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The two Mars orbiters captured the closest images of the comet among all ESA spacecraft. When the interstellar comet reached its closest point to the Red Planet on October 3, it was 30 million kilometers away from them.

Both spacecraft used their specialized cameras to track the comet’s passage. The cameras were designed to photograph the bright surface of Mars from distances ranging from a few hundred to several thousand kilometers. The scientists were unsure of what to expect from observing such a relatively faint target at such a distance.

ExoMars TGO, using the Colour and Stereo Surface Imaging System (CaSSIS), captured a series of images displayed in the GIF below. The comet 3I/ATLAS is the slightly blurred white dot moving downwards in the center of the image. This dot is the center of the comet, consisting of its icy rocky core and the surrounding coma of cometary dust.

CaSSIS could not distinguish the core from the coma since 3I/ATLAS was too far away. Imaging the core from such a distance would be as impossible as seeing a mobile phone on the Moon from Earth.

However, the coma, several thousand kilometers across, was clearly visible. The coma forms as 3I/ATLAS approaches the Sun. The Sun’s heat and radiation excite the comet, causing it to release gases and dust, which accumulate in a halo surrounding the core.

CaSSIS couldn’t measure the exact size of the coma because the brightness of the dust rapidly decreased as it moved away from the core. This meant the coma merged with the noise in the image.

Typically, the material from the coma transforms into a long tail, which can extend millions of kilometers in length as the comet nears the Sun. The tail is much dimmer than the coma. In CaSSIS images, we cannot see the tail, but as the comet heats up and releases more ice, it may become more visible in future observations.

Nick Thomas, principal investigator of the CaSSIS camera, explains: “This was a very challenging observation for our instrument. The comet was about 10,000 to 100,000 times dimmer than our usual targets.”

Thomas noted that 3I/ATLAS had not yet shown up in Mars Express images, explaining that this was because the images from Mars Express were taken with a 0.5-second exposure time (the maximum for Mars Express), compared to 5 seconds for ExoMars TGO.

Scientists will continue to analyze the data gathered from both orbiters. By combining several images from Mars Express, they will see if they can detect the faint tail of the comet.

Additionally, they used the OMEGA and SPICAM spectrometers on Mars Express and the NOMAD spectrometer on ExoMars TGO to measure the light spectrum of 3I/ATLAS. According to the scientists, it is still unclear whether the coma and tail of the comet are bright enough for spectral characterization.

Scientists will continue to analyze the data in the coming weeks and months to learn more about the structure of 3I/ATLAS and how it behaves as it approaches the Sun.

ESA’s Mars Express and ExoMars project scientist Colin Wilson said:

“While our Mars orbiters continue to make impressive contributions to Mars science, it’s always extra exciting to see them respond to such unexpected situations. I’m eagerly awaiting what the data will reveal after further analysis.”