During its closest approach to Mars on October 3, 2025, 3I/ATLAS was 30 million km away from ESA’s ExoMars Trace Gas Orbiter (TGO).
This image of the 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 of 3I/ATLAS with its Colour 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 fuzzy white dot moving downwards near the center of the images.”
“This dot is the center of the comet, comprising its icy-rocky nucleus and its surrounding coma.”
“CaSSIS could not distinguish the nucleus from the coma, because 3I/ATLAS was too far away.”
“The angular resolution of the CaSSIS camera is 11.36 micro-radians (equivalent to 2.34 arcseconds) per pixel,” said Harvard University’s Professor Avi Loeb.
“At the shortest distance from 3I/ATLAS of about 30 million km, this resolution translates to 340 km.”
“This pixel size is 1-2 orders of magnitude larger than the expected diameter of the nucleus of 3I/ATLAS, which ranges between a minimum value of 5 km and the maximum diameter value of 46 km.”
“Some of that extension is visible in the CaSSIS images,” he said.
“The passage of 3I/ATLAS over the Martian sky is viewed by Mars orbiters at a nearly perpendicular direction to the 3I/ATLAS-Sun axis, allowing a sideway view of the scale of the extension of the glow around 3I/ATLAS.”
“The width of the bright glow around 3I/ATLAS in the CaSSIS image is about twice the diameter of bright stars which 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 nucleus of 3I/ATLAS, while missing the lower surface-brightness envelope which 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.
“Though our Mars orbiters continue to make impressive contributions to Mars science, it’s always extra exciting to see them responding to unexpected situations like this one,” said Dr. Colin Wilson, Mars Express and ExoMars project scientist at ESA.
“I look forward to seeing what the data reveals following further analysis.”
Next month, the ESA researchers will observe 3I/ATLAS with their Jupiter Icy Moons Explorer (Juice).
Though Juice will be further from 3I/ATLAS than the Mars orbiters were last week, the spacecraft will see the comet just after its closest approach to the Sun, meaning that it will be in a more active state.
“We don’t expect to receive data from Juice’s observations until February 2026,” they said.
