European Space Agency's Mars orbiters have observed the third known interstellar comet, 3I/ATLAS, as it passed near the Red Planet. The faint visitor, discovered in July 2025, revealed a glowing coma in images taken from about 30 million kilometers away. Researchers are analyzing the data to uncover its composition and origins.
Between October 1 and 7, 2025, ESA's ExoMars Trace Gas Orbiter (TGO) and Mars Express spacecraft directed their instruments toward comet 3I/ATLAS during its closest approach to Mars on October 3, at a distance of about 30 million km. The two Mars orbiters provided the best vantage point among ESA's fleet for this rare event.
ExoMars TGO's Colour and Stereo Surface Imaging System (CaSSIS) successfully captured a series of images, showing the comet as a faint white spot—the icy, rocky nucleus enveloped in a glowing coma stretching several thousand kilometers. The coma forms as sunlight warms the comet, releasing gas and dust into a hazy halo, though the nucleus itself could not be resolved due to the distance. No tail was visible in these images, but it may appear as the comet nears the Sun and sheds more ice.
Mars Express has not yet detected the comet in its images, limited by 0.5-second exposures compared to TGO's five-second ones. Efforts continue to combine multiple exposures for better detection. Spectrometers on both orbiters—OMEGA and SPICAM on Mars Express, and NOMAD on TGO—attempted to analyze the comet's light spectrum, but success in identifying its chemical makeup remains uncertain.
Nick Thomas, principal investigator for CaSSIS, noted, "This was a very challenging observation for the instrument. The comet is around 10,000 to 100,000 times fainter than our usual target."
Comet 3I/ATLAS, first spotted on July 1, 2025, by the ATLAS telescope in Río Hurtado, Chile, is the third interstellar object observed, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Astronomers suspect it could be three billion years older than the 4.6-billion-year-old Solar System.
Upcoming observations by ESA's Jupiter Icy Moons Explorer (Juice) are planned next month, with data expected in February 2026. This event highlights preparations for the Comet Interceptor mission, set to launch in 2029 to intercept such visitors. Colin Wilson, Mars Express and ExoMars project scientist, said, "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. I look forward to seeing what the data reveals following further analysis."
Michael Küppers, Comet Interceptor project scientist, added, "When Comet Interceptor was selected in 2019, we only knew of one interstellar object—1I/ʻOumuamua, discovered in 2017. Since then, two more such objects have been discovered, showing large diversity in their appearance. Visiting one could provide a breakthrough in understanding their nature."