During its closest encounter with Mars on October 3, 2025, comet 3I/ATLAS was situated 30 million km from the ESA’s ExoMars Trace Gas Orbiter (TGO).
The image of interstellar comet 3I/ATLAS was taken on October 3, 2025, by the CaSSIS instrument aboard the ESA’s Trace Gas Orbiter. Image credit: ESA/TGO/CaSSIS.
TGO acquired new images of 3I/ATLAS utilizing the Color and Stereo Surface Imaging System (CaSSIS).
“This observation posed significant challenges for this instrument,” noted Dr. Nick Thomas, Principal Investigator of ESA’s CaSSIS instrument.
“3I/ATLAS appears as a slightly blurred white dot that descends toward the center of the image.”
“This point represents the nucleus of the comet, which comprises an icy, rocky core surrounded by a coma.”
“Due to the distance, CaSSIS couldn’t differentiate between a nuclear and a coma state.”
“The CaSSIS camera has an angular resolution of 11.36 microradians (equivalent to 2.34 arc seconds) per pixel,” explained Professor Avi Loeb from Harvard University.
“At a minimum distance of approximately 30 million km from 3I/ATLAS, this resolution translates to 340 km.”
“This pixel size is one to two orders of magnitude larger than the anticipated core diameters of 3I/ATLAS, which range from a minimum of 5 km to a maximum of 46 km.”
“Some of the expansion can be observed in CaSSIS images,” he mentioned.
“The passage of 3I/ATLAS across the Martian sky will be viewed by the Mars rover from an angle nearly perpendicular to the 3I/ATLAS-Sun axis, allowing for a side view of the glow surrounding 3I/ATLAS.”
“The width of the luminous glow around 3I/ATLAS in the CaSSIS image is approximately twice that of a bright star appearing as a background point source in the same image.”
“This span corresponds to a scale of 680 km, which is an order of magnitude smaller than the width seen in Hubble images.”
“Thus, it’s evident that CaSSIS only captures the brightest regions surrounding the core of 3I/ATLAS and cannot detect the low surface brightness envelope visible in Hubble images.”
From November 2 to 25, 2025, ESA’s Jupiter Icy Satellites probe will observe 3I/ATLAS with a range of instruments. Image credit: ESA.
“Our Mars rovers continue to contribute significantly to Mars science, and it’s always thrilling to see them respond to unforeseen scenarios like this,” remarked Dr. Colin Wilson, ESA’s Mars Express and ExoMars project scientist.
“We eagerly await the insights the data will reveal following further analysis.”
Next month, ESA researchers are set to observe 3I/ATLAS with the Jupiter Icy Satellite Orbiter (JUICE).
While JUICE will be located further from 3I/ATLAS than last week’s Mars rover, the rover will detect the comet shortly after its closest approach to the Sun, indicating heightened activity.
“Observation data for JUICE is anticipated to be received by February 2026,” they noted.
Source: www.sci.news
