ESA's ExoMars and Mars Express Observe Comet 3I/ATLAS

Each spacecraft utilized its specialized cameras to capture images of the comet's passage. These cameras are generally used to photograph the surface of Mars, which lies just a few hundred to a few thousand kilometers below. Scientists had uncertain expectations regarding observations of such a relatively dim target from a significant distance. ExoMars TGO obtained a series of images using its Colour and Stereo Surface Imaging System (CaSSIS), showing Comet 3I/ATLAS as a faint white dot, representing the icy-rocky nucleus encased in its coma.

Due to the distance, CaSSIS could not distinguish the nucleus from the coma, making it impractical to identify the kilometer-wide nucleus as one would not see a mobile phone on the Moon from Earth. The coma, which extends several thousand kilometers, is visible and forms as the comet approaches the Sun. The Sun's heat and radiation trigger gas and dust release from the comet, creating a halo around the nucleus. However, CaSSIS could not fully measure the coma's size due to its brightness diminishing with distance.

Typically, the coma's material forms a long tail, which can stretch millions of kilometers when the comet nears the Sun, but this tail remains too faint to see in the CaSSIS images. Future observations may reveal the tail as the comet emits more ice.

Nick Thomas, Principal Investigator of the CaSSIS camera, noted the challenge of capturing this observation, stating that the comet was considerably fainter than typical targets, estimated to be around 10,000 to 100,000 times dimmer.

The Mars Express, with a shorter exposure time of just 0.5 seconds, has not yet captured clear imagery of 3I/ATLAS. Researchers plan to pool multiple images from Mars Express to identify the comet. They also attempted to analyze the spectrum of light from 3I/ATLAS using spectrometers aboard both Mars Express and ExoMars TGO. It remains uncertain if the coma and tail were bright enough for effective spectral characterization.

Colin Wilson, project scientist for Mars Express and ExoMars, expressed excitement about the unexpected observations made by the Mars orbiters, looking forward to further data analysis.

Originating outside the Solar System, Comet 3I/ATLAS is the third interstellar comet observed, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Unlike Solar System bodies, interstellar comets offer insights into world formations beyond our own. First spotted on July 1, 2025, by the ATLAS telescope in Chile, 3I/ATLAS may be the oldest comet observed, potentially three billion years older than the Solar System itself.

In upcoming observations, ESA’s Jupiter Icy Moons Explorer (Juice) will study Comet 3I/ATLAS next month after its closest approach to the Sun, likely capturing data in a more active state. Results from these observations are expected in February 2026. Comet 3I/ATLAS presents a unique connection to the galaxy, and in preparation for future explorations, ESA is developing the Comet Interceptor mission, scheduled for a 2029 launch, aiming to target a pristine comet from the Oort Cloud or possibly another interstellar object.