the bright stars that serve as background point sources in the same image.”
“This extent corresponds to a scale of 680 km, an order of magnitude smaller than the width displayed in the Hubble image.”
“It is therefore evident that CaSSIS can only detect the brightest region around the core of 3I/ATLAS, while missing the lower surface brightness envelope that is apparent in the Hubble image.”
Between November 2 and 25, 2025, ESA’s Jupiter Icy Moons Explorer will observe 3I/ATLAS with various instruments. Image credit: ESA.
“Although our Mars orbiters continue to make impressive contributions to Martian science, it is always very exciting to see them respond to unexpected situations like this,” said Dr Colin Wilson, Mars Express and ExoMars project scientist at ESA.
“I can’t wait to see what the data reveals upon further analysis.”
Next month, ESA researchers will observe 3I/ATLAS with their Jupiter Icy Moons Explorer (Juice).
Although Juice is further away from 3I/ATLAS than Mars orbiters were last week, the spacecraft will see the comet just after its closest approach to the Sun, meaning it will be in a more active state.
“We do not expect to receive data from Juice observations until February 2026,” they said.
